Here are some of the common complaints that I've found on the
Internet. However, it's worth noting that the GPR-90 actually has more "five star" ratings
than it does ratings with some negatives mentioned. Here are
some of the "three star" complaints along with a couple of my
own observations.
1. Dial Resolution versus Tuning Accuracy
and the Whole GPR-90 Tuning Design
- Although most reviewers rate the GPR-90 dial accuracy as "excellent"
or "very good," it does depend on exactly what is considered "accurate."
There is a difference between "accurate" and then having usable dial
resolution. After alignment, most analog dial receivers will be accurate
on "marker" frequencies. That would be tuning in known frequency
stations such as WWV. Dial resolution allows an "accurate" receiver to
tune "to the kilocycle" since the dial can resolve frequency to the
accuracy of one
kilocycle. The Collins 75A and 51J receivers were analog dial tuning
systems that could accurately resolve "to the kilocycle." Any receiver
that covers several megacycles in each of its selected tuning ranges
isn't going to have a reasonable-size tuning dial that can resolve to the kilocycle.
AND, add to that,...any receiver that has a Bandspread variable-C tuning in
parallel with
the Main Tuning variable-C tuning is going to have potential dial readout accuracy
problems. And, that goes for both dials. The 100kc Calibrator found on
most GPR-90 Series receivers can be used to accurately set-up the Bandspread dial but the resolution is still somewhat limited. Also,
using the 100kc Calibrator, the Main Tuning dial can sometimes be
accurately set-up using the Bandspread dial for a frequency-trim adjustment.
But, sometimes the Main Tuning calibration reads lower than desired with
the BS set to 100. In that particular condition, the BS can't be used to
"trim in" the Main Tuning dial calibration (RF tracking
alignment needed.)
2.
The
Hammarlund Super Pro Inspiration - GPR-90 tuning system
uses "Pinch-Wheel" drives - The WWII Hammarlund Super Pro
seems to have been the "model" that TMC chose for designing the tuning
system used in the GPR-90. The dial layout versus placement of the
S-meter follows the WWII Super-Pro and many of the Hammarlund HQ-models.
The pinch-wheel, rim drive dial tuning system, including the dial
guides, are exactly like those used in the 1935 to 1949 Hammarlund
Super-Pro receivers (before the SP-600.) As to why TMC copied the WWII
Super-Pro in 1955 when it was obvious then that even the "new" Hammarlund
SP-600 was a "dated" design when compared to the receivers designed and
produced by Collins, is a mystery. It does seem like the TMC mechanical engineers
were copying designs that were already two decades old,...but, certainly
the cost factor must have been an important consideration on TMC's part.
Incidentally, the National NC-183D also used a pinch-wheel rim-drive
tuning as did the Hammarlund HQ-180,...probably because when all of the
parts are in good condition and clean, this design results in very
smooth tuning and an excellent tuning ratio reduction in the drive. In
fairness to the NC-183D, those are edge-illuminated Lucite dials mounted
to copper-plated metal backing plates and the pinch-wheel actually works
against the metal backing plate that then drives the main or bandspread
tuning condensers through a gear-reduction set up,...a first class mechanical design. The Hammarlunds
and TCM used the pinch-wheel to drive the rim of the plastic dial rim
directly. Dial warping was a problem on the pre-WWII designs but
post-WWII plastics were much better at resisting warping, so it's
usually not a problem to drive the plastic rim directly on these later
receivers. Although the plastic used for the dials for the GPR-90
doesn't
usually discolor from excessive exposure sunlight, I did recently
inspect a GPR-90R receiver that had a very noticeable tan discoloration that was
limited to the portion of the dial that had been exposed to sunlight
through the dial bezel opening. This discoloration was on both the main
dial and the bandspread dial. It's a seldom seen problem,...but it
can happen.
3. TMC's Defense Regarding
Tuning Accuracy - TMC seemed to get somewhat irritated
whenever the GPR-90's tuning resolution was criticized. TMC actually
published testy comments in their service bulletins (SUP-1 GPR 9-56) defending the variable-C
tuning with analog dials against the "to the kilocycle
accurate" Collins tuning systems. The TMC arguments were, "The
GPR-90 is a general coverage receiver, and a real good
one, but it is NOT a frequency meter." This was followed by,
"People who require frequency meters should buy frequency meters - not
make them double as communications receivers." These are certainly the best known
of their peevish comments that all seem to be directed precisely at TMC's
"big-time" military contract competitor, Collins Radio Company.
