[TMC] GPR90-RXD Receiver & SSB

[John Poulton]

Mark, and group,

I've worked on a half-dozen or so of these radios, both the GPR-90, and it's
cousin the GPR-90RXD.  I'm afraid I have to agree with the consensus of the
group:  they're among the handsomest radios of the period, but not stellar
performers.  They are quite adequate for AM use and are reasonably sensitive,
so if that's how you plan to use yours, FB.  For CW, and particularly for
SSB, there are many radios that are better.

That said, I recently 'went through' a GPR-90RXD that a friend pursuaded
me to modify by adding a product detector.  I used exactly the circuit
on the later production GPR-90RXD's and 'RXDS's.  In those radios, the BFO
tube is gone, and instead there's an octal socket into which a plug-in
can is inserted.  There's a 6BE6 on the top of the can, and various parts
inside to implement a product detector.  In these radios, the SPST BFO
switch is replaced with a DPDT switch (very hard to find an original
Carling replacement nowadays for this item!).  This switch contrived
to switch the audio from the AM detector to the PD, turn on the B+ for
the PD, and increase the AVC time constant.

The early production 90RXD's didn't have this PD, of course.  There appear
to have been at least 4 different versions of the 'RX receiver, but that's
another story.

You can find details about this PD in the manual for the late production
GPR-90RXD's, available from Robert Downs.  If anyone is interested, I
could write up what I did, which was obvious:  I replaced the BFO tube
with a 6BE6, and did a bit of rewiring with a few odd added components. 
I even managed to find one of the elusive Carling DPDT's.  In England.

The product detector "mod" works pretty well.  You can copy CW very well
at full RF gain.  I found it necessary to back off the RF gain a bit to
copy SSB, as the PD still doesn't have quite enough injection, but it
works reasonably well in that mode, too.

A GSB-1 is a good, though spendy, solution; a CV-591/MSR, even better.

BTW:  the radio I described above is for sale; if anyone's interested,
be happy to provide details about it.  It's a beauty!

73, John K4OZY

On Sun, 14 Dec 2008, Mark Johnson wrote:

> I recently acquired a GPR90-RXD receiver and did a pretty thorough overhaul and alignment. I replaced some weak tubes, a few leaky capacitors and as mentioned, I then did a very thorough alignment using better than average test gear.
>
> While the sensitivity is good and the receiver is OK, I find that using it for SSB or even CW, it is not as good as other receivers I have of similar genre. I have to back the RF gain control off to the point where sensitivity suffers or it will overload. I increased the value of the coupling capacitor from the BFO circuit and that helped a bit but as mentioned, other receivers of similar genre are better even though they, like the GPR90-RXD, don't have product detectors.