I'm struggling with poor TX audio quality on my Icom 736, bought
used several months ago. The mikes I use (an included HM-35, a
homebrewed preamped mike, a Radio Shack amplified mike) all get
reports of audio with no lows, pinched, not terrible but worse
than from other similar radios (e.g. an IC 735 with the same HM).
It might be good for pileups, but it's uncomfortable for normal
ragchewing.

I've read a lot about insufficient mike gain in older Icoms, and
about the possibility of plugging into the acc socket in the back.
But I'd like to first determine whether my mike circuitry might
have a problem.

I put a scope on the mic in and mic gnd pins of the connector,
and on top of the normal 7 or so V DC (for electret biasing) I
see an almost 1 V p-p sine wave, about 110 kHz, and very "noisy".
The trace gets clean and sharp after shunting the mic ground to
the regular chassis ground with a 10 nF cap. It's there whether
the mike is plugged in or not, and whether the radio is receiving
or transmitting.

This high frequency signal should not interfere with the audio,
but who knows. It might also indicate some other problem, e.g.
the built-in switching supply bleeding noise into the rest of
the radio.

I haven't noticed any other glitches with this rig whatsoever. I
do not have any excessive birdies from the PS. So I'm looking for
ideas/opinions. Ideally, some kind OM with a 736 might be able to
see whether his has that strange signal on the mic input too, in
which case I could rest assured that it isn't unique to my radio.

Thanks a lot in advance for any hints or advice. 73, KC9DUX

--
Eric Behr | NIU Mathematical Sciences | (815) 753 6727
| http://www.math.niu.edu/~behr/ | fax: 753 1112