However, TMC's comments do illustrate that when
comparing the GPR-90RXD to the Collins R-388 in an unbiased evaluation,
maybe there's something to be considered besides tuning accuracy. TMC
probably thought that really ALL the R-388 offered the end-user
was essentially a frequency meter that happened to also work as a
receiver. Beyond the R-388's front-end, the rest of the receiver's
circuitry is standard and in some places actually deficient in
performance, e.g., the detector, the AVC, the noise limiter and even the
audio quality. So, maybe if the R-388 was looked at "critically" then
TMC probably had a point in their "frequency meter" comparisons.
Nowadays, 65+ years later, when vintage equipment from either company
can be easily found, rebuilt and used, we have a different perspective
on vintage receiver performance and TMC's comments advocating buying a
heterodyne frequency meter to go with your GPR-90 receiver seems kind
of humorous,...although I don't think it was originally intended to be.
As an aside,...Beckman/Berkeley must have taken TMC's "freq meter"
comments "to heart" because one of their products was an elaborate
Beckman Frequency Measuring System that
had a Collins 51J-4 receiver (Beckman/Berkeley Model 7700) as the main component of the instrument.
4. Cheap Construction?
- This appears to be a common complaint but most reviewers are comparing
the GPR-90 to other more expensive, built-for-the-military (or
government) TMC products or to other expensive
Collins military receivers. Hardly a fair comparison. The passive components used in the GPR-90 receivers appear to be excellent quality. Ceramic disk capacitors
are used throughout the receiver circuitry. Component boards are used in
a few locations to reduce chassis component congestion and to provide a
better mounting platform for the circuitry. The build quality seems
to be better than average if the GPR-90 is actually compared to
other $400 to $500 receivers built for the ham market. That price would
put the GPR-90 mid-way between a Collins 75A-4 on the high end (about
$695) and a National NC-183D on the low end (about $385.) The GPR-90
sheet metal fit and finish are quite
good. The GPR-90 table cabinet is gorgeous with dark blue wrinkle finish
paint and a very low height compared to the usual 1950s cabinets (of
course, that's an opinion, isn't it?) However, if and when a GPR-90
receiver is extensively disassembled it becomes apparent that
maintenance wasn't at the forefront of the mechanical design
considerations,...but then, in the commercial environment, professional
repair technicians took care of the disassembly and reassembly.
5. Not Selective Enough
- This is another common complaint that seems rooted in comparisons to
either modern receivers or to receivers equipped with Collins mechanical
filters. The GPR-90 Crystal Filter is the only control that the
operator has over the bandwidth. One has to wonder if the reviewers
actually adjusted the PHASING control for minimum bandwidth or just
expected that the bandwidth showing on the switch nomenclature just
"happened." Like any standard Crystal Filter, the GPR-90's can
also eliminate heterodynes quite well but one does have to adjust the
PHASING to accomplish that. The NON XTAL position will have about 7kc
bandwidth at -6db. There are also a couple of audio filters that can be
utilized, one for voice operations and one for CW operations. On the whole, the GPR-90 is a little better
than most of its similarly priced competition for selectivity options (and
at least it doesn't have a Q-multiplier for a selectivity control!)
6. Not Mechanically Stable
- Another common complaint that certainly does have some merit,...but then,
how many people bang on the side of their receiver just to see what
happens? At any rate, there are a few things that contribute to the
mechanical instability issue and that would be that the coils mount on and
under the chassis directly below the tuning condensers that mount on the
top of the chassis. Any flexing of the chassis will skew the tuned
frequency by the mechanical movement of the LC involved. It's a common
problem in ALL receivers that are variable-C tuned with L coils mounted on
common chassis. When the receiver was mounted in a communications rack,
there wasn't access for the operator to reach in and "flex" anything, so
it's kind of a mute point when the receivers were properly installed and
properly used (and there are dial locks on both dials that could be
utilized.) However, mechanical instability problems that are unusual
can usually be traced to the "all too common" maintenance
technician practice of not
installing all of the shielding and not installing all of the screws,
washers and nuts. Mechanical stability did require that all of the
assembly mountings were secure and tight. Leaving
out screws (and even leaving out shields and covers) was a common occurrence with some types of maintenance
technicians. Also, component replacement soldering problems can be a
cause of instability. Wholesale
capacitor replacement isn't necessary on the GPR-90 because of the
ceramic disk capacitors used but the filter capacitor and a couple of
the other electrolytics might have installation problems. Also, it was
common to have to replace the power transformer and proper soldering
with that installation would be very important. So, there
are lots of ways to create your own mechanical instability problems. When
in excellent condition, with minimal mechanical wear and with a
competent going over of what really needs to be rebuilt or
replaced, the GPR-90 is as stable as,...say,...the Hammarlund HQ-180 or the
National NC-183D.
