[TenTec] RF

[Jim Brown]

On Fri, 10 Sep 2004 17:14:40 -0400, George Allgood wrote:

> My OMNI VI+ with computer wired to the Key jack swamps when
>I strike a key on the keyboard. The problem goes away if I reduce 
>power on the OMNI to about one half.   

I would be willing to bet that you have a simple pin 1 problem. In 
case you have missed my previous rantings on the subject, the 
problem is that the cable shield for the keying line is not getting 
connected straight to the chassis on either end, but is going to the 
circuit board first. In this condition, any RF current induced on the 
shield will flow on the circuit board, and the voltage drop across the 
inductance of the circuit trace shows up in the electronics. 

The fix is VERY simple. At the Omni, connect the shield of the keying 
line to the chassis, NOT to the RCA connector (which, on my Omni V 
and Omni A goes to the circuit board, not to the chassis).  At the 
computer, connect the shield to the shell of the D-connector. 

It also helps to use twisted pair for the keying line. The best twisted 
pair is shielded CAT5. Connect one side of the pair to the keying 
line at both ends, one side to the chassis at both ends, and connect 
the shield to the chassis at both ends.  

Second best twisted pair is braid-shielded cable made by Belden 
and Gepco for digital audio. I have tested all of the cables and wiring 
configurations in my shack on my K2/100 being keyed by my IBM 
T22, with an end fed antenna that puts a LOT of RF in my shack on 
all bands from 160M through 10M. Before I went to the cables and 
wirings described, the computer would stop with a solid key-down at 
above about 12 watts output. Changing to the wiring and shielded 
CAT5 completely eliminates problems on ALL bands. The braid 
shielded cable fixes everything but 10 and 15 meters. 

The difference in cable construction affects signal coupled onto the 
signal pair (the key line). The wiring on each end affects signal 
coupled onto the circuit board "ground" trace. Two separate 
problems. 


Jim Brown  K9YC