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Radio Boulevard NATIONAL COMPANY, INC. HRO - COMMUNICATION RECEIVERS - "The Cream of the Crop" PART 3 Guide to Restoring the HRO Receiver, PW-Gear Box, PW-Dial, Hints & Suggestions Restorations in Detail - 1935 HRO SN: E-50, 1935 HRO SN: F-16, 1935 HRO H-103 |
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PART 3 |
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Guide to Restoring the HRO Receiver Though much of the restoration process is similar to any other vintage receiver's restoration, the HRO does have some specific parts the require special knowledge and techniques to restore or to duplicate. The PW-D Micrometer Dial section below is very detailed since it is quite easy to reassemble this device incorrectly. The PW-Gear Drive below is also detailed for the same reasons. In addition to the restoration and rebuilding information, five specific HRO restorations are profiled. These sections show the various restoration problems that are encountered during the process and each restoration is shown from beginning to end with lots of photographs. |
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PW-Gear Drive and PW-D Micrometer Dial - Detailed Information The heart of the HRO is its fabulous PW-Gear Drive and its PW-D Micrometer Dial. These incredibly engineered components working with the precision tuning condenser and the innovative coil sets account for the HRO's top performance, both electronically and mechanically. Unfortunately, almost every HRO receiver encountered will have problems with the PW-Gear Drive or the PW-D Micrometer Dial, or both. Most problems found will be traced to incorrect reassembly by former owners. The following section gives the HRO owner detailed information on how to correctly disassemble, service and reassemble the PW-Gear Drive and the PW-D Micrometer Dial. "PW" was National's designation based on the gear drive's use of a "precision worm." |
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Lubrication and Assembly of the PW Gear Drive |
| The Cast Metal Gear Box
- Every HRO I've worked on has needed servicing of the PW
Drive Gear Box. Usually, when the top cover is removed, it will be
apparent that the old grease is very dry and not coating where the gears
mesh. Also, you might find that the split-ring anti-backlash gears are
stuck from old dried grease that prevents the anti-backlash from
working. You might even find that the worm gear shaft is assembled
incorrectly. Down where the worm gear is located can't be seen from the
top. To fully service the PW Gear Drive requires disassembly. To check
the worm gear assembly the elliptic bearing hub has to be removed. Be
careful when removing the last screw that holds the elliptic bearing hub
in place because the worm gear shaft thrust is spring-loaded. Hold the
split-ring gear together with your fingers as you remove the worm gear
for inspection and lubrication because the split-ring gear is also
spring-loaded for anti-backlash. As you remove the worm gear, note how
many teeth backlash is set on the split-gear - it shouldn't be more than
two teeth. Clean all of the old grease off using a stiff tooth brush
with WD-40 or some kind of light oil. Then apply a light-weight grease
like Lubriplate to the gears. A small amount of light oil should be
worked into the split-ring gears so they can easily move. Be careful of
the three springs that load the split-gear. They sometimes come off
their mounting pegs but they are easy to reinstall.
Note how the worm gear shaft is assembled. It's not uncommon to find the HRO gear drives assembled incorrectly. Look at the National artwork to the right. This shows the correct assembly of the worm gear shaft, washers, spring and the ball bearing. Note from the worm gear flange there is a ball bearing, then a washer, a spring and another washer. The assembly is held in place by the elliptic bearing hub. The ball bearing appears to have no support but the design is for the ball bearing to apply thrust force against the shaft flange without an excessive amount of friction. The worm gear thrust bearing can be assembled in reverse order and the drive works okay but National's design provided minimum friction from the thrust force. >>> |
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| >>> It's also possible to assemble the worm gear thrust
bearing with the ball bearing reversed. This will not allow the ball
bearing to rotate properly. Usually the side of the ball bearing with
the stamped identification should be facing the flange. However, it's
best to test whether or not the ball bearing will rotate correctly. Do
this by assembling the thrust bearing on to the shaft. Then hold the
pointed-end thrust bearing against a piece of wood by pressing against
the spring with your fingers. Then rotate the worm gear shaft with the
fingers of the other hand. If the thrust ball bearing is mounted
correctly, the outer housing of the ball bearing will not rotate but the
inner race will. If the bearing is incorrectly reverse mounted then the
outer housing of the ball bearing will rotate with the shaft.
To reassemble the gear box first apply some light-weight grease (lots of lithium-based greases available - just don't use Lubriplate because it is a "sliding" type of grease, not for gears) to the conical thrust bearing of the worm gear as this is spring loaded against a mating conical thrust bearing that is machined into the cast gear box housing. Don't over lubricate any of the individual parts by packing the gear box full of grease since the thrust ball bearing and the split-gear must be able to move easily which is difficult when either part is loaded with grease. Be sure to thoroughly clean the thrust ball bearing as it is essential to the design that this bearing moves easily. Use a very light oil and grease mixture for the ball bearing. Lube the worm gear with light-weight grease. The split-gear looses its anti-backlash position for spring loading when the worm gear is removed (the "loaded" position is held by the worm gear) - just rotate the two gears in opposite directions a couple of gear teeth while engaging the worm gear. Be sure to watch the split-gears as it is common to have the anti-back lash setting slip as the worm gear is inserted into position. Check the anti-backlash tensioning springs to be sure they are installed on their pegs. Lube the inside of the elliptic hub with light-weight grease. Reinstall the elliptic bearing hub casting which spring-loads the thrust on the worm gear. Note that one of the flanges is marked "TOP" so be sure to mount the the elliptic bearing hub housing with this flange up. This is important because it places the elliptic hub in the correct position to function with the PW-D micrometer dial. Check the gear drive for smooth operation and zero backlash. The Cast Metal Gear Box is used from 1935 up to around 1945. The replacement Stamped Sheet Metal Gear Box was introduced with the HRO-5 and HRO-W receivers. |
The Stamped Sheet Metal
Gear Box - During WWII, the cast metal gear box was
replaced with a stamped sheet metal housing. This was a cost-reduction
that eliminated the difficult to machine cast metal gear box. Since the
new sheet metal gear box was thin metal, it couldn't be machined to
provide a thrust bearing at the rear. Instead a bronze bearing was
pressed into the sheet metal to provide a thrust bearing. Also, since
the sheet metal housing was thinner than the cast metal housing, the
condenser supports were integrated into the assembly support of the gear
box. This then reduced the clearance inside the sheet metal box and the
thrust ball bearing assembly on the worm gear shaft had to be
redesigned. With the sheet metal gear box, the thrust bearing assembly
now had the ball bearing at the front of the shaft and the spring
butting up against the worm gear flange. Also, the washers were
eliminated and a brass fitting was installed to allow the spring force
to be applied to the inner race of the ball bearing. If you try to
assemble the thrust bearing like it was for the old cast metal gear box,
it will not fit into the sheet metal gear box.
This reversal of the thrust ball bearing assembly on the worm gear shaft works essentially the same as the older assembly does and does provide a reduction in friction for the thrust force applied. The sheet metal gear box is introduced with the HRO-5 and HRO-W models (the new octal tube versions of the HRO for WWII production.) This dates from about 1945. All post-WWII gear box housings are similar in construction and include the HRO-5A, HRO-7, HRO-50 and HRO-60 versions. The later version of the split-gear anti-backlash drive is heavily spring-loaded. Usually the split-gears can only be set to "one tooth" of backlash. Since the springs are very strong this "one tooth" loading is sufficient for removing any backlash. |
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PW-D Micrometer Dial |
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PW-D Micrometer Dial Design - The famous "micrometer"
dial used on the HRO and several other National receivers was based on a
Sperry Gyroscope design. The National "Micrometer Dial," or PW-D (PW-D
is the National designation for the complete Micrometer Dial) doesn't
change during many years of manufacture except for two very minor
mechanical modifications made during WWII. Only two moving parts and
eight stationary parts are used in this elegantly simple design. The
following are the parts used: 1 - Index Dial (or Outer Dial - moving
part 1) How the PW-D Works - The PW outer Index dial by itself will not not turn the inner Number dial - it must be used with the PW Gear Box that has the elliptic hub mounted on the front of the gear box. The elliptic hub actually provides the eccentric that always forces engagement of the inner Number dial gear teeth to the gear teeth of the outer Index dial as the PW-D is rotated. The gear teeth are located on an embossed rim inside the outer Index dial and are cast into a circular recess in the inner Number dial. All of the parts must be assembled together correctly and the PW-D must be mounted on the elliptic hub of the PW Gear Box in order for the micrometer dial to operate and indicate properly. |
PW-D Mechanics
- Internally, all PW-D dials are the same and work by rotating the
Number Dial on an elliptic hub. The Number Dial is gear-driven by the
Index Dial that is also set-screw coupled to the main tuning shaft of
the PW gearbox drive. Turning the Index Dial simultaneously turns the
Number Dial and the HRO Tuning Condenser via the PW gearbox. The Index
Dial has five windows with ten index marks between each window. The
Number Dial displays sequential numbers that increment by 10 as each
window comes into view at the top of the dial. Only the when a window is
at the top of rotation is a particular number centered and easily
readable. The Number Dial has 0 to 500 numbers incremented by 10 and
each revolution of the Index Dial increments the numbers by 50 (five
windows viewed with each revolution.) With ten revolutions total of
numbers displayed is 0 to 500 incremented by 10. Since the Index Dial is
4.625" in diameter, its circumference is 14.53" and with ten revolutions
the total span is 145.3" or just over 12 feet (12.1'.) When HRO coil
sets A, B, C or D are set to band spread then the ham bands each span 50
to 450 on the PW-D. This is 80% of the full 500 divisions thus the ham
bands equate to a linear dial that is 9.68 feet long.
As to the two moving parts (Index Dial and Number Dial) and seven stationary parts - besides the Index Dial and the Number Dial there are two springs that hold the two dials together, three flat-head screws to mount the HRK-style knob to the Index Dial and a single set screw on early versions. During WWII the set screws were increased to two. Elegant in its simplicity, about the only thing that ever goes wrong with a PW-D is caused by technicians who carelessly disassemble the dial and then reassemble it incorrectly. More details on how to disassemble, service and reassemble the micrometer dial in the "Lubrication of the PW Gear Drive and PW-D Micrometer Dial Assembly" section further below. PW-D Evolution - The first PW-D had a flat nickel finish on the outer Index dial with black index graduations. The inner Number dial was also nickel plated with black numerals. German Silver was used on the smaller HRO knob skirts. German Silver is an alloy made of copper, nickel and zinc. Since the PW-D dial is cast metal, it can't be solid German Silver but is actually German Silver plating. Generally, the PW-D was supposed to appear "flat white-silver." By 1936, with the introduction of the NC-100, a number of different lacquer finishes were used for the PW-D. The HRO Senior and Junior PW-Ds will be found with paint that varies from dark gun-metal gray lacquer finish to a dark bronze lacquer finish. These varying paint colors appear black except under intense light. NOTE: In the photos below of both the HRO Sr and Jr PW-Ds. These colors will generally appear black in normal room light but under the intense light of the camera "flash" the true color shading is seen. The white index lines are very narrow on early PW-Ds and seem to become slightly wider during WWII. |
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The PW-D for the NC-100 was blue-gray on the outer dial and red with
white numerals on the Number dial. Some NC-100X used a gray Index dial
and the HRO type black Number dial. The gray PW-D was also used on the
National Airway Communication Receivers that were based on the NC-100
receiver. These receivers generally have gray PW-D Index dials with
black Number dials with white numerals.
