"You have hum on your signal"

This was a comment made by another ham during a QSO on 2m. The rig I was using was my old but trustworthy Icom IC3200A dual band.
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I ran up the HP 8924C test set and sure enough, there was hum on my signal. This is an oldie but a goody as far as test sets go. Analogue, CRT screen has Spectrum analyser with tracking generator, just the thing for filters or cavity tuning. Anyhow, the monster test set (requires 2 people to lift it!!) said " I agree, there is hum on your signal.
 
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So I tried all the usual suspects. The 20 amp PSU tested out at 20mv AC on the meter so that should be fine.I tested the earths, and they seemed to be fine. I put a ferrite choke on the power feed, no difference. Then I unplugged the microphone and keyed the transmitter up, no hum, the scope had a wee bit of noise, but no hum. It was coming from the michrohone.
 
I consulted the circuit diagram for the HM12 micophone. I tested all the connections sheilds for connectivity, no issues there. As you can see to the right the bias for the electret microhone insert goes throuh a 2K ohm resistor and filtered by a 10 mFd electrolytic. The audio pin is connected to a DC blocking coupling capacitor, in my case a .33 Mfd. The likely culperit is the 10 mFd shunt capacitor. So I replaced it.
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On testing, clean signal, that was the problem, a 20 year old shrivled up electro.