[TMC] GPR-90 Redesign

[Richard Knoppow]

----- Original Message ----- 
From: "Chris Kepus" <ckepus at comcast.net>
To: "'Dave McDonald'" <jdmcd at ij.net>; <TMC at mailman.qth.net>
Sent: Monday, August 27, 2012 8:57 AM
Subject: Re: [TMC] GPR-90 Redesign


> Hi Dave,
>
> Thanks for your response.  It is great to hear from 
> someone who actually did
> the research on the differences among the receivers 
> available back in the
> day.
>
> After I get my op desk with the GPR's going, I'll report 
> back.
>
> Thanks again!
>
> 73,
> Chris
> W7JPG

      Comparisons are indeed interesting.  The GPR-90 is a 
curious combination of quite conventional design with a few 
innovations that were advanced at the time. The receiver was 
obviously meant to appeal to the amateur market was well as 
being a foundation receiver for commercial and military 
receiving systems. It has jacks on the back for an SSB 
adaptor but could have been built with a product detector 
and provisions for a narrower IF filter. Cost was probably 
one object, the GPR-90 was by no means a cheap receiver but 
was still half the price of either a 51J or SP-600-JX 
neither of which had SSB capability without an external 
adaptor of some sort.
     TMC used a low-noise front end with a cathode coupled 
pentode mixer. Collins did this in the S line well after 
TMC.  National also used pentode mixers in the HRO but with 
LO injection on the screen grid rather than the cathode. 
These mixers are much quieter than the usual pentagrid or 
hexode types.
     I think TMC fell down in their advertising for the 
GPR-90, they created a controversy about dial calibration 
and stability that I think put questions into the minds of 
prospective buyers that were unnecessary.  At the time the 
competition included the Collins 75A and 51J receivers which 
had extremely good stability and calibration but, from the 
numbers in TMC specs, the stability of the GPR-90 was as 
good as the SP-600-JX, maybe better.  The GPR-92 specs are 
the same but indicate a warm-up time of 12 hours to achieve 
0.003% drift. This is actually pretty good being 90hz at 
30mhz.


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