It seems probable that Doehler supplied the PW-D castings to National and the painting was done at National. There are so many color variations that it seems unlikely Doehler would have stocked the paint and been able to keep track of the order requirements. It's more likely that National painted the PW-Ds as required. This probably also applies to the German Silver plated PW-D since National did their own plating. When the NC-101X was introduced, it usually was fitted with the same "black lacquer" PW-D as the HRO, although early versions are sometimes seen fitted with gray Index dials. Some pre-WWII military HROs were fitted with light gray PW-D dials. During WWII, some HRO receivers were moisture and fungus proofed (MFP) which sometimes included coating all of the knobs and the PW-D Index dial. MFP gives the outer Index dial an olive-drab appearance. |
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| After WWII, the HRO-5A series goes back to the dark bronze finish
for the PW-D with some exceptions that may have resulted from National
"cleaning house" and thus early dark gun-metal gray dials do show up on
post-WWII HRO receivers. When the HRO-7 was introduced the PW-D was
given a light gray paint job to match the changes that the HRO-7 cabinet
featured. The HRK knob was also changed to an HRT style knob. Some rack
mount HRO-7R receivers were finished in wrinkle black and these
receivers will have the standard "black" PW-D dial installed. The HRO-50
returned to a nickel or chrome finish to the outer dial with the inner
dial black with white numerals. The HRT style knob was retained. The
HRO-60 returned the HRK knob to the PW-D assembly. The HRK knob was also
usually installed on the HRO-50 if it was rack mounted.
Throughout this evolution only one minor mechanical change to the design was incorporated. During WWII a guide pin was added to the inner wheel which helped with alignment and made installing the PW-D onto the elliptic hub somewhat easier. Additionally, a second set screw was added, probably to meet "military specifications." National, at times, seemed to use whatever PW-Ds they had in stock - or maybe, in "excess" stock. It's common to find late-thirties HRO receivers with gray NC-100-type PW-Ds. Gray PW-Ds will also show up on early NC-101X receivers and on all National Airway Receivers. Examine such variations for authenticity but don't be surprised to find that the PW-D is original to the receiver. |
Repainting the PW-D - A lot of the PW-D Index dials will be in poor condition with chips, scratches and worn surfaces. Additionally, sometimes a receiver will be found that obviously has a later PW-D installed. The upshot is the PW-D Index dial will need repainting. I've been able to get automotive paint shops to successfully custom match a PW-D Index dial, even the dark bronze-black color. Only use high-quality auto-motive paint for this restoration process. Use Artist's Acrylic white + raw sienna + black to create a beige manila color for the index lines. Don't use lacquer stix or bright white paint. The manila color will look correct. Be sure to let the PW-D paint cure for at least four or five days before doing the index lines. It's rare for the Number dial to need a repaint. |
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PW-D Micrometer Dial Assembly |
| The PW-D Micrometer dial only has two moving parts and seven stationary parts. Elegant in its simplicity of design, it is very easy to work on if you know the "tricks." About the only thing that goes wrong with the dial is that it becomes noisy in operation and might have a rough feel when tuning. Synchronization might be a problem too - if the dial was reassembled incorrectly. The inner dial has gear teeth cast into its inner perimeter and also has the bearing that fits on the elliptic hub of the tuning shaft bearing housing. The outer dial has mating gear teeth that are cast into a rim projection and a hub that is set screw attached to the tuning shaft. By having the inner dial ride on the elliptic hub cast into the tuning shaft bearing housing, the gears of the two dials engage and as the outer dial is turned, by way of the mating gears, it turns the inner dial and since the outer dial is coupled to the tuning shaft, it also moves the tuning condenser. The elliptic hub is actually round but since the tuning condenser shaft exits the hub "off-center," the hub bearing surface appears as an ellipse to the tuning condenser shaft. This eccentric placement forces engagement of the inner and outer dial gears, thus driving the inner dial with the rotation of the outer dial. >>> | >>> To disassemble the Micrometer dial, first tune the receiver to "250" then loosen the dial set screw. The dial should come off of the elliptic bearing hub and tuning condenser shaft with very little effort. If the PW-D dial is hopelessly stuck, read the section below "Dismounting a Stubborn PW-D" for the removal procedure if this problem is encountered. With the PW-D dismounted, look at the back of the dial and, if the PW-D was synchronized prior to removal, you will notice that the three screws that hold the knob to the dial appear through cast holes in the inner dial. Also note that the two springs that hold the inner dial to the outer dial are pointing straight up. Remove the knob. Then remove the two springs - their ends fit into slots at the end of the shaft hub of the outer dial (where the set screw is.) Now the dials can be separated. Examine the cast gear teeth for condition. Usually they are in good shape and all that is present is old dried grease. Use a tooth brush and light oil to remove all of the old grease and then apply a light coating of Lubriplate to the inner dial gear only. Position the inner dial to the "250" position with the cast holes lined up for the knob screws. Then install the two retaining springs. >>> |
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>>> Before putting the PW-D back on the receiver,
lightly grease the elliptic bearing hub (since the inner dial rides on
this it should be clean, smooth and lightly greased.) Verify that the
elliptic hub has the "TOP" embossing on the flange at the top. This
places the elliptic in the correct position for the PW-D gears to
engage. The PW-D should already be set to "250" and the receiver
condenser should not have been moved. Place the PW-D back on the
receiver by carefully noting how the elliptic hub requires the inner
dial to be slightly down from center. If you have assembled the PW-D
correctly, it will just slip right on to the hub. Tighten the set screw
and test the operation - it should be ultra-smooth and very quite in
operation. If your PW-D dial doesn't just slip onto the elliptic hub
then it isn't assembled correctly. The PW-D can be assembled several
ways but only one way is correct. There are no exceptions to this
- if the PW-D doesn't slip right onto the hub then it is assembled
incorrectly. If your PW-D doesn't just "slip onto" the bearing hub, read
the section below "Correct Assembly of the PW-D Micrometer Dial
- Details." Around the beginning of WWII, a guide pin was added to the inner dial, located just above the inspection/grease hole. This pin limits the vertical movement of the inner dial and was probably installed to keep the gear alignment fairly tight and allow easier installation of the PW-D onto the elliptic hub. Later production dials will have this guide pin molded in the casting of the inner dial.
photo left: Two moving parts and seven stationary parts are all that are used in the PW-D. A second set screw was used during and after WWII - total of eight stationary parts then. |
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Correct Assembly of the PW-D Micrometer Dial - Details - When the PW-D is assembled correctly, it will just slip onto the elliptic hub. If you are trying to force the PW-D onto the hub, then the PW-D is assembled incorrectly - no exceptions. Look carefully at the photograph to the right noting the location of the smaller round hole, the two oval holes, the "NATIONAL CO., INC." embossing and the location of the screws that are for mounting the knob. If you orient the reverse side of your assembled PW-D in this position and then look at the outer dial, you should see "250" centered in the window that is directly behind the "NATIONAL CO., INC." embossing. To check your assembly,...position PW-D as shown in photograph to the right. You should see the following: 1. "NATIONAL CO., INC" embossing should be at 12 o'clock 3. Two oval holes should be at 2 o'clock and 4 o'clock 4. Note that all three knob screws are directly centered in the small round hole and the two oval holes 5. Turn dial over from right to left and you'll see "250" centered in the window at 12 o'clock (the window should be directly behind the embossing "NATIONAL CO., INC" on the back of the inner dial.) 6. Push the inner dial down to make the "250" appear closer to the bottom of the window. You may hear a slight click as the gear teeth mesh but you might only "feel" the engagement. It helps to "rock" the inner dial while pushing it down. If your assembled PW-D meets this criteria, then it will slip right onto the elliptic hub. You may find that, for some reason, the PW-D just doesn't want to fit onto the elliptic hub. Don't force it. When assembled correctly the PW-D will slip right on to the hub - no exceptions. If it doesn't, remove the PW-D and look carefully at the Number Dial bearing and its relationship to the tuning shaft hole in the Index Dial and note that the "off-center" nature of the opening is correct for the elliptic nature of the hub. Also, verify that the elliptic hub is installed correctly with "TOP" in the upper position of the mounting flange. If the all of these relationships are correct, then again, on the PW-D, very slightly push the Number Dial down to engage the gear teeth of the Index Dial to see if a better fit can be achieved between the gear teeth of both dials. You should hear a slight "click" when the "alignment slips into place" and is correct. Verify that "250" is still centered in the window of the Index Dial. At that point the PW-D will just slip right onto the elliptic hub. |
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| Dismounting a Stubborn PW-D - Sometimes removing the PW-D Micrometer dial proves to be a very difficult operation. Almost always, the difficulty is caused by excessive marring of the tuning shaft by the knob set screw(s.) If everything seems it indicate that the PW-D is not coming off of the shaft no matter what, this is the procedure necessary to remove the dial. First, for pre-WWII HROs, you'll need a couple of right-angle screw drivers. I usually have to make them because everything commercial is too big to fit between the back of the front panel and the tuning shaft bearing hub screws. Make at least two that have different angles for the blade engagement. Next, remove the upper rail that is mounted to the back of the front panel and forms the front top piece of the cabinet. Now, using the right-angle screw drivers, loosen and remove the four screws that mount the elliptic bearing hub screws. This bearing hub is spring loaded for thrust so be careful removing the last screw. Now, remove the entire PW-D and the bearing hub and the worm gear out through the square hole in the receiver's front panel. Now, remove the knob from the PW-D assembly. This provides access to the front of the tuning shaft. Then, using a soft metal drift, gently tap out the shaft from the PW-D dial assembly. You will have to reassemble the gear box per the instructions in the "Lubrication of the PW Gear Drive" section above. Also, be sure to dress down the marring on the tuning shaft so the next time the PW-D has to be removed, it will be an easy job. >>> | >>> For the WWII and later HROs, you'll need an open-end 1/4"
wrench. Removing the top two hex head screws that hold the bearing hub
is pretty easy but the two bottom screws will take patience. Usually
only one hand will fit into the space between the back of the front
panel and the tuning condenser, so, to remove the lower right screw, I
use a small thin rod manipulated with my left hand fingers to lift the
wrench onto the hex head and then turn the screw one flat with the
wrench manipulated with my right hand fingers. Reposition, and again
using the thin rod to help position the wrench, loosen the screw another
flat. On and on, it takes awhile but you can get both of the bottom
screws out that way. You'll have to change hands depending on which of
