Right, pin 8 is mike, and pins 5 & 7 are the grounds.
So, leaving pin 5 unconnected seems to have helped a bit, but there's still a hum.? After some troubleshooting on the local repeater, we noticed that placing my hand on the transceiver itself while transmitting (holding the mike) made the hum go away, indicating a capacitive problem somewhere.? Bridging an extra decoupling cap across the power internal to the mike didn't change anything, however.
The new theory is that the problem might be a shorted cap in the microphone line (pin 8).? Guessing C104, according to That would explain the high voice volume, as well as perhaps the hum.
I'm measuring about 28k ohms across it (in-circuit), with the probes in either polarity.? So, it's not shorted, but perhaps really leaky?
Before digging in (it's a more invasive fix for me), does that sound plausible?
Thanks for the help!
Greg? KO6TH
Omni wrote:
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I think you have the mic wired wrong. Pin-8 is the mic input not ground, which would explain the hum. Also, do not tie the mic ground (Pin-7) and the "other" ground (Pin-5) together. They are separated for a reason, so keep them that way. The POT controls the DTMF signal level and I would not suggest messing with it.
