At 07:51 AM 2/21/02 -0500, John Allen wrote:
...
>I first tried all sorts of ferrites on the keying cables etc. BUT (and
>don't ask me how I got to this discovery - I don't remember!) THE SOLUTION
>WAS to pull the phono plug from the CT LPT interface to the rig/keyer OUT
>until the shield was not connected, only the center pin of the phono plug.
>Presto Magic - no more lockup. Since there were multiple ground paths,
>including the safety ground for the power line, the keying still worked.
>
>Why did this work? I don't know for sure, but I surmise that it was a RF
>related "ground loop" where current was flowing through that shield onto the
>computer chassis and causing the problem. Why didn't the ferrite clamp ons
>stop the ground loop? I don't know, maybe not enough impedance - I did not
>try a real large toroid that could take several turns, just about 8 1 turn
>clamp ons. Note - the antennas are 100 feet away and 100 feet up.
Buried in this Month's QST article on "A Quality Sound Card Interface for
ICOM rigs" on page 35 is an explanation of how this technique works. It's
the first such I've ever seen, and suggests why some ac hum and/or RFI
problems may be intractable even when we think we've done all the right
things (such as isolation transformers in audio lines). Recommended!
73, Pete N4ZR
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