[TMC] TMC GPR-90

[Richard Knoppow]

----- Original Message ----- 
From: <wb9tow at egr.msu.edu>
To: <tmc at mailman.qth.net>
Sent: Sunday, June 30, 2013 7:03 PM
Subject: Re: [TMC] TMC GPR-90


> Just wondering,I've recently been using my TMC GPR-90 on 
> CW
> more with a newly acquired GSB-1. I find the cw function 
> with the
> GSB-1 much improved, but the rig did need to stay on a 
> good 24 hrs before
> it was as stable as I like for cw.
>
> I also note the old problem with the "90" of over-loading 
> on
> freq above 7 Mhz especially at night with a busy band and 
> big
> signals...
>
> Has anyone modified the 90's front end to the GPR-92 front 
> end circuit?
>
> 73 all Steve w8tow

   After a careful look at both versions I think 
modification is not practical. Here is why; the GPR-90 has 
an impedance matching network between the antenna and first 
RF cathode. Not much selectivity there.  The RXD has 
conventional tuned transformers between antenna and grid of 
the first RF. The added selectivity of this tuned stage has 
to improve the IMD performance. Generally, the more 
selectivity between the antenna and mixer the better the IMD 
will be. I think so much change would have to be made as to 
make the mod impractical. However, some sort of tuner ahead 
of the receiver would help. I don't think this has to be 
active, i.e. there is plenty of gain there, but it needs to 
be selective enough to reduce nearby signals.  It would have 
to be manually tuned but many modern receivers have 
pre-selectors that are not ganged to the main tuning. 
Receiving matching networks are nothing new, I think even 
Heathkit made one for use on cheap receivers that had no RF 
stage. However, one would probably improve even a receiver 
with two RF stages by adding just a little selectivity.


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com