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 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.
The
Hammarlund Super Pro Connection - This
is something that I've noticed about the GPR-90 tuning system - 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 use the pinch-wheel to drive the rim of the plastic dial
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.
TMC's Defense Regarding
Tuning Accuracy - TMC seemed to get somewhat peeved
whenever the GPR-90's tuning resolution was criticized. TMC actually
published in their service bulletins comments defending the variable-C
tuning with analog dials against the superior "to the kilocycle
accurate" Collins tuning systems. The TMC argument "If you want frequency meter accuracy then buy a frequency
meter, don't ask the receiver to double as both" is certainly the best known
of the comments that all seem to be directed precisely at TMC's
"big-time" military contract competitor, Collins Radio Company.
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-90RX receiver seems
humorous,...although I don't think it was originally intended to be.
2. 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. 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.
3. 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 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!)
4. 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 problem 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.
5. Demodulating SSB Signals is
Difficult - A common complaint and for a "stand alone" GPR-90
it's
somewhat true. The BFO coupling capacitor is only 3pf so the ratio of
BFO injection to signal level at the detector is quite low. This low
level of BFO injection helped with weak CW signals that might be
"masked" by higher levels of BFO injection to the detector. This design
then 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
significant reduction of the RF was to get the ratio of BFO level to
signal level for proper copy. This method was used in all communication
receivers from the 1930s up into the early-1950s 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 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. I haven't done this because my GPR-90RXD 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.
More details on the MSR-6 further down this article in the "TMC SSB
Adapters" section. The GSB-1 will also improve the standard
GPR-90's SSB and CW capabilities.
6. 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. Also, hamsters always seemed to drill and
"hack-in" a non-original receptacle rather than
simply change the connector 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. 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 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 with 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 better overall but it naturally depended on the owner,...and, after
60 to 70 years, there have probably been a lot of those.
7. 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.)
8. 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 and lack thoroughness. The
installation and operational guide is 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 provided
with 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." 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 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 description 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.
9. 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.
When operating the GPR-90, do you have to "ride the RF
Gain" for CW and SSB? Yes,...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. ~ Do you need to use a matched or resonant
antenna for best performance? Yes. All communication receivers will
function best with a resonant antenna and an antenna with some gain is even
better, especially a yagi or a quad on the higher frequency bands.
~ Is the receiver only moderately
stable? Yes. The GPR-90 isn't the worst receiver for stability 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.
~ On and on,...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.
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