7. Demodulating SSB Signals is
Difficult - A common complaint and, for a "stand alone"
GPR-90, it's somewhat true but it's especially true for the GPR-90RXD receivers. The BFO coupling capacitor is 15pf in the standard GPR-90
and the GPR-90RXD uses only a 3pf capacitor. The ratio of BFO injection to signal level at the detector is
about average for the time period for the standard GPR-90. However, the
method of controlling RF/IF gain using the cathode resistance doesn't
allow enough sensitivity control to cope with the strong SSB signals
found on the ham bands nowadays. The RXD's BFO
injection is very low and this low
level of BFO injection may have helped with weak CW signals that might have
been
"masked" by higher levels of BFO injection to the detector.
The low BFO injection required the operator to turn off the receiver's AVC, increase the
AF Gain to a high level and reduce the RF Gain to a low level. The
RXD requires a significant reduction of the RF gain to get the ratio of BFO injection to
signal level correct for proper copy (so much so that overall
sensitivity is reduced.) For almost all communications receivers,
from the 1930s up into the early-1950s, this method
of somewhat reduced RF gain was used for good CW copy. This
BFO to signal level ratio also applied to demodulating the "new SSB
signals" as SSB became more popular in the early-fifties. At that
time most
hams still had older receivers with diode detectors so to get the proper BFO
ratio they reduced
the RF Gain so the SSB signals would demodulate correctly. Some hams went
further and increased the BFO injection by increasing the value of the BFO coupling
capacitor. Later, modifications for a product detector to replace the
diode detector came on the scene. The standard GPR-90 manual (in Section
7) suggests that the value of C59, the BFO coupling C, can be increased for better SSB
demodulation. The standard GPR-90 is already using 15pf but certainly going up to about 45pf
would help with SSB demodulation. I haven't changed the 3pf BFO cap in my GPR-90RXD
because it came with its
matching (same serial number) MSR-6 SSB Adapter. The MSR-6 (or any
compatible TMC SSB Adapter) makes all the difference in copying SSB
or CW
effectively with the GPR-90RXD. The MSR provides another IF conversion (455kc
to 17kc) along with a product detector and
its own BFO.
More details on the MSR-6 further down this article in the "TMC SSB
Adapters" section. The GSB-1 SSB Adapter will also provide a tremendous improvement
in SSB and CW reception capabilities for the standard GPR-90 but the
GSB-1 is a rare and expensive accessory for the standard GPR-90.
Connecting a temporary 30pf capacitor in parallel with C59 will provide
a 45pf BFO coupling capacitance that should help with SSB demodulation
until a GSB-1 is found (or one of the 455kc MSR SSB adapters.) Until
either C59 is changed or a SSB adapter is found, one can use the ANT
TUNE control to reduce the signal level and that will allow better
demodulation of powerful SSB stations.
8. The GPR-90's Achilles' Heel
- Maybe it's just me but it seems like a large number of GPR-90
receivers are either missing the Z-matching antenna input transformer or
the one that's still in the receiver is defective. Unless there's some
protection provided to the antenna input, any RF, even from a relatively
low power transmitter, getting to the 75Z or 300Z inputs will have a
direct path through a few turns of #28 magnet wire to chassis-ground. An
arc inside an older style Dow-Key relay could provide sufficient RF to
fry the Z-matching transformer. Many other destructive scenarios can
certainly come to the imagination. Of course, TMC expected that the ham
user would provide a T-R switching arrangement that not only
disconnected but also grounded the GPR-90's antenna input upon transmit.
