[TMC] GPR-90RXD stuff

[Richard Knoppow]

    I have a feeling the receiver was meant to be  used with 
external adapters of various sorts and the inherent bandwidth 
wide enough not to get in the way. For instance the MSR series of 
sidband adapters have quite good filters. One version of the RXD 
has 15Khz bandwidth, evidently wide enough for multiple signal 
transmission (I have lost the proper name of this for the moment) 
again for use with an external converter. The IF's in all of 
these seem to be critically coupled transformers perhaps with 
some attention to relatively flat phase response. Other receivers 
use overcoupled transformers to either increase bandwidth or 
result in a fairly flat passband with sharper skirts. For 
instance the  51J has three overcoupled transformers and is tuned 
using a detuning attachment. Some receivers use variable 
coupling. I suspect the GPR-90 was designed to be either a stand 
alone receiver or part of a system, such as a diversity system, 
with external means to obtain more stable frequency control than 
the basic receiver could deliver or better IF selectivity or some 
other special feature. The other receiver suitable for that 
application is the Hammarlund SP-600, either the modified version 
from Northern Electric or the JX-17 (which still needs some 
modification). The SP-600 cost nearly twice what the GPR-90 did.
    I am wandering here and speculating. Its just that the GPR-90 
and its relatives have always fascinated me.

On 8/14/2021 6:11 AM, Les Locklear wrote:
> I've been reading the TMC thread with interest.
>
> Having owned both a GPR-90 s/n 1120 and GPR-90RX s/n 7514 I have come to the conclusion that the GPR-90 series is one of the most visually appealing
> general coverage receivers built.
>
> Dial readout is quite good, stability good enough for that period. My only complaint is the if filters. They are barn-door wide
> and very poor for any kind of critical listening.
>
> The GPR-90RX was much better on sideband than the GPR-90.
>
> Both were sold/traded years ago and I never missed them.
>
> Les Locklear
>
> ----- Original Message -----
> From: Richard Knoppow <1oldlens1 at ix.netcom.com>
> To: <tmc at mailman.qth.net>
> Sent: 8/14/2021 2:53:45 AM
> Subject: Re: [TMC] GPR-90RXD stuff
> ________________________________________________________________________________
>
>     Some more GPR-90 questions.  Because of the previous thread I
> began to read over the various GPR-90 manuals at the TMC site. In
> particular I wanted to know what differences there were between
> the 90 and the 90RXD. I discovered a surprise to me that the RXD
> had a built in product detector. Its activated by the SSB switch
> on the back apron. It has a 6BE6 which operates either as a BFO
> supplying the diode detector or as a product detector taking a
> signal from the IF and connecting to the AF amp. There does not
> seem to be any provision for a  slow release AVC. This is a
> puzzle to me since the receiver is set up to work with an
> external SSB adaptor like the MSR-6 and could work with a more
> comprehensive one like the MSR-9 which has means for connecting
> to a receiver's AVC. So, what was the purpose for this detector?
> Without a slow release AVC it can't have been too satisfactory.
> In addition the IF bandwidth is a little wide for SSB. What was
> going on there?  The receiver has other changes, such as a double
> RF amplifier and perhaps some others. Its a very frustrating
> design to analyze without any idea of the intended market.
>

-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL