This is a long-shot, but easy to try...
A "loose" connection. Maybe in the tray connector or
headphone/speaker wiring or somewhere in the antenna coax run. New
radio may may not be mating completely with tray connector.
With engine off (and radio on of course), tune in a ATIS or AWOS or
whatever you can find that is transmitting. Then, shake, pound,
wiggle the tray, antenna, coax, all wiring (including
headphone/speaker wiring).

Sid Knox
Velocity N199RS
Starduster N666SK
KR2 N24TC
W7QJQ


"Eugene Wendland" wrote in message ...
I just installed a Michel TKM MX 11r which is a direct slide-in replacement
for my Narco 120 Comm. Because I have an intercom, I had to add a resistor
to the circuit board. Having done this, I installed the radio in the old
Narco tray and tested the radio:

Findings:

1/ Comm works great with just power on.
2/ With engine (lycoming O-320 with new mag leads) on at low r.p.m.s, the
received audio has interference (very garbled).
3/ Once the r.p.m's increase to about 1500 and beyond, the recieved
transmissions seem great.

Because TKM is closed, I haven't called the tech rep about this, but could
anyone tell me what's happening? Everything else about the Comm seems to be
working well. My transmissions to the tower are 5 squared. My Narco didn't
do this, so I'm assuming there's something wrong with the radio.

Thanks

Eugene Wendland