the bottom screws you are removing, right or left. I usually take the
bottom ones out first and then the top ones. Reassembly is easy since
you can access the hex screws through the square opening in the front
panel using a 1/4" nut driver. NOTE: When remounting the bearing hub note that "TOP" is embossed on one of the flanges. This flange obviously must be mounted up. This is important because it positions the elliptic bearing correctly for the PW-D dial. |
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HRO Restoration - Hints and Suggestions |
| Manuals - Fortunately, the HRO didn't change very much from 1935 up to 1945 (including some obvious errors and outdated chassis photos that were not changed for years.) The receiver was so well thought out and so well designed, only minor tweaking of some component values were necessary but the manuals were very slow a showing any updates. As a result of this, all of the manuals will have minor glitches but almost any manual will provide enough correct information for operation, repair and alignment of most HRO receivers. Be aware that a few changes in resistor values occurred when the 6.3vac tube HRO Seniors became standard (~4/39.) The screen divider (R10 & R15) was changed slightly to provide a lower screen voltage for the newer manufacture 6C6/6D6 tubes. In the 1940s, National issued manuals that were usable for all of the HROs, e.g., the HRO, Senior, Junior, M, MX, RR. Later, the HRO-5 was also included in the compilation. These manuals are available as copies from several sources. If you are working on an early 2.5vac HRO be sure to use the earlier compilation for the correct resistor values (but don't be surprised if your receiver has original components that are slightly different than the parts list values.) As for original manuals, they are interesting to own but not absolutely necessary as an information source. Even the Rider's Perpetual Troubleshooters Manual Vol. VIII has enough information on the early HRO for repair and alignment. | Capacitors - All paper-wax capacitors need to be replaced in any vintage receiver if you intend to operate the set more than just casually. In the HRO, all the capacitors are easy to access and replace. In fact they are so easy to get to, I usually go a step further in the restoration process and restuff the original capacitor shells with modern film caps. This is just a cosmetic issue and doesn't make any difference to the final performance but it preserves the original under-chassis appearance. The two cathode bypass electrolytic caps are riveted to the chassis with one of the rivet heads under an IF transformer shield. They can be dismounted, rebuilt and re-riveted - if you are a purest. Also they can be left in place, disconnected and the new replacement electrolytic cap mounted nearby if under chassis appearance isn't an issue. Actually, these two electrolytics were well built and some of the originals might be usable. They were very low voltage (25vdc) and were well sealed - just test, reform and, if good, use. |
| Ceramic Lead-end Resistors - National made their own resistors. They consist of a piece of ceramic round stock that has been drilled lengthwise to accept small diameter carbon rods. The rods protrude out the ends and when the lead-end was cast, contact was made with the carbon rods. Usually two rods are used on larger resistors and one rod on the small size. Over the years the contact to the carbon rod(s) may become compromised or maybe the rod(s) will crack - either will cause the resistor value to change significantly. Usually a restorer will find at least a few of these National resistors that need to be replaced. If under chassis appearance is not important than just replace the resistor with a modern equivalent. If you want to preserve the original appearance of the National resistors then a reproduction has to be made. These are actually easy to replicate using the lead-ends removed from the original resistor - they just twist off without too much effort. Select the proper value IRC resistor (International Resistance Co. - the type made in the forties are the correct diameter.) Drill 1/16" holes at the center of the lead end-caps and insert the resistor leads through the holes, sliding the end-cap on to the resistor body. Solder the leads to secure. Paint the resistor body white and you're done. Install the repro resistor. For larger types, use the closest size resistor and if necessary slightly enlarge the end-cap opening with the correct size drill bit to get the proper fit. See the "1935 HRO H-103 Restoration" section further below for photos and more details.>>> | Crystal Filter Problems - Most of the time, the Crystal Filter works quite well and we hardly ever think about a problem developing inside. The "ON" switch is incorporated into the SELECTIVITY control and actuates the filter when advanced. You can usually feel a slight "thump" as the switch changes position since it is cam-actuated. Sometimes the cam wears or other mechanical problems happen requiring some adjustment to the action of the cam and switch. Geoff Wooster (in the UK) ran into a problem involving the crystals themselves. This problem showed up on two different filters. Oxidation on the plates inside the crystal housing caused the crystal activity to be much lower than normal. The filters acted more like attenuators than anything else. Geoff's solution was to disassemble the crystal and thoroughly clean everything including polishing the metal plates inside. This cured his filter problems. It is possible in very humid areas that oxidation can work its way inside the crystal. It can take years but these crystals are over 70 years old now. James Millen mentioned the HRO crystal filter in one of his QST letters in the 1930s. His recommendation was to clean the crystal with denatured alcohol. He also mentioned that the crystal should be slightly loose in the holder. This would be the older plug-in style crystal used in the pre-war HRO. |
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Cables for the HRO - If your HRO doesn't have its
original power cable, examine the one installed carefully. The amount of
current required to operate the tube heaters will cause a significant
voltage drop unless the proper gauge wire is used in the cable. The
2.5vac tube heaters draw a total of 9.55 AMPS, or about 23 watts! Also,
there is a center tap resistor to eliminate hum that is also across the
tube heater line. The 2.5vac HRO cable should have 10 gauge wire in the
cable for the tube heaters. All reproduction brown cloth covered cables
(for battery radios) are inadequate due to the small gauge wire used in
the cables, (there are two types of repro cables, one has six wires and
one has eight wires.) Even connecting two or three wires in parallel
will not lower the resistance enough, resulting in too much IR
drop for proper heater voltage (along with heating up the wires in the
cable.)
The first option is to find another HRO power cable by salvaging one from a derelict HRO. It requires finding a "parts set" and that is becoming more and more difficult as almost all HRO receivers are becoming desirable and expensive,...even when they're "junkers." Another solution is to search for an authentic vintage battery radio cable of sufficient length that has heavy gauge wires for the A battery connection. Most of the Atwater Kent battery sets from 1925 and 1926 have two ten gauge wires for the A battery and then several small gauge wires for the B and C battery connections. If one can be salvaged from a derelict A-K radio, leave two small gauge wires for B+ and B- and then cut the remaining small gauge wires close to the braid. The cable will have two 10 gauge wires and two ~ 18 gauge wires. When installed, it will look very authentic. The other option is to build the power cable using two 10 gauge wires for the heaters and two 18 gauge wires for the B+ and B-(chassis.) The 6.3vac HRO Seniors are a different matter as they only draw 3.1 amps at 6.3vac for the tube heaters. By connecting two or three of the wires in parallel in the reproduction cables, adequate current carrying ability will result and the IR drop will be minimal. Vintage battery radio cables will also work fine with the 6.3vac HRO receivers. >>> |
>>> The National power supplies do have an increased voltage for
the heaters at the transformer. Usually about 25% increase to compensate
for the IR drop across the power cable but this was designed to
compensate for cables having large diameter heater wires. The best test
is to check the voltages with the HRO operating on the intended power
supply and measure the heater voltage across the hum elimination
resistor. Heaters should be within 5 - 10% of the specified voltage.
With low heater voltage (>20% drop,) the HRO seems to work okay but
you'll notice a general lack of sensitivity requiring the RF Gain to be
advanced more than usual. Also, check B+ at the tie point where the
cable connects. With low B+, the HRO also might seem to work okay on the
lower bands but the S-meter will not work correctly and the RF Gain will
have to be near maximum for CW reception. B+ should be 230vdc at about
70ma. In actual measurements the B+ will vary depending on if the HRO is
receiving AM or CW signals, how loud the set is playing, etc. - it is
normal for the B+ to vary from 220vdc up to about 245vdc depending on
the load. National stated that the HRO would work on as little as 135vdc
B+, but they did add, "at some sacrifice in performance."
Usually, the B+ IR drop is never a problem because the current
draw is so low. Cable Connections - The schematics were always vague about the plug connections and the wire colors. The cable used a 10 gauge wire, colored red for FIL AC connection to pin 1 of the plug. The cable used an 18 gauge wire, colored blue for B+ connection to pin 2 of the plug. The cable used an 18 gauge wire, colored either green on early cables or yellow on later cables for B- /Chassis connection to pin 3 of the plug. The cable used a 10 gauge wire, colored black for FIL AC connection to pin 4 of the plug. Inside the chassis, the 10 gauge, red wire connects to pin 1 of the BFO tube socket. The 10 gauge, black wire connects to pin 6 of the BFO tube socket. Also, between pins 1 and 6 of the BFO tube socket is connected the wire wound hum reducing resistor. The 18 gauge, blue wire connects to the same tie strip (but not the same terminal) that also provides a ground connection for C28. The 18 gauge, green (or yellow) wire is connected to a chassis ground lug that's mounted at the same tie strip. Early cables had the B- wire colored green. Later cables had the B- wire colored yellow. |
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Building a New Power Cable - Even if the original
cable is tattered and worn, it is best to keep it since it has the
correct current carrying ability for that HRO - but if the restorer
wants to replace a missing old cable on a 2.5vac HRO with
a NEW cable of the same original length, it will have to be
built. The original HRO power cables were about four to five feet in
length.
In order to have the new cable be flexible and to have very low resistance you will have to search for proper wire. There are a number of companies that make replica cloth covered wire. Usually this is for vintage lighting but also vintage-looking automotive wire is being produced. You'll need 10 gauge and 18 gauge wires to make the new HRO cable. The next problem is the outer covering of the repro cable. All of the braiding that I've seen doesn't look vintage and sometimes looks like plastic. There is cloth tape but it never seems to look correct either. Some enthusiasts report the some braided tubular shoe laces can be expanded and used for cloth braiding. There are expandable woven cable covers available in black that look very nice - just not original. It's also possible that a special build from one of the antique automobile electrical harness builders could supply an authentic HRO power cable with the correct gauge wires and the proper cloth covering. No doubt an expensive solution. |
2.5vac Tubes
versus 6.3vac Tubes - The Myth - James Millen wrote, in
his monthly QST letter of March 1937, a recommendation that when the HRO
was operated on an AC power supply, the 2.5vac tubes should always be
used. Millen stated that the 6.3vac tubes produced a noticeable increase
in the hum level and noise that would in some cases mask weak signals.
