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

[Guy Atkins]

OK, you talked me into it, Walt <g>

I owe my success with narrower roofing filters in the RA6790GM to Frank
Carcia's helpful advice. I wanted 8 kHz bandwidths, but all I could locate
were 10 kHz modules. I found them on the 2nd IF boards from Cubic R-3030s,
both via Ebay ($35 & $50 each, if I recall). The metal cases were the same
size as the stock Racal roofing filters, but the pinouts were reversed. Both
were installed at an approx. 45 degree angle to the original orientation on
the Racal boards, with a couple carefully drilled holes in the PCB and a
short jumper wire.

As discussed recently on the list, roofing filters are part of a carefully
balanced system in the front end of a receiver. The new filters upset the
AGC apple cart, but with Frank and Dallas Lankford's advice I eventually got
some AGC loop pots adjusted so that strong (local) AM signals wouldn't
distort.

The result was lower noise and the ability to DX a weak station close to a
very strong one. The improvement was most noticeable with weak trans-Pacific
mediumwave stations, which I chase between the 10-kHz North American
channels on DXpeditions and from home. For whatever reason, the mod also
tightened up the 8 kHz IF filter so that the Racal measured it as 5.5 kHz.
The net effect was clearly improved shape factor in that filter position;
stations really "snapped" in and out of the selectivity window as I tuned
around with the modified receiver.

Guy Atkins
Puyallup, WA USA


-----Original Message-----
From: Walter (Volodya) Salmaniw [mailto:salmaniw at shaw.ca]
Sent: Thursday, February 17, 2005 5:28 PM
To: premium-rx at ml.skirrow.org
Cc: dx at guyatkins.com
Subject: Re: [Premium-Rx] RE: Mods for Racal 6790GM

<SNIP>

Guy Atkins recently did the roofing filter thing with his 6790 which vastly
improved it's performance.  This rig was also recently sold on
eBay.  Perhaps Guy can make some comments on the improvement in
performance.......Walt.