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
======================================
BP40IQ 500 KHz - 10-GHz www.kl7uw.com
EME: 50-1.1kw?, 144-1.4kw, 432-QRT, 1296-?, 3400-?
DUBUS Magazine USA Rep
[hidden email]
Coming Soon - "Kits made by KL7UW"
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