Omni-6 RF Feedback Problem

[DAN]

Greetings to fellow Ten-Tec enthusiasts!

A few months ago, I replaced my Kenwood TS-850 with an Omni-6.  I'm very glad I made 
that choice (I don't say this lightly, because the 850 is a very nice radio).  The 
Omni-6 has been masterfully engineered, and we're fortunate to have such a company as 
Ten-Tec in our country.

Now for the problem....  When I pulled the TS-850 out, I put the Omni-6 into the empty 
slot.  Then I hooked up my computer (25 pin serial port), key, Heil mike, external 
speaker, antenna (via SB-220), and a footswitch to the Mute & PTT jacks.  It checked out 
on CW just fine.  When I tried it on SSB, it worked OK on 160, 80, & 40, but had RF 
feedback in the audio circuit on 20 and 15.

Here are some observations:
1) The feedback was power sensitive--as I decreased the power, the feedback decreased.
2) With my SB-220 engaged, the feedback was unbearable.
3) When I connected a dummy load (either directly to Omni-6 or with SB-220 engaged, the 
feedback stopped).
4) The feedback was the same with either an R7 or a longwire connected.
5) The feedback was so bad that it would motorboat with mike disconnected (if the mike 
gain was turned up high)!
6) I did not have the problem with the 850.
7) I tried several different mikes, including the Ten-Tec.

I have a good, low impedance, single point ground to which all of my equipment is 
connected.  I rebuilt my operating desk to put more space between the SB-220 and the 
Omni--no help.

Finally, I removed each connection from the Omni, one at a time, checking for the 
feedback.  When I removed the last one, the computer serial interface (25 pin), the 
feedback disappeared, and the audio was beautiful.

As per the Ten-Tec manual, I was using a commercial serial port cable, about 5 feet 
long (I had been using a homebrew interface and homebrew cable on the 850).  Before I 
make up a new cable (the old fashioned way) and proceed with the resolution of this 
problem, I decided to ask you fellows if you have been down this path.  (Is the answer a 
 new cable, isolation, filtering?)

Thanks for any comments you may have.

73,  Dan - W4TQ (Dunnellon, Florida)

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