In the April 1939 QST letter, Millen recanted what the earlier letter
had recommended. Millen stated that the 6.3vac tube quality had improved
significantly and there was no longer any advantage to using the 2.5vac
tubes. The letter elaborated that a modulated hum around 15MC had been
an earlier problem with the 6.3vac tubes. Additionally, the letter
mentioned the new Model 697 power supply for the HRO which provided
230vdc B+ and 6.3vac. Also mentioned was the fact that earlier HRO
battery operated receivers could not be operated on the new 697 due to
the B+ being too high unless the receiver was modified. Some former
National employees speculate that the actual reason for continued use of
the 2.5vac tubes was due to National being "over-stocked" with power
transformers that had the 2.5vac winding which were essentially useless
unless used in the HRO power supply.
Unfortunately, many hams and collectors only remember James Millen's original, 1937 remarks. This had led to an "Urban Ham Legend" that the 2.5vac tubes are superior and re-tubing a later HRO Senior with early style tubes will give great performance results. This isn't true. The first problem encountered might be insufficient current carrying ability of the power cable for the 9.5 amps required to operate the 2.5vac tube heaters. It depends on the particular HRO, although most do use the same power cable regardless of the heater voltage. Also, the screen divider was changed for the 6v HRO Senior to provide a slightly lower screen voltage required for the newer manufacture 6C6/6D6 tubes. A somewhat higher screen voltage was used on the old 57/58 tubes and early 6C6/6D6 tubes. The later HRO Senior, with the 6.3vac tube IDs on the chassis, will perform great "tubed" as it came from National. The early HROs that were originally 2.5vac receivers should be operated "as designed" - with the 2.5vac tubes. Although, the early 2.5 vac HRO can be re-tubed to 6.3vac and it will operate okay, you usually can't re-tube a 6.3vac HRO to 2.5vac tubes and expect it to work well without some modifications. |
| Loudspeakers - The HRO normally does not have an audio output transformer installed in the receiver. This was common practice when the HRO was designed. Most manufacturers mounted the audio output transformer on the speaker. This practice does allow full B+ to be present the speaker pin jacks at the receiver - always a source of unnecessary worry for electrophobics. Even the Navy worried so much about the B+ on the speaker pin jacks, they wanted output transformers installed on the HROs that they ordered (all military National receivers, in fact.) If you don't have the "NC" speaker, or you have the box with a non-original speaker and need an audio output transformer, the HRO output Z is 7000 ohms on the primary (single ended) and the speaker Z is usually 8 ohms. Most universal audio output transformers will have a range on the primary, e.g., 4K to 10K ohms Z and a selectable secondary of 4 or 8 ohms Z. These usually work fine with the HRO. >>> | >>> Check the DC resistance of the primary, it should be around 150 to 300 ohms but this depends on the power rating of the transformer. Higher power transformers will have larger gauge wire used and therefore a lower DC resistance. You should go by the impedance first and just reference the DC resistance. There are many "universal type" audio output transformers that will work fine with the HRO. The Triad S-53X is an excellent choice (but certainly not the only one,) providing 7K ohm primary with an 8 ohm secondary with the primary DC R at 300 ohms. (Actually, the S-53X is a universal audio output transformer with the option of selecting many different combinations of primary and secondary impedances but a 7000 ohm Z primary and 8 ohm Z secondary are options that are provided.) |
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Modifications - Unfortunately, the HRO and HRO Senior receivers were built during the Depression. This means that many hams - out of economic necessity - had to keep their receivers as long as possible. The temptation to modify a great design was going to be there. Also, during WWII there was a complete absence of parts for those hams who could still operate - the only way to legally operate was as part of the Civil Defense emergency nets that were on usually once a week during WWII (transmitters had to be registered with the Navy to be legally used during that time.) This also led to some modifications out of necessity in keeping the receiver operational. After WWII, cheap HROs were around and subject to more abuse during the "modification mania" of the fifties. Very few pre-WWII HROs are found today that don't have some kind of mods installed. Whether the receiver can be returned to original will require a detailed examination. Unfortunately, many hams drilled holes and cut panels or chassis to install their so-called upgrades. Certainly, the rarity of the HRO will dictate whether the effort is justified to return the great old receiver back to its original design and appearance. Be aware that National performed their own upgrades on receivers that were returned to the factory for repairs or "updating." Usually the caliber of workmanship and the engineering thought process is far superior to that encountered from the average "hamster." >>> |
>>> Post-WWII production, that is the HRO-5TA1, HRO-7, HRO-50 or HRO-60, are less likely to be found extensively modified. The abundance of surplus gear to modify and the fact that the receivers were new or fairly new seemed to curb "cutting and hacking" activity. Sometimes product detectors will be found installed in the NBFM accessory socket of the HRO-50/60 but these are normally homebuilt and based on an article that was published in CQ magazine in the mid-sixties (later in Electric Radio.) The problem with the HRO-50/60 product detector plug-in mod is that the BFO is normally disabled when in NBFM and the stock National switch is a special build that doesn't provide a terminal to actuate the BFO when in NBFM. A separate BFO switch is necessary although some mods have the BFO wired to be "on" all the time - okay for CW and SSB (the main concern at the time) but a problem if AM reception is desired. These mods are easy to remove and return the circuit to an original configuration. Operation today using the HRO-50/60 is normally for vintage AM nets and the need for a product detector is nil. However, if CW or SSB operation is desired using a stock HRO-50/60, just reduce the RF Gain and increase the AF Gain for the proper ratio of signal to BFO injection at the second detector - that's the way it was done before product detectors came along. |
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HRO Restorations in
Detail |
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1935 HRO Receiver SN E-50 - Restoration E-50 was donated to the Western Historic Radio Museum by Jim Larsen AL7FS of Anchorage, Alaska in September 2011. E-50 was given to Jim by Val Anderson of Seward, Alaska. In the 1940s, E-50 was owned by Anderson's brother-in-law, Carl Carlson of Anchorage, Alaska. Carlson may have gotten E-50 through his son-in-law who worked for the CAA in Alaska at the time. By the 1950s, E-50 had gone to Cordova, Alaska and when Carlson died in the 1950s, the receiver was shipped to Val Anderson. E-50 had spent most (if not all) of its existence in Alaska. The receiver was stored for many years in Anderson's basement and, due to the cold, humid coastal environment of Seward, E-50 ended up with some rust issues. Additionally, several years of modifications had left E-50's originality somewhat compromised. A couple of the modifications suggest that the receiver was rack mounted (possibly when used by the CAA.) This article is rather lengthy and changes style towards the end when I had started using a "journal" approach to these restoration write-ups that have the work progress as a daily entry, like a log. Since the restoration of E-50 took seven years from start to finish, I guess some evolution in my writing style must have also happened. |
| Pre-restoration - The photos to the right show E-50 as received. The condition is actually not too bad when considering the storage and history of E-50. Obvious problems are the rust issues which will require extensive disassembly. The modifications to the Mixer and Detector tube sockets will require obtaining original type sockets. Six-volt tubes are installed. The pilot lamp shouldn't be installed on a second production run receiver. The PW-D is the incorrect vintage and is missing the internal number dial. |
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The S-meter is the later Marior Electric type (1938) and meter
switch isn't original either. The dial pointer is not the correct
vintage. The RF Gain knob is a later style. The coil set installed is a
late-WWII or post-WWII vintage. Note that the lid finger projection has
been flattened, probably to have the receiver installed in a rack.
The photo left shows the underneath of the chassis of E-50 as received. The five orange paper capacitors are the only original Micamold brand types that haven't been replaced with other types. There are a handful of original white ceramic leaded-end resistors still in the circuit. Most resistors appear to have been replaced just after WWII judging by the types used. Wires that aren't black are non-original. Note the Mixer socket and the Detector socket are octals and all of the associated parts are replacements. The shielded wire from the Antenna terminal is not original. Despite the "rough" look of E-50, it's a very restorable receiver. However, E-50 is a "long term" project. That is, the missing parts will take some time to find. The extensive rework necessary will also take time. Some of the missing or incorrect parts will need to be replicated which might require developing a methodology to achieve the desired results. Most likely, as certain original parts turn up, some rework will take place. This process will go on until E-50's restoration is finished (Who knew then it was going to take seven years!) |
| Disassembly and Chassis
Restoration - Note the photo to the right showing the
severity of the rust. It was going to be necessary to completely
disassembly the E-50 chassis right down to the rivets in order to fix
the rust problems. With careful drilling the rolled ends of the rivets
can be removed and then the rivet driven out with a small punch. This
allowed complete removal of all parts on top of the chassis.
