[TMC] Why GPR-90s Look So Good

[Les Locklear]

-----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

-----Original Message----- 
From: Richard Knoppow
Sent: Friday, December 12, 2014 3:41 PM
To: Les Locklear
Subject: Re: [TMC] Why GPR-90s Look So Good


----- Original Message ----- 
From: "Les Locklear" <leslocklear at hotmail.com>
To: "Richard Knoppow" <1oldlens1 at ix.netcom.com>; "John
Poulton" <jp at cs.unc.edu>; "Chris Kepus" <ckepus at comcast.net>
Cc: "tmc collector's group" <TMC at mailman.qth.net>
Sent: Friday, December 12, 2014 12:04 PM
Subject: Re: [TMC] Why GPR-90s Look So Good


> It's obvious that engineers from Hammarlund were involved in the cosmetic 
> design of the GPR - 90 series.
>
> It's also obvious that someone else had a hand in the electrical and 
> mechanical design of that same series.
>
> I have a near mint GPR-90 that looks great and works ok. The mechanical 
> stability when attempting to utilize the antenna tuning control shows the 
> shortcomings of that particular design feature. It has more warbles and 
> birdies that a Drake R7.
>
> I like it, but seldom use it. But, it looks nice sitting on the desk.
>
> Les Locklear
>

    I remember the first one I saw, brand new at Henry Radio
when they first came out. Very impressive looking but I
discovered right away that it was microphonic. Most
receivers using conventional tuning capacitors are. That was
one of the things about the Collins receivers that set them
apart.  I have not had my hands on a working GPR-90 for so
long I don't have much sense of how they work.
    The Antenna Tuner is curious because its not in the
antenna stage at all. Rather its in the grid of the second
RF so it acts more to trim the tracking than anything else
although it may be an actual antenna compensator on the
broadcast band where the first RF is by-passed. In the
GPR-90RXD its in the grid of the first RF. This stage is
essentially a manually tuned pre-selector with the antenna
tune control to tune it. I drew out the input circuit of the
GPR-90 at some time, its a series of high pass filters, no
band pass function, at least that I can see, so it would not
have much effect on image response or on cross modulation
from nearby signals. I think its the cross-mod that was the
target of the tuned RF on the RXD version.
    The grounded-grid RF input should be very low noise but
I suspect its mainly an impedance converter. I have tried to
calculate how much gain, if any it has.  It would be
interesting to know the design philosophy of this
arrangement.


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