[TMC] GPR-90 RXD Tuning

Fri Mar 14 13:59:02 EDT 2014

Hi all,

Unfortunately, much of this discussion is over my head, but I wanted to ask the following.  I have a GPR-90/GSB-1 set up (several of them!) I am just now building a new shack and redesigning everything. As part of the redesign, I am considering putting a multicoupler in front of all of the receivers. I have two, one is a CU-1280/FRD-10A and the other is a TMC AMC-8.  I don't have the technical specs of either of these in front of me.  But I believe the AMC-8 has a BCB notch filter in it (again, not 100% sure).  If this turns out to be correct, should this improve the receiver's performance and would there have been any technical benefit in re-instating the first RF stage for this band now the overload from BCB stations should have been reduced? Not to say that I would attempt to modify the radio, but if TMC was designing the radio with the expectation of an AMC-8 in front of it, for example.

And unrelated to this topic, does anyone have any documentation on the CU-1280 and whether it might have any notching in it?

Thanks
73 Eugene W2HX

-----Original Message-----
From: tmc-bounces at mailman.qth.net [mailto:tmc-bounces at mailman.qth.net] On Behalf Of jvendely at cfl.rr.com
Sent: Wednesday, March 12, 2014 6:07 PM
To: tmc; Richard Knoppow
Subject: Re: [TMC] GPR-90 RXD Tuning

It's interesting to compare the specified intermod performance of the GPR-92, which has a tuned 1st RF stage almost identical to the GPR-90RXD, and the GPR-92A, which has the same grounded grid broadband RF stage as the GPR-90.  The GPR-92 intermod rejection is spec'd at 70 dB, whereas the -92A is spec'd at just 35 dB.  35 dB intermod rejection is surprisingly poor performance, and there seems no reason to assume the GPR-90 would perform much differently than the GPR-92A in this respect.  Intermod performance is not even spec'd for the GPR-90. I have a GPR-90RXD, and I must admit it often has fairly severe intermod problems on 40M at night.  Of course, the broadcast stations are quite strong here on the Atlantic coast of Florida.  

But one can easily imagine the problems the big point-to-point HF receiver sites might have had when these receivers were used with large antenna systems.  An interesting example is the NASA HF network back in the days of Project Mercury.  Western Electric was awarded the contract for the Mercury Ground Network and subcontracted the HF portions of it to TMC, who supplied a number of GPT-10K and 40K transmitters, and diversity receivers.  The receivers had a pair of GPR-90RXDs with a VMO, and two SBC/AFC-2 ISB converters.  Some receivers had the same setup but with two GPR-92s and its accessory crystal oscillator units instead of the GPR-90RXD.  Someone who was "on the scene" at the time told me that HF interference problems occurred frequently, which were obviously receiver intermod problems.  A bad case of cross modulation occurred during John Glenn's orbital flight, and Voice of America audio could be heard.  Official phone calls were quickly made instructing VOA to cease t  ransmitting on the problem frequency, the only rapidly available solution.  

NASA greatly expanded the network starting in 1965 to support Project Gemini and the upcoming Project Apollo. New remote-tuned TMC TSTE-10K transmitters were installed, and the older receivers were replaced with the giant remote-tuned DDR-506, which had much better front ends.  These systems were used at all ground stations and on board the Apollo Tracking Ships until well into the 1980s.  

It's remarkable that the GPR-90RXD and GPR-92, with their obvious ham radio pedigree, were used as extensively as they were in large government and military systems.  Overall, they apparently provided acceptable service.

73,

John K9WT

---- Richard Knoppow <1oldlens1 at ix.netcom.com> wrote: 
> 
> ----- Original Message -----
> From: <jvendely at cfl.rr.com>
> To: "tmc" <tmc at mailman.qth.net>; "Richard Knoppow" 
> <1oldlens1 at ix.netcom.com>
> Sent: Wednesday, March 12, 2014 6:47 AM
> Subject: Re: [TMC] GPR-90 RXD Tuning
> 
> 
> > Richard,
> >
> > There is a 5 pole highpass filter at the input of the GPR-90's 
> > grounded grid 1st RF stage, which I assume cuts off just above the 
> > standard broadcast band, since this stage is not operational in that 
> > range.  Eliminating strong BCB interferers probably improved HF 
> > performance significantly.  The GPR-90 front end has the advantage 
> > of providing a well controlled input impedance (75 or 300 ohms)over 
> > a wide frequency range.  The input impedance of most HF receivers of 
> > the day varied all over the map, even at the tuned RF stage center 
> > freq.  Return loss "off channel" was usually very poor, though this 
> > really was not a problem in most applications. On the broadcast 
> > band, the tuned 2nd RF stage appears arranged to provide a 
> > relatively high impedance antenna input which varies considerably 
> > with frequency.  This allows good sensitivity with short random wire 
> > antennas.
> >
> > Of course, the drawback is that the GPR-90 1st RF stage gets 
> > bombarded by everything coming in the antenna, hence the receiver's 
> > reputation for intermod problems.  As broadband tube front ends go, 
> > it's actually pretty linear, though not nearly good enough for 
> > demanding environments.
> > I suspect the main reason for the broadband RF stage was cost 
> > reduction, as it eliminated an extra gang on the tuning capacitor 
> > and a bank of bandswitched coils. The combination of the 1st and 2nd 
> > RF stage plate tuning was enough as-is to provide reasonably good 
> > image rejection.
> > But in the process, they traded off intermod performance.
> >
> > I don't recall the original selling price, but it was at the high 
> > end of the range for the amateur market.
> >
> > 73,
> >
> > John K9WT
> 
>      That pretty much confirms what I thought. I will have to draw out 
> the filter and see if I can calculate what it does.  The second tuned 
> RF in the RXD would also reduce LO radiation, which might have been 
> necessary for military or commercial multi-receiver applications.
> 
> 
> --
> Richard Knoppow
> Los Angeles
> WB6KBL
> dickburk at ix.netcom.com 
> 
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