[Elecraft] Subject: Re: K3 Hum on Audio...

[Edward R. Cole]

Phil,

The mic element is not connected to the mic preamplifier until PTT is 
engaged (on most radios), so it still implies the mic is the pickup 
path.  You said there is no hum with the mic removed from the front 
panel jack, right?  Change mics; is hum still there?

You also said you could suppress the hum by laying a hand on the 
KPA500.  Capacity of your body is enough to bypass the hum 
energy.  Could be time to check the resistance of your ground system.

Two tests you can do to see if it returns:
1)  Connect a 4-5 foot piece of wire to the mic pin that connects to 
the mic element and key the radio - hum?
2)  Disconnect the ground wire.  More hum or less hum?

A high resistance ground can lead to RF getting into a mic and could 
just sound like hum, though AC pickup is more likely the source.

Like you said it probably is lurking to strike again.  GL, Ed

*Note:  my old workplace had such bad ground loops that it was nearly 
impossible to route audio or computer network wiring thru the comm 
room.  I measured about 20vac on the ground system to my RF 
ground.  Telephone circuits with twisted wire had so much 60 and 120 
Hz hum it interfered with normal tracing tones.  Unfortunately the 
company was unwilling to try fixing this.  The building had 480, 277, 
and 120vac circuits and the 480 mains input transformer was 20-feet 
away.  The building formerly was used as a welding shop.  I resorted 
to using isolation transformers for my test bench ac supply to 
isolate from the ground system.


73, Ed - KL7UW, WD2XSH/45
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