[Elecraft] Hum on TX audio w/ CM500 headset on KX3

[Frank O'Donnell]

Thanks very much for the suggestions, Jim -- all sounds like good advice.

On 5/10/20 2:24 PM, Jim Brown wrote:
 > 1) Is what you hear HUM (pure 60 Hz) or BUZZ (harmonics of 60 Hz)?

Without routing the KX3's audio out to a soundcard and looking at the 
waveform(s) on a PC, I'm going to say it sounds to my ear like a pretty 
pure hum, not a buzz.

 > 2) Make sure that the adapters you got are actually doing what you 
think they are ...
 >
 > 3) Make sure that bias is turned on ...
 >
 > 4) Make sure that Mic Gain and Comp are not turned up too high. ...
 >
 > 5) For TXEQ, set three lowest bands (50, 100, 200 Hz) for max cut, 
set next
 >    band (400 Hz) for 6dB cut), set the top two bands for about 6dB 
boost.

All those points were good.

 > 6) Make sure that everything in your station that connects to AC 
power, ... has their
 >   chassis bonded together AND powered from the same outlet, ...

In the initial test, which produced the hum, the power source for the 
KX3 was a Pro Audio Engineering PAE-Kx33 low-RFI supply. This PSU was 
plugged into the same outlet as a variety of other shack gear, some 
powered on. The only other things connected to the KX3 were (a) an 
antenna intended for field use, and (b) the CM500.

I noticed that when I touched the shield on the antenna coax's BNC 
connector with my finger, the hum diminished substantially.

I then unplugged the PSU and replaced it with a car battery charged to 
12.01v. Now the hum appeared to be completely gone.

So, if I were to keep using the KX3 within the shack, it sounds as 
though a thorough grounding/bonding workup would probably fix the 
problem. I'm in the process of working through
http://k9yc.com/GroundingAndAudio.pdf.

I was mostly curious why the CM500's mic produced the hum, but the 
handheld mic that came with the KX3 didn't. The CM500 has a quite long 
cable, and I wondered if that could be part of the problem.

Thanks again and 73,

Frank K6FOD