[TMC] GPR-90 Redesign

[John Vendely]

Interesting comments, Richard.  It's worth noting that the GPR-90 RF 
stage is common-grid, with a broad-band ferrite core input transformer.  
This arrangement provided a fairly stable input impedance over the 
frequency range, compared to receivers with conventional tuned-input RF 
stages. But this also resulted in an RF stage that was rather 
broad-banded, and thus susceptible to intermod problems.  The GPR-90RX 
was probably aimed more at the commercial and low-end military markets 
versus the "stock" GPR-90, which straddled the amateur and low-end 
commercial markets.  The GPR-90RX series were therefore more likely to 
have been connected to large, broadband antennas with significant gain, 
such as log periodics and rhombics used in the big point-to-point 
receiver plants.  This would have caused serious intermod problems in 
the GPR-90's relatively broad RF stage, so TMC may have gone to the more 
conventional common-cathode, tuned-input RF stage in the RX version to 
mitigate this.  This would likely have improved image rejection, as well.

I have a GPR-90RXD, but do not have a GPR-90, so I can't provide a 
comparison based on operating experience or measurement.  But complaints 
of intermod problems in the GPR-90 are very well known, which may lend 
credence to this theory.  I know a fellow who bought a GPR-90 brand-new 
in the early 1960s, and sent it back to the factory multiple times 
complaining of intermod problems.  TMC repeatedly assured him the 
receiver was performing properly.   He finally resigned himself to this, 
but complains bitterly about it to this very day!

Perhaps someone who owns both versions of the receiver can enlighten us 
on this matter.  It would be interesting to make some intermod and 
image-rejection measurements as a point of comparison...

73,

John K9WT



On 8/27/2012 12:56 PM, Richard Knoppow wrote:
>
>        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
>   
>
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