[Premium-Rx] RE: Mods for Racal 6790GM

[Dan Rae]

Dan Seven wrote:

>I am interested to know if there are any documented mods available to
>improve the performance of the Ra6790GM.
>For instance, has anyone tried a preselector with this unit, or outboard
>preamp?
>

Dan, to start with, not all RA6790/GMs  are created equal.  I have one 
here (a civilian one, not ex-military) that has all Collins Mechanical 
filters fitted, as well as two 8 kHz wide 40.455 MHz roofing filters and 
the LF version 1st mixer, and the performance of that on a crowded band, 
say in a contest,  is markedly better than the standard one. 

In my opinion, the biggest failing of the 6790/GM is the rather weak 
second mixer, where a lot of intermodulation products are generated.  
This is probably why my LF/MF/HF version sounds better.  Secondly the 
synthesizer has rather more phase noise than I would like, but there is 
not much that can be easily done about this, other than making sure it's 
set up correctly.

There was a second mixer board Racal made that is stronger (high dynamic 
range), but they are hard to find, I've only ever seen one...

Some people fit the A1 RF amplifier / LP filter box from the RA1792 to 
the 6790/GM, but in my experience, with a properly aligned set there is 
no point in adding this as the noise figure is adequate as it stands, 
for HF work.  Adding it will also reduce the IIP3 numbers somewhat below 
the +20 dBm it should have.    But if you were only interested in weak 
signal work above say, 21 MHz then it might be worth trying. 

Ever since Racal introduced the high first IF and quad FET mixer in the 
RA1772, way back when, I've never found a preselector to be necessary, 
unless you are co-sited with lots of  high power transmitters.  Even on 
40 meters in Europe at night these radios work OK and that's saying 
something.   My 1772 is permanently in the broadband position, but that 
one's off topic :^)  The Royal Navy used tracking preselectors with 
their 1792s, but they have 1 kW transmitters on the same boat...

Others will have other opinions, I'm sure, this is just my experience.  
If the set is aligned correctly, it's pretty darn good for a 20 + year 
old set.

Dan
ac6ao / g3ncr