But, older style DowKey relays (and most other types of coaxial
antenna relays) didn't do this and many hams just ignored what could
happen if the relay arced internally. What is unfortunate is that when
T1 is "fried," the receiver will still seem to function okay. An
inexperienced seller might even list the receiver as working fine. The
problems experienced with a defective T1 are subtle in some modes of
reception (AM, for instance.) In checking out a
prospective GPR-90 purchase, if possible, measure the isolation between
the primary and secondary windings of T1 and measure the winding's DCR
to verify that it will function correctly. However, many times you won't
have the opportunity to measure T1's DCR because it won't even be there but will be entirely missing. Luckily,
T1 wasn't used in the GPR-90RXD versions of the receiver. However, I do
have to mention that my RXD had Band 3 RF transformer's primary winding
"burned open" so putting RF into a receiver's antenna input does seem to
happen a lot more often than one would think.
9. Extensive Use by Commercial Users
and Reworked by Hamsters - This isn't specifically
mentioned on the Internet but it is implied by some of the reviews. How
a receiver was used and maintained in the past can result in a litany of
problems after 60+ years of existence on the Planet. Many GPR-90 Series
receivers have extensive wear, especially those receivers used
commercially or by the military. Many receivers were maintained by
commercially employed technicians that had no regard for the equipment they were
employed to maintain. Many receivers were virtually destroyed by commercial
technicians with various types of rush-job, hacked repairs or non-engineered mods
that were for specific needs at the time by the end-user. Many of the
shields and top covers along the screws that mounted them were removed and
discarded. Many of the receivers, after surviving the gauntlet of
commercial technician maintenance, went on to ham ownership. That
sometimes ended up with the receiver being worked on by inexperienced hamsters who
lacked any ability but always seemed to be eager to modify the receiver rather than
actually repair it correctly. Installing thread-cutting, sheet metal
screws in place of the proper, original machine screws seems to be a
common hamster solution for dealing with lost original screws. Also, hamsters always seemed eager to drill and
"hack-in" a non-original receptacle rather than
simply change the plug on the cable or accessory they wanted to use. So, expect to
find RCA phono jacks for antenna connections, speaker connections and
other circuit access points or Cinch-Jones receptacles installed just to
make it difficult to "de-mod" the receiver. The upshot is not too many of the
commercial GPR-90 receivers
have survived in original, pristine condition. Some will be found that
have been competently rebuilt, look great and function to specifications,...but many others will be
non-functional, worn-out "hack jobs." To a certain extent, this
has led to impetuous judgments being made on receivers that haven't been
serviced in decades and certainly have corrupted circuits,
worn-out components and environmental contamination ingression from poor storage
conditions. The GPR-90, that is, not the RX, RXD or other
commercial-military models, being that it was a ham receiver,
has probably faired much better overall. Mainly because these receivers
weren't subjected to the
rigors of commercial-military use or those types of maintenance programs. Naturally, it depended on the
ham owner,...and, after
60 to 70 years, there have probably been a lot of those. But, typically
the GPR-90 will be found in better physical/cosmetic condition than any of the
commercial-military variants.
10. Power Transformer Failures
- This wasn't one of the Internet complaints but something I've noticed
in examining many GPR-90 receivers. Most of the replacement power
transformers were installed in commercial or military versions of the
the GPR-90. That would be the GPR-90RX and RXD primarily, but I've also
seen a replacement power transformer installed in a GRP-90R receiver.
The conclusion drawn would be that since the standard GPR-90 receiver
was a ham receiver it wasn't normally operated 24/7 and, therefore, if
there was a component failure, it was immediately noticed and the power
shut off so the power transformer never had a chance to over-heat and
fail.
However, the commercial-military receivers were operated long hours,
perhaps 24/7. They were commonly left unattended for long time periods while still powered up.
This appears to have taken a toll on the power transformers due probably
to the transformer over-heating possibly from a component failure that
wasn't noticed for several hours. So, inspect any "prospective-purchase" RX, RXD, R
and any other GPR-90 version that was intended for commercial-military
operation,...you'll probably discover that the power transformer has
already been
replaced (or it might still be defective.)
