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

[Jim Brown]

On 5/10/2020 12:05 PM, Frank O'Donnell wrote:
> For my CM500 headset with the KX3, I referred to Walter Underwood 
> K6WRU's blog posts to order the adapter allowing the CM500 mic plug and 
> a PTT switch to share the radio's mic jack. I also followed his 
> checklist from a second blog post to set TX EQ and mic bias.
> 
> While adjusting the mic gain with the TX power set to 0.0, I noticed hum 
> on my audio from the CM500 mic. This was audible regardless of whether I 
> had the PTT cable attached, or if I left it detached and tapped the XMIT 
> button. The hum also appeared regardless of whether I used the CM500 
> phones vs other headphones. If I unplug the adapter and use the handheld 
> mic that came with the KX3, the hum does *not* appear.
> 
> Any suggestions on what I should look at to run this down?

1) Is what you hear HUM (pure 60 Hz) or BUZZ (harmonics of 60 Hz)? In 
general, most hum is magnetically coupled, while buzz is power line 
leakage currents, aggravated by by Pin One Problems and failure to do 
proper bonding.

2) Make sure that the adapters you got are actually doing what you think 
they are doing (or that you want them to do), checkin them out with an 
Ohmmeter.

3) Make sure that bias is turned on -- the mic in CM500s is an electret, 
which requires bias.

4) Make sure that Mic Gain and Comp are not turned up too high. To set 
them, turn Comp all the way down, set TXEQ (see next step), then set Mic 
Gain to get indicated 100% modulation on the meter.

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.

6) Make sure that everything in your station that connects to AC power, 
including any computer CONNECTED to your radio and anything connected to 
the computer, has their chassis bonded together AND powered from the 
same outlet, or from outlets whose green wires are bonded together, and 
one of those chassis connections bonded to your station ground , AND all 
grounds in your home all bonded together. These slides have drawings 
that illustrate conceptually how to do it right.

http://k9yc.com/GroundingAndAudio.pdf

73, Jim K9YC