Additionally, with the rivets removed the complete circuitry from under
the chassis can be removed intact. This will make reassembly of the
circuitry much easier than if all of the parts were removed
individually. The chassis is painted with nitrocellulose lacquer and we are going to repaint using the original type of black nitro-lacquer paint. This means the complete removal of the old paint is not entirely necessary. The new paint will not react with the old paint and the places where the new paint covers old paint will not show since the edges are sanded and feathered. The easiest way to get at all of the rust is to just use sandpaper and wire brushes to remove it. The end result can be seen in the photo below. The chassis then needed the miniature tube socket hole filled. It can be seen in the "before" photos above and in the "sanded" photo below. Bondo was used to fill the hole. After sanding down the Bondo, it has to be primed with lacquer and then sanded lightly again. This seals the Bondo for final painting. If I was doing this repair today I'd use "USC All-Metal" which is a metal-filled body repair paste that's activated with a catalyst. It's very strong though easy to work. It does require priming before painting. |
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| The aluminum shields for the IF transformers, the BFO
coil and the tube shields all required a chemical treatment to return
the flat-satin finish that the aluminum pieces originally had. I had to
disassemble the IF and BFO units so only the shield-cans would be
involved in the chem-treatment. I use sodium hydroxide which is easily
found in Easy Off Oven Cleaner (EOOC.) The active chemical is basically
lye and it will react fairly quickly with aluminum. I spray on a heavy
coat of EOOC on the aluminum piece and then wait about one minute. If
there is a lot of corrosion, I'll work the EOOC around the aluminum
piece with an acid brush. After about two minutes, I rinse the piece in
cold water. You can't rub the aluminum after this treatment or it will
slightly polish and ruin the flat finish we are trying to achieve. Let
the pieces "air dry" and reassemble after that. The photo to the right
shows E-50's chassis assembled with the EOOC treated aluminum pieces
installed. HRO Cabinet and Panel Painting - The original black wrinkle finish used on all early radio equipment was a two-part process that used a base coat of Nitrocellulose Lacquer followed by a spray-on catalyst that was heat-activated, usually requiring oven-baking to produce the finish. Today, all we have available is a single-part process that can work quite well if the proper conditions are provided for the wrinkling process. First, one has to observe how the original cabinet and panel were painted. Since the base coat was smooth nitrocellulose lacquer, this will appear as gloss black. Note that all inside surfaces of the cabinet and panel are gloss while the outer surfaces are wrinkle. The outer surfaces was where the catalyst was sprayed. When baked to activate the catalyst, the result was gloss black on the inside and wrinkle finish on the exterior. Before painting, wipe down all of the surfaces with lacquer thinner. This is to remove all grease, oil or other contaminates. To duplicate the original finish appearance, we first have to spray the gloss black lacquer on all of the inside surfaces of the cabinet, the lid support lips and the back of the front panel. On the removable front rail, spray gloss black on the inside surface and only on the lip that supports the lid. Let the gloss black lacquer dry overnight. To do the wrinkle finish successfully you should only do one surface at a time. If you try to paint the entire external surface of the cabinet, you're sure to have a problem heating the entire paint job enough for the wrinkle to appear evenly. Before painting, I set up at least two heat lamps that are nothing more than 100W incandescent lamps with the aluminum bell reflectors as the primary heat source. I also have a handheld heat-gun to apply heat to specific zones that are the usual problem areas - like edges or corners. |
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| I apply the wrinkle paint in at least four heavy coats with about
one minute waiting time between each coat. Each coat is applied in a
different direction to avoid a "striped" pattern. Once the surface is
painted, it is placed it under the lamps. After about five minutes, I
start using the heat-gun to generally heat the entire surface but to
also heat areas that aren't receiving a lot of heat from the lamps. I
usually will apply the heat-gun from underneath of the painted surface
because too much heat directly onto the wrinkle paint will cause it to
"gloss" and not match the texture. I always keep the heat-gun moving to
keep the heat distributed and to avoid burning the paint. After a few
minutes, the wrinkle pattern will begin and then, using the heat-gun, I
apply heat just ahead of the pattern to move it along. The entire
surface should be wrinkled in about ten minutes total time. Now, I move on to the next surface. At the edges, I shoot the paint from over the surface being painted rather than from the opposite side. That way the edges will blend and not have an "over lap" of paint that won't match correctly. When I do the top lid, I set it up so that the lid is down on the supporting lips. That way, the gloss black will be protected from the wrinkle finish and the end result will look like the original application did - see photo below. The wrinkle finish is very, very soft at first so it's best to avoid any contact with the wrinkle finish "surface." I have to move the pieces carefully as I set up to shoot each side. When the painting is finished, I'll let all the pieces cool down for awhile. It's best to let everything set undisturbed for about one hour. After an hour, the wrinkle has set up enough to handle and move the pieces indoors but the wrinkle finish will take about two days to develop it's hard, durable surface. The longer it ages, the harder it gets. Avoid mounting anything to the wrinkle finished surface for at least two days. |
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| Replicating the
S-Meter - Close study of a photo sent to me by K6GLH of
his "E-run" HRO allowed me to see how the unique E-run S-meter looked. I
found that the same meter was used on the RME-69 and I had a parts set
'69 that could provide the proper meter. The next step was to replicate
the scale. I used the original scale from the S-meter in D-65 to make a
copy on photographic paper. The photo-paper results in a slight gloss to
the scale. The DCR of the HRO meter is 35 ohms and the RME meter is
32 ohms. The RME meter movement works fine for the early HRO receiver. Replicating the Pearl Button Switch - I tried to paint an old ivory push button switch with Revlon Perfect Pearl fingernail polish. The Revlon "paint" never looked correct so I was looking for an original pearl button switch. Thanks for the Parts - Thanks to Scott McDonald KA9P for the small pointer knob for the Selectivity control. Thanks to Bill Fizette W2DGB for the two National ceramic tube sockets needed. Thanks to Jim Allen NU6AM for the small red "NC" pointer mounting screw. |
White Graph B Coil
- I was able to build up an early version B coil thanks to the
acquisition of D-65 with its matched coils. I was able to copy all of
the white background graphs from D-65's coils. I had the coil assemblies
for an early B coil set, although from a F-run set so the contacts were
the larger type. I had a thick panel with wrinkle finish left over from
the F coil that came with E-50 (those F coil assemblies were mounted to
the correct type steel panel to build up an F coil set for my HRO-W.)
After assembling all of the early style parts, I now have a complete
white graph B coil to go with the early, small contact, white graph C
coil for E-50 (I already had the early C coil as an orphan with the
spare parts.)
Paper Capacitor Update - A thorough examination of HRO D-65 has lead me to believe that all early HRO receivers came with Micamold brand paper caps. D-65 has 15 Micamolds with the remaining 6 caps being mostly Sprague (one Tobe.) E-50 has 5 Micamolds remaining but E-50 has an extensive repair history. It's unlikely that so many Micamolds would turn up in two different HROs unless they were original caps. A check of the underchassis photo from the 1935 and 1936 HRO manuals is clear enough to read the "Micamold" logo on the capacitors in that receiver which seems to confirm that they are the original brand used. |
| Nickel Plating the PW-D
- I thought I might be able to create an early style nickel plated PW-D
by using a later painted PW-D. The painted dial is slightly different
with thicker index lines on the outer dial and thicker numbers on the
inner dial but with the correct color coming from the plating, these
slight differences wouldn't be objectionable.
Nickel plating generally should be done over copper plating or brass. I had to try to nickel plate pot metal. I've plated aluminum with no problems, so I thought the pot metal would do okay. Well, the first attempt turned splotchy and dark, a sure indication that too much current was being used for the electroplating. The second attempt, at reduced current, also turned dark. >>> |
>>> The third attempt reduced the current even more and the results were okay. There was still some splotching but overall the dial looked close to original. I felt that the plating might have turned out better had I used a different kind of anode. I was using pure nickel for the anode and I'm pretty sure the original PW-D dials were German Silver plated. Maybe I'll use a nickel coin next time for the anode. I think the best solution would be to find an original plated PW-D dial for E-50. |
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All of the work described in the above sections was done while I was living in Virginia City. The 2012 move to Dayton, Nevada certainly seemed to stop this project for awhile. Original PW-D Found - Finally, after over two years of "keeping an eye out" for a nice condition German Silver PW-D, one has turned up. I fitted this correct PW-D to E-50 and also installed the correct red "NC" diamond to the pointer and the transformation was stunning. What a difference an original German Silver PW-D makes. Sometime later, I was going thru some junk and ran across a Supreme Diagnometer that happened to have the exact pearl push-button that was also used in the early HROs. I swapped the ivory (painted pearl) push-button that had been in E-50 for the "real" pearl button in November 2016. UPDATE: NOV. 2016 - Just so readers know that I am still interested in finishing E-50, I recently found a correct "short boss" RF Gain knob for E-50. I've also found a roll of the correct wire with black cloth insulation for replacement of some of the wiring. An original-type pearl push button is installed and the S-meter scale is now the correct, early-style scale. |
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| Getting Started
Again - March 26, 2017 - E-50 had been languishing around
since before the Dayton move in 2012. Not that there hadn't been some
progress on the ultimate completion of E-50. All of the parts that were
missing in 2012 have now been found. E-50 is now complete with a very
nice condition German Silver PW-D, a "NC" dial index pointer, a genuine
pearl push-button S-meter switch, the correct early S-meter scale and
all correct, short-boss bar knobs. I have two coil sets, B and C, that
are for E-50 (white graphs and one with early-style contact buttons.)
To get restarted, I stripped out all non-original components and non-original wire. This allowed me to see that about 75% of the wiring was original and the majority of resistors were the correct National type. Only five of the capacitors were Micamolds. I still haven't exactly worked out how to get the paper shell to look like a Micamold but I think orange paper labels made from copying the original Micamold labels might work. Experimentation will be necessary when we get to that part of the restoration work. As a last resort, I could use Sprague shells since National went to that brand in 1936. |
Using a Wiring Model
- The easiest and quickest way to determine what is original and what
isn't is to compare E-50 to another early HRO receiver. Additionally,
the 1936 HRO manual has a very good under-chassis photo that provides
another look at the early wiring of HRO receivers. The schematic and
parts list can also be used for confirmation of component values and
hookup. So, with both receivers upside-down on the bench, the
restoration of E-50 could get underway.
Wire - Almost all the hook-up wire in an HRO receiver is black cloth-covered stranded wire that is about 18 gauge. Since I had pulled the harness out intact for the chassis repaint, most of the original wires that were left went back into the chassis as a semi-complete harness. This meant that the tube heaters and many other wires were already present and connected correctly. All that was needed was to replace non-original, replacement wiring and to install missing wires. There were two non-original tube sockets that were mods from the past. I had replaced these with original National ceramic sockets. I was able to use the original heater wiring to connect up these sockets. |
| Assemblies - Since the IF transformers, the BFO assembly, the Crystal Filter assembly, Audio Gain pot, S-meter and S-meter switch had to all be removed for the chassis restoration, these assemblies needed to be connected back into the circuit. These components do provide some of the wires that aren't black cloth insulated. Once the chassis mounted component-assemblies were connected into the circuit the next step was to replace non-original wires and missing wires with duplicate of original black cloth insulated wire. | Replicating National Hook-up Wire - The old wire roll I had looked very close to the National HRO hook-up wire except the wire itself was solid conductor where the original National wire is stranded. When I needed an original-type wire I would measure the length needed and then strip off that length of cloth insulation from the solid conductor wire. Next, I would cut a slightly longer length of modern 20 gauge stranded wire and feed it thru the cloth insulation with the 20 gauge wire insulation not removed. Once the wire was thru the cloth insulation, about one inch of the modern insulation was removed on each end. Then the cloth insulation was pulled tight around the modern wire by pulling it thru my fingers. This results in the wire looking like National-correct stranded hook-up wire but with old black cloth insulation over modern insulation that can't be seen. Each time I needed an original-looking wire this process had to be used to create a wire that would have the correct under-chassis appearance. I was able to nearly complete the under-chassis wiring by this method and the appearance is authentic. |
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Well,... here we are again, the beginning of Summer 2017, and the various "new" radio restoration and documentation projects are getting backed up. I had to set aside E-50 due to beginning a major restoration project,...the 1935 USN RAA-3 Long Wave Superheterodyne receiver. This ultra-rare, 465 pound, three-unit receiver will consume a lot of restoration time for a while,...at least until the major work is finished. Then, during the winter, I should be able to insert E-50 into the work schedule. E-50 is close to completion with only resistors and capacitors remaining. The next session of work on E-50 will complete this long-running project. To be continued,.... |
| More Parts Turn up for E-50
- Oct. 13, 2018 - I received an e-mail from collector
Terry N3GTE who had an early style D coil set that he wanted to trade
for a late-thirties type D coil set. I went through the spare coil sets
and found a decent D coil set that tested good and looked presentable
but could still use cleaning and new chart covers. We each sent our
trades out and the early D coil set arrived here a week later. This now
gives E-50 three coil sets, B, C & D. Still need to find an early style
A coil set to have the complete (standard) frequency coverage.
