[TMC] Why GPR-90s Look So Good

[Les Locklear]

Probably a poor choice of wording regarding "birdies" but, that's what it 
sounds like when tweaking the antenna control on my GPR-90. I did own a 
GPR-90RX  many years ago, but it was quickly turned around in a trade that 
someone drooled over, I don't even recall what it was now. Yes, the HQ-120 
and 129 had the same sort of set-up, but they never seemed to cause any 
problems that I can recall.. A possibility could be the 2nd rf tube, 1st 
converter and oscillator centered in between the two tuning capacitors on 
the GPR-90. The plates that the tuning capacitors and the tuning capacitor 
on the GPR-90 are thinner metal and probably flex somewhat and/or capacitive 
coupled by the tubes. I don't know.

I'm not bitching, you learn with every different receiver owned/operated and 
this has been a learning experience. I'll stick with what I have and leave 
it when I'm gone for someone else to figure out.

I also can remember the original advertising and photos touting the SP-600X 
as it was originally referred to and it was quite different from the 
original ads, specs, etc..

Les Locklear





-----Original Message----- 
From: Richard Knoppow
Sent: Friday, December 12, 2014 6:24 PM
To: Les Locklear ; TMC at mailman.qth.net
Subject: Re: [TMC] Why GPR-90s Look So Good


----- Original Message ----- 
From: "Les Locklear" <leslocklear at hotmail.com>
To: <TMC at mailman.qth.net>
Sent: Friday, December 12, 2014 4:03 PM
Subject: [TMC] Why GPR-90s Look So Good


>
>
> -----Original Message----- 
> From: leslocklear at hotmail.com
> Sent: Friday, December 12, 2014 5:59 PM
> To: Richard Knoppow
> Subject: Re: [TMC] Why GPR-90s Look So Good
>
> If you turn it on and lets say tune to WWV at 15.0 MHz or MC zero beat it 
> at
> .25 KC  and wait 10 or 15 minutes, turn on the bfo and guess what, it will
> usually be zero beat. OK, good stability and good bfo stability. Go
> somewhere, it doesn't matter, 80, 40 or 20 meters ssb and do the same then
> tune the antenna control for maximum signal and the birdies circle the
> receiver.
>
> I didn't intend this to be a rant of the receiver or make it a contest vs.
> other receivers such as the SP-600, but, I find myself listening to my
> SP-600 JX-21 much more often. True, the SP-600 drifts a bit when turned on
> and the GPR-90 doesn't seem to, but in the end, tuning a ssb signal on
> either is not a problem with the SP-600 and the GPR-90 the band spread
> tuning coupled with the antenna tuning is very unstable. Even with the 
> band
> Spread tuning capacitor being a separate capacitor, it is much more
> difficult (touchy) than on the slower tuning of the SP-600 single 
> capacitor.
> I have tightened every screw on the receiver to no avail. Having a air
> variable tuning capacitor for the antenna between the main tuning and band
> spread capacitors is a piss poor design IMHO. That is where the problems
> lies and I see no cure for it.
>
> I just think somebody missed the boat on a chance to manufacture a better
> receiver.
>
> No complaints as long as I listen to AM BCB or Shortwave stations. But, I
> can receiver ssb signals on the GPR-90 with the rf gain full up and copy 
> ssb
> with no problem as long as I do not try to adjust the antenna
> control/tuner/compensator. Where on the SP-600 I am using I have a 
> separate
> SSB adaptor, a PD-2. And, the SP-600 has no antenna
> control.tuner.compensator.
>
> YMMV
>
> Les Locklear

     I wonder what's going on here. Birdies are from
spurious responses. It may be that the front end design does
not have enough selectivity to eliminate them. Do they seem
to appear at frequencies that are related in some way to the
IF?  It would be interesting to figure out what the problem
is.  Also, do you know if the RXD behaves the same way?
     FWIW, the mechanical design of the HQ-120-X and
HQ-129-X are similar. The antenna trimmer is mounted
inbetween the main tuning and bandspread caps. It does not
seem to have problems with birdies. Something coupling that
shouldn't be.
     At the moment, for various reasons, I am not sure I
want to acquire any more old radios but would probably give
house space to a GPR-90 if someone wanted to throw one away.
    Also, I have two SP-600s which have been on the back
burner for a long time. I had to replace the tuning cap on
one because the plates had been bent to the point where one
came loose. The other has also been bent but I think I can
straighten that one. I think part of the trouble with the
SP-600 is that the stators are clampled in the frame and can
drift. If they are not exactly centered and parallel the
capacitor will not follow the correct tuning law. They are
easy enough to remove and can be adjusted with shims go get
them right. This is a job I must do on my JX17.  My main
problem is that I have fairly severe back problems and must
be very careful about lifting anything. What a bore.
    The SP-600 is really a much more sophisticated design
with its rotary turret and adjustable coupling IF.  If you
look at the very early advertising for it you will find the
specs quite different from the production receiver. I think
the design was nearly complete and was then pulled and
re-designed. I would love to know the story behind this. Not
quite the same as Hammarlund and the HQ-129-X which was put
on the market and then pulled because they could not meet
the price committment ($129).  It is also well to remember
that the GPR-90 was a little more than half the price of an
SP-600 or 51J.


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com