[RFI] Keyboard

[Jim Brown]

On 3/3/2018 12:09 PM, Joe wrote:
> BUT I may have fixed it by accident. I have been in the process or 
> bringing an old dead SB-220 back to life, and have put it inplace this 
> morning, so cabling has changed greatly in the shack. And the problem 
> may be gone now by just changing out all the cables? 

Lots of problems like this are caused by a bad piece of coax, or coax 
with a poor shield connection (or no shield connection). Under normal 
operation, current on the center conductor returns on the shield, so the 
field stays within the coax. When there's no shield connection, current 
returns on whatever path Mother Nature finds, and there's a strong field 
OUTSIDE the coax. That field radiates, and also magnetically 
(inductively) couples to anything around it.

What is the RF path for antenna switching in the SB220? Many unwashed 
engineers used the chassis as return rather than running coax. This 
creates the same sort of problem inside the amp -- current returns on 
the chassis rather than the coax shield, so it produces EM and magnetic 
fields. GOOD amp designers use coax for everything in the RF path.

73, Jim K9YC