I went over to KB6SCO's storage unit on October 13th and was helping him clear out some junk. John gave me a cigar box that was "filled" (literally) with nothing but leaded-end resistors. About half of them were white ceramic with blue ink "hand-written" values while the rest were painted color-coded or paper labels over the white ceramic body. Just what I needed to complete the restoration of E-50. The next work session on E-50 should complete the restoration. I've decided to just replicate the Micamolds as close as I can by building them by using salvaged paper capacitor shells and covering the shells with replica (well,...close copies) orange paper "Micamold" labels. Sounds bad but should look good,...I think. The resistors hopefully can be found in the cigar box but if the values needed aren't there then the leaded-ends can be used to make correct value replicas. More details when the work gets started again in a couple of months - winter project. |
| Back at E-50 -
November 26, 2018 - Winter was early! Sorted thru the
ceramic resistors to find the values needed. Ten resistors were needed.
Found one that would work. The remaining missing resistors will need to
have replicas made. I use defective ceramic leaded-end resistors and
remove the ends. These are installed on IRC type carbons that are the
correct value and approximately the same physical size as the original
ceramic body. Then the new resistor body is painted white. Looks good
from a distance and works fine. The procedure is covered in the
restoration write-up on HRO SN: H-103 above. Finished up installing the few remaining wires that were missing. Also completed the connections to the 'phone jack and to the S-meter and S-meter push button switch. |
November 29, 2018 - After carefully checking values only two of the ceramic resistors can be used. I went thru my vintage IRC carbon resistors to find the correct resistances necessary and that the resistor was the correct dissipation so it would be the proper physical size. All values required were found and double-checked using a DMM. I salvaged the required leaded-ends from defective ceramic resistors first and the remaining were salvaged from ceramic resistors that were duplicates of unusable values. Finished building the nine ceramic leaded-end resistors Dec 1st. Ended up needing to build three more leaded-end resistors (besides the nine) to complete the circuit - total of 12 needed. |
| December 2, 2018 - Different Circuit Wiring? - I ran into a difference between the wiring of the 2B7 tube in H-103 and the early version manual schematic I'm using. It involved the 1st AF Amp tube biasing and the AF Gain control wiring. My thought was that certainly the D-run receivers are wired as the early version schematic and probably the circuit wasn't revised until mid-E run at the earliest. However, I pulled the bottom covers on both HROs D-65 and on F-16. I found both receivers were wired just like H-103,...hmmmm. It's likely that D-65 had been sent back to National for upgrading around 1937 and the 2B7 wiring upgraded at that time resulting in the stock-original appearance of the rework. F-16 wiring appears to be original. Finally, I happened to remember that D-76 was up for auction on eBay. I took a look and the seller did have a good photo of the under chassis. Obvious was the early wiring of the 2B7 in D-76. So, it seems that National changed the 2B7 wiring by run-F but, although it's hard to imagine, National didn't change the schematic or photos in their manuals until after run-N. I initially thought I'd go ahead and wire E-50 per the early schematic. But, a closer inspection of E-50 revealed that the amount of rework involved (taking out some of the wiring I had already installed) would be substantial for such a minor deviation in the circuit wiring. I decided to go with the later version schematic for the 2B7 wiring and leave the wiring that I had already finished installed. | Dec 4, 2018
- Discovered that I hadn't installed the B+ wire to the LO. This was
because on H-103 the wire running under some components. When using D-65
as the reference, the missing wire in E-50 was pretty obvious. Also
needed another wire that connected the 800 ohm resistor to the 2B7
cathode. I had to rob the 100pf, 250pf and 500pf mica capacitors out of
a HRO "parts set" to install these components into E-50. I also had to
replace the LO grid leak resistor and make a 20K leaded-end resistor for
that component. All leaded-end resistors and mica capacitors are now
installed.
The next step is recreating the Micamold paper capacitors. I have created the artwork for Micamold paper labels using the computer. I plan on printing the labels (probably a few per page) on orange colored paper. These will then be cut out and applied to vintage paper capacitor shells. Then a new capacitor installed in the shell and the whole assembly covered with bee's wax. This isn't how the original Micamolds were made - they used the "rolled end" method and their caps weren't wax coated. But, I think the end result will look pretty authentic and at least the caps will have "Micamold" on the label. I might try dipping the assembled replica capacitor into yellow-tinted lacquer as see if that looks more like the original Micamold caps. But, if that doesn't work out, then there's still the bee's wax option. Dec 6, 2018 - Pulled from "stock" the new capacitors needed. 14 - 0.1uf-600vdc, 7 - 0.01uf-600vdc and one .22uf-600vdc. These are polyethylene dielectric types and all of the caps are rated at 630vdc. Their small size allows insertion into vintage paper shells. I also had to account for C7 being listed as the wrong value in the parts list. It's listed as 0.1uf-200vdc but it should be 0.01uf-400vdc. This subtracts one .1uf 200vdc from the parts list and adds one .01uf 400vdc to the list totals. Although all of the caps used will be 630vdc rated, the replica labels will have the original values shown. I'm using all vintage late-thirties Zenith Radio Company capacitor shells. No particular reason other than that I have a lot of them and they are orange. When finished, the replica Micamold caps will look moderately authentic. |
| Dec 8, 2018
- Fine tuned the capacitor artwork for best paper use. I had to do a
"cut and paste" in order to have the "OUTSIDE FOIL" and indicating line
to be perpendicular to the Micamold logo and the capacitor
values. I ended up with four masters for the four different values of
capacitors used in the HRO. Dec 9, 2018 - Made all of the Micamold labels needed. 11 - .1uf 200wvdc, 3 - .1uf 400wvdc, 7 - .01uf 400wvdc and 1 - .25uf 400wvdc. The artwork was copied onto orange paper with three labels per sheet except for the seventh .01uf 400vdc and the one .25uf which used a single sheet of orange paper each. Next was to melt-out all of the old capacitors from the Zenith shells. That process took about 30 minutes. The capacitor wax is melted using a heat gun. I use long needle-nose pliers to hold the cap by one lead end and heat it up until the wax runs off into a metal wastepaper basket. Then the shell can be pulled off and the junk cap insides dropped into the basket. Then the shell is wiped down with a paper towel to remove as much wax as possible. This has to be done quickly while the shell is still hot. The shells, the repro labels and the new capacitors were all sorted into plastic bags to keep everything straight and in order. Dec 11, 2018 - Stuffed all cap shells with proper poly caps. Secured new caps inside shells using hot melt glue. Then the cap shell ends are filled with dark brown sealing wax. This process goes along pretty quickly because I'm essentially making new capacitors rather than removing an old cap from a receiver, rebuilding it and then reinstalling. Probably took about an hour and a half to stuff 22 capacitors. |
Dec 12, 2018
- Glued replica labels on 16 of the capacitors using 3M spray glue. On
the .25uf 400vdc, the three .1uf 400vdc and the six .01uf
400vdc,...these labels were sprayed with yellow-tinted lacquer to seal
the paper and give a darker color with a slight sheen. On a few of the
labels the lacquer caused the seam to lift a bit. I just re-glued the
seam to correct.
Found a complete set of 2.5vac filament tubes - 4 - type 58, 3 - type 57, 1 - type 2B7 and 1 - type 2A5. Tested all of the tubes to ensure they were all functional. Installed the tubes into E-50. E-50 is now ready to test. |
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Dec 15, 2018
- Before applying power to E-50 I checked the DC resistance of the B+
just to make sure there were no shorts or very low resistances. The B+
resistance to ground was in excess of 10K which is okay. I connected up
the 5897AB to E-50 and connected a MCS-8 speaker. I just used a six foot
wire for an antenna. I flipped on the switch on the power supply and saw
that the tube filaments were beginning to light up. Within about 30
seconds I could hear that E-50 was receiving something. I had the C coil
installed and it was set for general coverage. At around 480 I heard a
British Columbia ham on CW. I decided to connect up the regular ham
antenna adjusted for 40M. I was able to hear several hams on both CW and
SSB. I tweaked the IF a bit and signals were stronger. Now, E-50 at this
moment has not been aligned. The coil sets haven't been aligned. But,
right off E-50 is working pretty well. More details after the alignment.
photo left: The underneath of the E-50 chassis now. My restoration attempts to "recreate" the original appearance of the early HRO receivers. The capacitors are very close replicas externally but have modern polyfilm caps inside. The white ceramic resistor are either originals or recreations that use vintage leaded-ends installed on correct value vintage carbon resistors that are painted white and labeled with blue ink. The wire is either original or recreated replica type wire. The only deviation in the actual wiring is the Audio gain pot and 2B7 grid bias follows the later production change (run F.) |
| IF Alignment
- I rough aligned to 456kc first to see that everything did adjust. I
then "rang" the crystal frequency and found it to be 455.7kc. I then
aligned the IF again to this frequency. This assures that the crystal
filter will function correctly. I also aligned the BFO to zero beat at
"6" on the CW OSC scale of the knob. This allows selecting either
sideband of a signal. This completed the IF alignment.
RF Alignment -
Coil set C adjusted with no problems. This coil set is an "F" prefix
serial number and, while still an early coil set with white charts, it
does have the larger contacts that started with the F-run. Coil set B
was an early coil set with the small contacts but it did align with no
specific problems. Checked D coil set, aligned, already pretty close.
Worked well, listened to Pacific NW AM hams.
photo right: looking into the cabinet with the lid up shows the top of the chassis details of E-50. All aluminum pieces were treated with Sodium Hydroxide (Easy Off Oven Cleaner) to clean and then impart the matte finish to the metal surface. The tuning condenser was totally removed when the chassis was restored and repainted. At that time, the condenser was thoroughly cleaned, detailed and adjusted. The gold finish on the crystal filter housing was accomplished using amber-tinted lacquer. Note the interior of the cabinet is gloss black as is the lip around the top opening while the exterior of the cabinet is wrinkle finish. |
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| AVC Problem
- AVC not working as expected. Signals present with AVC off but
receiver is blocked with AVC on. Problem caused by unsoldered end of
R-14 (diode equalizing resistor) on detector tube socket. Also found
that the wire from the arm of the Audio pot wasn't soldered on the
terminal strip under the chassis. I thoroughly went over all of the
solder joints and the connections to confirm everything had been
soldered. A couple of other questionable solder joints were fixed.