11. Manuals - Again,
Internet reviews don't mention the TMC manuals. The GPR-90 manual just
okay since not a lot of information is provided. The circuit
descriptions are very basic. The installation and operational guide are fine with lots of drawings. Voltage and resistance
charts are provided. Alignment information is fine. The biggest problem
is that no under the chassis component identification or location is
provided, not even a photograph. TMC does provide a slight hint at a component's
location with a brief notation in the parts list in the column "Function
& Location." For example, looking at C11 the notation is "Cathode
Bypass" and "V2" for the location. I guess it's better than nothing at all. The GPR-90RXD manual
is much more detailed, probably because the receiver was
commercial-military in its expected use. There are several enlarged
schematics that show just certain sections of the circuit that are used
along with a better, more detailed circuit descriptions overall. Still, although there
are photographs of the top and bottom of the chassis, none of the small
individual component locations are identified. Even the "Function &
Location" in the Parts List is reduced to just "Function." You're on
your own to trace out the wiring versus the schematic to locate any of
the individual small components. Interestingly, the
CV-591A/MCR-4 manual has the components that are mounted on the numerous terminal boards
identified. However, individual small components under the chassis
aren't identified in this manual either. Although all of the TMC manuals are pretty good,
don't expect totally complete information,...but that's typically the way most
equipment manuals are anyway.
| 12. Conclusions - I've
used the GPR-90 and the GPR-90RXD on-the-air several times over the
years. Although
this is on-the-air operating on 75M where almost any receiver has the capability of
providing decent reception. The challenge on 75M is to provide enough selectivity to have the ability to copy weak signals through the noise and QRM. I've found that
the Crystal Filter is a major factor in being able to use the GPR-90s as
a ham receiver. The audio is very nice for AM and the receiver's ease at
being able to set up the remote standby is a big help. So, the GPR-90s can
be used successfully in a vintage ham station provided you operate
mainly on 75M in the AM mode. I'm sure 40M wouldn't be a problem either. 20M through
10M signals seem just fine but I've never operated a GPR-90
on-the-air on those bands,...just in the listening mode,...and in that
mode the receivers do fine up into
the 10M band. BUT, "doing fine" is a judgment based on
comparison to other 1950s vintage receivers, not to modern transceivers or SDR receivers.
Finally,...Things to Keep In Mind When Operating
a GPR-90
A - Is the GPR-90 really incapable of receiving SSB? Do you have to "ride the RF
Gain" for both CW and SSB? Yes,...receiving SSB on a GPR-90 is
difficult unless you have the TMC GSB-1 or one of
the TMC
CV-591A/MSR-4, MSR-6 or other 455kc input SSB adapters. These SSB Adapters make ALL the DIFFERENCE in
being able to successfully listen to all types of SSB or CW signals.
This is especially true for the GPR-90RXD.
B - Do you need to use a matched or resonant antenna for best performance?
Yes, especially with the GPR-90 because of its
ferrite core Z-matching input transformer. An untuned end-fed wire will
generally not provide either 75Z or 300Z at the antenna input and the
GPR-90 ANT TUNE is actually connected as a "fine tuning" control, so it
won't compensate for the antenna Z mismatch. For best performance, an
antenna matching device (antenna tuner) should be used to assure
that 75Z (or close to it) is what the receiver "sees" as the
antenna impedance. All communications receivers will
function best with a resonant (or Z-matched) antenna and an antenna with some gain is even
better, especially a yagi or a quad on the higher frequency bands.
C - Is the GPR-90 receiver only moderately stable? Yes. The GPR-90
isn't the worst receiver for frequency drift but there
was a reason why the military wanted the RX and RXD versions
with the capability of a selectable crystal oscillator for the HFO. The
GPR-90 does settle down after about 20 minutes of operation (something
that can't be said for the HQ-180,...oh,...unless you leave it plugged
in so the HFO and BFO tube filaments stay powered up continuously, even
when the receiver is turned off.)
Essentially, the GPR-90 is a decent receiver that can
easily perform as well as a Hammarlund HQ-180 or a National NC-183D or
any of the other $400 to $500 receivers that were available in the late-1950s.
|
TMC's two-page advertisement in the 1956
ARRL Radio
Amateur's Handbook
No prices shown, you had to request the specific
bulletin for that information. Note that the loudspeaker shown
with the GPR-90 is the GPS-2 version (same cabinet depth as the
receiver.) And another thing,...wasn't A2 (MCW) illegal on the
HF ham bands? |
|
