Magneto/comm interference on TKM MX-R Narco 120 replacement

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

Problem Solved!

I tried to isolate the problem by disconnecting the magneto p-leads, then
the alternator. After disconnecting, I would fire up the engine to check the
radio and it still would be very garbled at low engine r.p.m. and then clear
itself up at higher r.p.m.

While sitting in the cockpit, I noticed that my antenna, which is connected
to my new TKM MX11 radio,was vibrating back and forth. As I increased
R.P.M's, the antenna would stop vibrating and the garbled sound at low
r.p.m. disappeared. My antenna is a stainless steel whip model that sits in
front of the windshield on the cowling. See attached link to view:
http://home.cogeco.ca/~pl2/p3.html

I quickly switched to my other comm antenna and now have no problems.
There's probably a ground fault at the antenna base - I haven't had a chance
to sort it out. I don't know why this didn't manifest itself with the old
Narco 120.

BTW, the TKM MX-11 radio works great. As a slide in replacement, it's
easier than removing the old tray and installing a new tray for a ICOM A200
or a Bendix/King KY76A. It should be noted however that, barring the
antenna problem, the MX-11 needed some work before it would work. It seems
that if you run your radio through an intercom, which I do, you have to add
a capacitor to the radio's computer board, which I did. This took a little
less that an hour, but after I did this the radio worked right away.
However having said that, apparently, if you run the radio through an audio
panel, you don't have to make this tiny modification. Strange. In my
opinion, I still saved hours of grief with this installation versus a new
tray, etc.

Blue Skies

Eugene Wendland