Receiver now working much better,...except for,... S-meter Problem - doesn't adjust and is "pegged" at FS when the button is pressed. Just to be sure that this meter (it came out of a junk RME-69) would work, I measured the DCR of the meter coil at 34 ohms. I then measured D-65's meter coil at 32 ohms. Close enough. Rechecked wiring and found that there was an error in how the push button switch was wired but correcting this didn't solve the problem. The meter movement is dependent on the plate current in the AVC controlled stages. The plate current is dependent to some extent on the value of the cathode resistors. The 1st RF cathode resistor should be 300 ohms but it was 1.2K. The 2nd RF should be 300 ohms but was 390 ohms (not bad.) The 1st IF 300 ohm was 490 ohms and the 2nd IF 300 ohm was 450 ohms (pretty far out.) Replacing the 1st RF cathode R helped. I later replaced all of the 300 cathode resistors with rebuilt white ceramics using 330 ohm CC resistors with leaded ends installed. In the later HRO manuals the values of the two S-meter bridge resistors are listed as anywhere from 200 ohms to 2500 ohms implying that they were selected during test. The values of the bridge resistors will determine the current through the S-meter. I checked H-103 and found the bridge resistors were 1.2K. I paralleled some 3.3K resistors to result in 1.2K for the bridge resistors but this still didn't correct the problem. Further checking found the problem. The screen voltage bleeder resistor R15 had been marked as a 100K (in blue ink) and when I checked its value it was around 100K so I thought it was okay. When checking the schematic closer, I saw that the value of R15 should have been 30K. I lifted one end of the resistor that was installed and it actually measured open. Luckily, in the cigar box of leaded-end resistors I found a 30K original that I then installed. This got the S-meter working correctly. The values of the bridge resistors actually function better at 1.2K as that allows the RF Gain to be fully advanced and the S-meter can still be set to zero. With the original 2K value, the RF Gain can't be set higher than 9.5 before the S-meter can't be adjusted to zero. I built two leaded-end resistors with the value of 1300 ohms and installed these for the two S-meter bridge resistors. Now the RF Gain can be fully advanced and the S-meter can be adjusted to zero. |
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Performance and Wrap-up
- Once I had all of the problems fixed, E-50 worked quite well. In
comparison to other HRO receivers, I find that E-50 functions very close
to H-103. High sensitivity allows receiving most signals "on the air."
The use of the 135' CF InvVee antenna results in strong signals since
the antenna is "tuned" for the particular frequency I am tuned to. Audio
is excellent using a MCS-8 type speaker. The push-button operation of
the S-meter is awkward although it functions fine and I don't really
hear any "popping" when the button is released (a common complaint about
the push-button.) It did take about one week of operating and aligning,
operating and re-aligning to get the receiver to become stable and
function in the same manner each time it was operated. Probably due to
switches, contacts, tube sockets, coil bay contacts, coil contacts,
tuning condenser rotor contacts all needing decades of oxidation "worn
off" (even DeOxit doesn't get it all.) A few things I'd do differently,...I wouldn't use "fake rivets" as I did on E-50. The original style rivets are available and the setting tools are also available and both are easy purchases on eBay. I do wrinkle finish a little differently now and I think that would have produced a "deeper" texture. I think I should have given the chassis more coats of lacquer. The originals seem to have very thick coats of paint. But, I think E-50 turned out great and it works very well. The total time was just over seven years! That's a long time for a restoration, but I wanted to use original parts for those that were missing and locating originals just takes whatever time it's going to take. I didn't do any advertising for parts, I just waited until what I needed turned up. I could have reduced the time by advertising but things like a "good condition, nickel-plated PW-D," or a "small red <NC> dial pointer" are hard to describe to "non-enthusiasts" when advertising for parts. Update: Dec 22, 2018 - In going through all of the spare coil sets, I found an A coil for E-50. It's not the earliest type but is one from late-1935 with a "J" prefix serial number. I replaced the black background charts with copies of the white background charts for an A coil. Tested the A coil set's operation in E-50 but, as would be expected, the coil set needed to be aligned to the receiver. Afterwards, E-50 was receiving SW BC stations around 15.3mc quite well. I'll keep watching for an early, small contact, coil set for A and also for coil C. E-50 has small contact D and B coils sets but coil sets A and C are large contact, later types. |
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"Detailing" 1935 HRO sn: F-16 "Detailing" is where a few minor things need to be done to a receiver but a full restoration isn't necessary. Cleaning, servicing and alignment may be required. Sometimes minor cosmetic corrections are required. Detailing doesn't require replacing capacitors or other components other than perhaps a few defective tubes. F-16 was that type of receiver. It was in good shape but certain things were incorrect. The following write-up covers what was needed to transform F-16 into a very nice condition (but not exactly original) early rack mount HRO receiver. |
| The Problems
- HRO F-16 originally came from National as a table model receiver.
Somewhere along the line, it was converted to a rack mount receiver. It
wasn't sent back to National for the conversion. My guess is that a
former owner was able to find a rack front panel and then decided to do
the conversion himself. This isn't difficult to accomplish since all of
the holes match and only the proper mounting hardware for the mounting
would be necessary (it appears they were obtained along with the panel.)
Rack mount receiver panels were painted black crackle finish (National
called it leatherette.) Since just the rack panel was changed, all of
the "matching serial number" coil sets still had their original wrinkle
finish panels. Additionally, the S-meter didn't fit correctly since the
rack panel was of a later vintage. The meter mounting holes were
utilized to hold the original S-meter to the panel by the rim of its
flange. When the panel was changed to a rack mount type, the original
table cabinet became the dust cover. Since it had been a table top, the
lid had the finger lift projection. Rack mounts had flat lids to allow
the receiver to be installed into the tight spaces of some rack
installations. That was the cosmetic issues. The electronics were typical of an old receiver that had been used up to the 1960s but never restored. The original capacitors had been replaced around 1960 using Sprague molded types with the red nomenclature, not black beauties (but similar.) The resistors were mostly original but many had small carbon resistors added in parallel to "trim" the value back to the proper resistance. There were no "cut and hack" modifications. photo right: F-16 "as found" with wrinkle finish coil panels, sticker adhesive and dirt |
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F-16 was a good example of a receiver that was "well taken care of."
The former owner must have liked how the receiver performed and
therefore maintained it in working condition. In fact, the former owner
built an impressive, custom rack mount spare coil storage box with "lift
up" front cover. It's entirely made out of plywood and masonite. The
exterior is painted black. It stores five coil sets. F-16 has its four
original A, B, C and D coil sets plus E and F coils sets that must have
been acquired later.
It was obvious that F-16 had been in storage for quite a while. Some surface corrosion was visible on a few parts but mostly the chassis was covered with lots of grease and dust. Another problem was the heavy glue residue from two stickers that was left on the front panel. With a little electronic cleaning and testing, I had F-16 working quite well. I suspected that the good quality capacitors from 1960 would probably still function. I listened to the receiver for a few weeks to see what the reliability of the somewhat vintage components would be. No problems were encountered. Even the "as-found" alignment was very good. Detailing was going to require the following: 1. Replace wrinkle finish coil set panels with crackle finish coil set panels on A, B, C and D. White frequency charts will also have to be transferred to the crackle finish panels. 2. Replicate a typical meter clamp for the original meter to mount correctly. Fill meter mounting holes in the front panel. Shim the meter to fit the panel hole correctly. 3. Clean front panel of label adhesive residue and then recondition the front panel paint. 4. Flatten the finger-lift projection to create a rack mount type dust cover. |
| Crackle Finish
Coil Set Panels - I had a complete spare set of black
leatherette finish coil panels that were mismatched to a 1936 table
model HRO receiver. By exchanging panels on the two complete coil sets,
I could have F-16 original coils assemblies installed on crackle finish
panels. The excellent wrinkle finish panels would then be used to create
a "matched set" of P-run coils that would be aligned for H-103 giving
that HRO a very nice set of "matched" coils.
The exchange is easy to accomplish although it's a little time consuming. First, all of the individual coil shielded assemblies are removed from their respective panels. Four coil assemblies for each panel. This only requires loosening the mounting screws. Then the coil frequency charts are removed and the coil panels cleaned and touched-up as necessary. Then the proper charts are installed onto the correct panels. Then the correct shielded coil assemblies are mounted to their new respective panels. It takes a couple of hours to do the exchange with cleaning taking up most of the time spent. |
Meter Clamp - I made the meter clamp out of a 1.25" wide strip of .030" aluminum. I notched the metal strip along one side and bent the resulting tabs at a 45 degree angle. I then bent the aluminum around a form that was the same size as the meter housing. I bent two 90 degree end tabs and drilled a hole in those tabs. This was to allow a 6-32 screw and nut to tighten the clamp onto the meter housing. I aligned the tabs to make sure that all were contacting a flat surface. I then painted the clamp black. Installation was to install the meter from the front of the panel and the clamp onto the meter body. Push the clamp all the way forward and tighten the screw to secure. Like all of the vintage clamps, this one doesn't keep the meter so tight that it can't be turned in the mounting hole. It keeps the meter secure and that's all the vintage ones did. |
| Meter Shim -
The easiest method to shim the .030" clearance was to use friction tape
that was cut to about .190" wide. This was wrapped around the meter body
up next to the flange. About four wraps allowed the meter a good, secure
fit into the mounting hole. Filling the Meter Mounting Holes - The easiest method is to use 5 minute epoxy as the filler. I used masking tape on the front panel to cover the holes. I then fill the holes with epoxy from the back of the panel. The tape assures that on the front of the panel the hole fills will have an absolutely flat surface. After the epoxy has set up, I remove the tape. On the back of the panel, I use a razor blade to "shave" the top of the epoxy fill so that it is flat. The front and rear surfaces of the fills are then painted using black nitrocellulose lacquer to match the original paint. |
Removal of the Finger Lift - Rack mount HRO receivers have a flat lid on the dust cover/cabinet. This was to allow easy installation of the receiver into a "crowded" rack. Removal of the lid finger lift projection first required removal of the dust cover from the receiver. Then I had to use some padding to protect the paint. I used heavy paper and inserted the lid into a very large vise that had brass jaw covers. Tightening the vise flattened the finger lift most of the way. To really get the area straight and flat required "body work." That is using a hammer and wooden protectors to allow the metal to assume a straight line. Since there was some distortion in the metal when the projection was formed, it won't reassume a perfectly flat shape now. I had to file the front edge somewhat to get it straight. The entire process took about 25 minutes to accomplish. Afterwards, I touched up the area with black nitrocellulose lacquer in combination with Mars Black acrylic paint to achieve the texture. |
| Cleaning PW-D Index Dial
and Knob Skirts - The knob skirts are German Silver which
is essentially nickel with a small amount of bronze in the form of
copper and tin. Generally, brass, which is bronze mixed with steel, will
not scratch nickel unless a lot of pressure is involved in the contact.
