I recently went though two issues with a MK12D and were resolved by
Narco. The first one was that I would also loose sidetone and not be
transmitting. We could never find the problem in the avionics shop on
the bench. It would not happen all the time. To comfirm that this
problem was in the radio and not elsewhere in the system, I swapped the
two MK12Ds in the radio stack and the problem followed the radio. It was
a specific radio problem. I sent the radio to Narco with my
troubleshooting and they found several issues with the old radio, one of
which could cause the problems I was having. I got the radio back and no
further "no sidetone" problems. However, I started having another
problem with the receive audio, where it would just drop out. I got in
the habit of pulling the volume knob out to be sure I still had audio.
And several times I would not. I contacted Narco and sent it back under
the repair wannenty. This just happened to be a defective volume
control. I got the radio back and now I have not had any problems in the
20 hours I have on it. Narco did a good job. Now to send in the other
radio for some needed R&R: bad display and frequecy not always indexing
when turning the knob.

Regards, Ross
C-172F 180HP
KSWI


three-eight-hotel wrote:
I had posted an issue I was having, regarding my Narco radios, back
before Osh and got some valuable feedback:

http://groups.google.com/group/rec.a...09e45844bccf31


What I was lacking was some decent troubleshooting! This weekend, I
took my brother-in-law up in the plane and handed him a handheld, and
we tried several different things... I'd like to throw them out to see
if this narrows down the possibilities to those of you with far more
expertise in avionics/electronics than I!

1. We departed our local airport, both of us plugged into the intercom
jacks and I made a radio call (and heard others making calls on the
same frequency). I had side-tone in my transmissions and everything
seemed fine

2. 5 minutes after take-off, I noticed a constant scratchy reception
(continuous static, with something resembling radio calls through the
fuzz)... I tuned to a local ATC frequency and switched over, noticing
no difference in static.

3. I had my passenger plug into the same ATC frequency and indicate
whether or not he was hearing any transmissions between ATC and other
aircraft, as I was hearing static... He was in fact listenting to coms
between ATC and other aircraft (loud and clear)

4. 5 or so minutes after that, there was silence on my radio! I tuned
to the local frequency and tried to make a "radio-check" call, and
there was no side-tone. From my point of view, plugged into the
intercom, I had a completely dead radio (complete silence with no
side-tone on transmission)

5. I had my passenger tune to the same local frequency (on the
hand-held), and performed another "radio-check" call. The passenger
heard me loud and clear, yet I had no side-tone and still had silence
on the radio.

6. I then tuned to the local ATC frequency and had the passenger tune
to the same frequency. I unplugged my headsets from the intercom jacks
and plugged them straight into the aircraft jacks. I had the passenger
give me an indication if he was hearing coms between ATC and other
aircraft. He was hearing coms, yet I was still experiencing silence.

7. I then plugged back into the intercom jacks and made a "radio-check"
call to ATC. My passenger heard my call on the ATC frequency and also
heard ATC respond to me... "LOUD AND CLEAR"!!! ??? I still had
silence, and no side-tone on transmission.

So it appears as though I am transmitting fine (without side-tone
though), but I am receiving nothing. Sometimes everything will work
normally for 30-45 minutes before these symptoms start to appear. The
last few threads of the original post were leading me to believe that
this could be a COAX issue between the radio and the antenna... Does
this still seem like a possibility?

The radio has been run up on a bench for 4-5 hours and supposedly,
everything is within specifications. (I also threw a working radio in
my plane and was able to encounter a failure with it). The antenna is
brand new and was replaced during this troubleshooting process.

Does anybody have an idea of what the issue is? I would really like to
have it narrowed down to one or two possibilities before I go back to
the mechanic and start taking things apart. My semi-shotgun approach
has already cost me a fair amount of money and I am no better off today
than I was when we started this process.

Thanks for listening and thanks in advance for any input!

Best Regards,
Todd