[TMC] GPR-90 RXD Tuning

[jvendely at cfl.rr.com]

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 


---- Richard Knoppow <1oldlens1 at ix.netcom.com> wrote: 
> 
> ----- Original Message ----- 
> From: "Richard Knoppow" <1oldlens1 at ix.netcom.com>
> To: <jvendely at cfl.rr.com>; "tmc" <tmc at mailman.qth.net>
> Sent: Tuesday, March 11, 2014 8:39 AM
> Subject: Re: [TMC] GPR-90 RXD Tuning
> 
>      Another question:  I am not sure I understand the RF 
> input circuit for the GPR-90.  I have not yet drawn it out 
> but it appears to use sections of LC high-pass filters.  How 
> is this supposed to work?
>      Also, I have never seen a price for the RXD but the 
> GPR-90 was a bit more than half the price of the SP-600 for 
> similar functions so I suspect the RXD was considerably 
> cheaper than the equivalent SP-600-JX-17 or the Northern 
> Radio modified receivers, which were very similar. Can 
> anyone confirm this?  The answer may be on the TMC price 
> lists which have been posted. I am off to work and can't do 
> that until this evening.
> 
> 
> --
> Richard Knoppow
> Los Angeles
> WB6KBL
> dickburk at ix.netcom.com 
> 
> ______________________________________________________________
> TMC mailing list
> Home: http://mailman.qth.net/mailman/listinfo/tmc
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:TMC at mailman.qth.net
> 
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html