I used a "new-unused" brass brush in combination with lots of Glass Plus
to clean the knob skirts. Whatever was on the skirts resisted normal
cleaning using either WD-40 or Glass Plus. By lightly brushing
the Glass Plus with the brass brush, the yellow (probably nicotine) was
easily removed without scratching the skirt. The end result was the flat
silver finish that is correct for the knob skirts. The same yellow discoloration was on the PW-D. This dial is German Silver plated, so you have to be careful. I disassembled the PW-D so I was only working with the Index dial. Again, lots of Glass Plus and a very light touch with the brass brush removed the yellow contaminate and left the Index Dial the correct flat silver finish. Be sure to test the reaction of the brass brush and the German Silver. In my experience, the brass seems to be a little softer so it doesn't scratch. But, brass brushes may vary in the type of brass used for the bristles. Some brass has a lot of steel in it and that might make it hard enough to scratch German Silver. Test on the back side of a knob skirt first.
Photo right shows F-16 after the detailing. It's difficult to see the crackle finish texture in this photo. |
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1935 HRO Receiver SN H-103 - Restoration |
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To the right is a photo of HRO sn H-103 taken when the receiver was first obtained (around 2007.) It is obvious that the cosmetics were in very good condition. The yellow "R-014-8" designation may have been an identification for commercial use but nothing was documented as to its meaning or of the receiver's former use. Most of the problems with H-103 were under the chassis. Foremost was the 1.25" hole that had been punched in the front left section of the chassis. This was going to be a challenge to repair. Also, the receiver had been totally re-capped using modern-style yellow plastic capacitors. Additionally, there were some non-original resistors, a few modifications with some extra non-original parts installed. The goal in the restoration was to bring H-103 back to full operation using the original design circuit and using parts that would have the appearance of the originals but were rebuilt new parts. The most time consuming part of this kind of restoration is restuffing the paper-wax capacitors. Since I didn't have the original caps, I ended up robbing a complete set of HRO "Sprague" paper-wax capacitors from an old HRO parts set. NOTE: Run-G HRO receivers probably used Micamold paper caps but these are difficult to find so I used Sprague which were used in all post-1935 HRO receivers. These caps were restuffed using modern "yellow jacket" caps installed into the original paper-wax shells. The shell ends were then sealed with colored wax. Proper orientation was maintained though this was for aesthetics only. While this is sometimes tedious work, the end results are a receiver that looks totally original and performs as new. Photo right: HRO SN H-103 before restoration |
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Below is a "before" photo of the underside of the chassis showing all of the non-original type capacitors along with other non-original parts and circuit modifications. |
Below is an "after" photo of the completed restoration of the underside of the chassis showing how restuffing caps preserves the original appearance. |
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Another challenge were the white ceramic resistors with lead end-caps that National used in the HRO receivers up to about mid-1936. I had to "recreate" three of these resistors to replace modern resistors that had been installed. I checked the parts boxes and found three correct size lead end-cap resistors. Of course, they weren't the correct value but all we needed were the lead end-caps to build our replicas. Next, I needed to find correct value, IRC (International Resistance Corp.)1/2 Watt Carbon resistors from the late thirties. These resistors are the correct physical size to fit into the lead end-caps. The lead end-caps will just "twist off" of the old resistors and then, by drilling a 1/16" hole, the lead end-caps can be fit onto the new resistor (with the leads protruding through the holes.) Soldering secures good electrical contact to the lead ends. All that remains is to paint the body white and install the resistor replica. Originally, the ceramic resistors were lettered by hand or stamped with the value, however it looks better to just leave the body white as nothing looks quite "correct" or "original" for putting the value on the resistor. The photo to the right shows one of the replicas installed next to a rebuilt (restuffed) paper-wax capacitor. |
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This photo shows the aluminum hole plug installed but not painted. Also note that the plastic cap is missing from the push-type "ANT" post. |
The 1.25" hole punched in the front-left part of the chassis was a
real problem. It appears that someone wanted to install yet another mod
into the receiver but fortunately stopped after they had done this
damage. The hole was so large an epoxy fill would not have had enough
strength. I decided to make a "custom-fit" plug and epoxy it in place.
The plug was made out of .050" aluminum since this was about the
thickness of the chassis steel. The rim interface would be filled,
sanded and then painted to match.
After the plug was fit and the interface filled, the paint matching was attempted. Since this was a rather large area, artist's acrylic would be too flat of a finish to match. The photo to the right shows one of the "black" paints used that was a fair match. Several "black" paints were tried but the best match came from Testor's Jet Black, plastic model paint that comes in the little glass jars at hobby stores. See the chassis photo below for the final appearance. Though not perfect by any comparisons, I think the fix is better than the gaping hole. On most HRO receivers, the gear box has probably never been opened since the receiver left National. The grease is usually dried up, the anti-backlash split-gears are tight and the other bearings also need lubricant. |
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I thoroughly cleaned the old grease out, then used "3 in 1" oil on
the split-gears to assure that they can move freely, which eliminates
backlash. I used "Lubriplate" for grease application - I don't use
Lubriplate anymore since it tends to dry up after a while. I use any
light-weight multi-purpose grease. The worm gear might also need to
be removed and lubricated if the gear box is particularly dry but most
of the time the worm gear can be lubed with oil drops to the end bearing
in the housing and by working grease to the worm gear by lubing the
split-gear. The rotor contacts on the tuning condenser should be sprayed
lightly with a contact cleaner that has some lubricating qualities. When
installing the micrometer dial, notice that the elliptic rides on the
boss of the tuning shaft bearing housing. I usually find that this area
is entirely devoid of lubricant which results in a "rough" feel or noisy
operation of the micrometer dial. I lightly grease this boss before
installing the micrometer dial. All of the tube sockets should be cleaned. I usually spray De-Oxit on the pins of the tube before inserting it into the socket a few times. I also clean the grid caps as they are always oxidized. On the HRO, you also have to clean the flex finger contacts for the coil sets. These are usually very dirty and if not clean will compromise the all-important contact with each coil in the coil set. After a good set of tubes are installed, all that remained was test and alignment. There were several circuit modifications that I had to remove on this HRO and then return the circuit to original. Testing was going to reveal whether I did this correctly or not. Fortunately, everything worked pretty much as it was supposed to. During test, the receiver's gain was up and down a lot so I removed the tubes and cleaned the sockets again which corrected that problem. |
| The S-meter seemed to be far too sensitive and the adjustment pot
had little effect. The problem was the R10, the screen load resistor
(which is part of the S-meter bridge,) was only about 8K and the correct
value should have been 20K. So, another lead end-cap resistor replica
was made to replace R10. All resistors were checked during the rebuild
but I obviously missed the low value R10 - at least it showed up as a
genuine problem during test. With R10 at 20K the S-meter circuit worked
correctly and the S-meter responded to signals in normal manner.
Early HRO receivers usually have R10 at 30K. Later receivers went to
15K. 20K seemed a good compromise. The next step was alignment of the IF, which is straight forward although the IF frequency is determined by the actual crystal frequency - in this case 456.5KC. After the IF is aligned, each coil set has to be aligned for proper tracking. Most HRO receivers don't have all of their original coil sets anymore. H-103 was no exception, having only a 40M coil set with it. We usually have to rely on "orphan" coils to make up a complete set of coils for each HRO receiver. These "orphan" coils must first be disassembled, cleaned and inspected. Many times dirt, dust, bugs and spider webs, along with broken wires or bent condenser plates will be found in each of the coil boxes. In fact, the 40M coil that came with H-103 had a broken connection inside the 1st RF coil that had to be repaired. >>> |
>>> After reassembly, the coil set needs to be aligned to the specific receiver that is going to be used with. There is a significant difference between each HRO receiver, especially between the 6.3vac tube type and the earlier 2.5vac tube type. While any coil set will function okay in almost any HRO, the alignment will assure greatest sensitivity, correct tracking and best performance of the receiver. When aligning HRO coil sets, the general coverage must be aligned first and then the bandspread. Switching to bandspread requires relocating four screws on the coil contact insulator blocks. The eight adjustment trimmers are accessed from the top of the chassis through holes just in front of the tuning condenser. It is easier to access these holes if the front-top piece of the cabinet is removed. It is held by the two top screws in each corner and the dial pointer "diamond." If the lower end of the tracking needs to be adjusted, the padders and inductance disks can be accessed through the holes in the back of the coil shields on each coil set. On coil sets A and B, the LO inductance is adjusted with a "loop" of wire inside the coil form that has its relative position moved to change total inductance. Once the coil set is aligned, the final adjustment is to readjust the 1st RF coil trimmer to the station antenna. I happened to also have correct vintage E and F coils for this HRO. The E coil provides tuning of the 160M band along with the upper part of the AM BC band. Coil F covers 480kc to 950kc for the lower part of the AM BC band. These coils do not bandspread. They also have a padder adjustment on the top of the coil set so when aligning, both high-end and low-end tracking can be adjusted from the top access holes. |
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The photo to the left is the finished result. Like all fully
restored HRO receivers, H-103 is very sensitive and the bandspread
capabilities make the amateur bands seem to tune on and on - 400
divisions in fact, or the equivalent of about nine and a half feet of
linear dial. The feel of the micrometer dial is smooth and the accuracy
of any reset is precise. Audio is very pleasing even though it is just a
single-ended 2A5. I am using an early 8" National speaker box with the
cloisonné "NC" emblem. Since the HRO receiver originally has no
internally mounted audio output transformer, National installed the
output transformer in the speaker box. UPDATE: Dec 2018 - When I did this restoration of H-103 (over ten years ago,) I used modern plastic insulated wires to make a power cable and that cable was covered with brown duct tape. It looked really awful but worked fine. I've acquired several HRO "parts sets" since doing H-103, so I decided to harvest an original power cable from one of the "junkers." The cable was cleaned, an appropriate plug installed (with rubber cover) and the cable installed. H-103 still works fine and now looks very original, even from the rear. Additionally, I acquired a complete, matching set of coils D, C, B and A from run-P. I decided to replace the "mismatched runs" coil sets I did have with H-103 and use this 1936 sets of coils. I aligned the four coil sets to the receiver without any issues. Although these P-run coil sets weren't originally with H-103, having a matching set of coils (all serial numbers match each other) is the next best thing.
photo left: HRO sn: H103 as it appears nowadays (Dec. 2018) |
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