[TMC] GPR-90RXD stuff

W2HX w2hx at w2hx.com
Thu Aug 19 12:53:37 EDT 2021 

George, thanks for directing us to Johns TMC history video. I'd never seen it before and, John, it is great. Thank you!

73 Eugene W2HX
Subscribe to my Youtube Channel: https://www.youtube.com/c/w2hx-channel/videos





-----Original Message-----
From: tmc-bounces at mailman.qth.net <tmc-bounces at mailman.qth.net> On Behalf Of George Maier via TMC
Sent: Tuesday, August 17, 2021 3:03 PM
To: 'Richard Knoppow' <1oldlens1 at ix.netcom.com>; 'Les Locklear' <leslocklear at hotmail.com>; tmc at mailman.qth.net
Subject: Re: [TMC] GPR-90RXD stuff

Richard, Les, and all:

Actually, it's the GPR-90RXDS that has the product detector built-in; the RXD does not; they're easy to confuse.  The selectivity is certainly not the tightest, but it's great for AM, and the crystal filter is one of the best and quite useable for SSB or CW.   As Richard noted, there were other receivers on the market that were superior but way more costly.  Also, the dial calibration from end to end is more accurate than anything I've ever seen in general coverage receivers from the likes of Hammarlund, National or Hallicrafters.   I have an R-388A/51J-4 and a Racal RA-17C12 here, but there's just something about the GPR that attracts me to it. 

The RXD and RXDS offered a lot of flexibility with all of the external interfaces for space and/or frequency diversity; chances are it was mostly frequency diversity as adequate decorrelation of HF antennas takes up a LOT of real estate. 

73, George - W1LSB

PS - If you'd like to a see a really good TMC history talk; navigate to www.ce-multiphase.com and scroll down to the 4th annual (2016) CE Boatanchor Event summary where you'll find a link to John - K4OZY's presentation; it's well worth seeing. 

-----Original Message-----
From: tmc-bounces at mailman.qth.net [mailto:tmc-bounces at mailman.qth.net] On Behalf Of Richard Knoppow
Sent: Saturday, August 14, 2021 9:23 AM
To: Les Locklear; tmc at mailman.qth.net
Subject: Re: [TMC] GPR-90RXD stuff

    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 frequ  ency 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

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