COMS dont transmit after flying in rain

[wburger1]

Interesting problem.  I have a Narco a Mark 12D and just a digital Narco COM.  After flying through a rain both radios will not transmit.  Just get a static noise.  I can hear controllers OK though. Tried both mic inputs, both transmit buttons and two different head sets.  I do not see any rain dripping under the panel.  I siliconed the outside and still get the same result.  So I land and after about an hour they both work again.  Any comments?  I am thinking it is my old audio panel since it is on top and maybe getting a couple of drops on it.  By the time I take out all those mooney screws to take a look there either was no water of it has all gone.  Have had two different avionics guys look at it and they just covered up the top of the avionics and that did not work

[jetdriven]

If those avionics panels aren't sealed down and screwed down wet they will leak. They really are a bad idea, no other airplane leaks water onto radios, yet owners are reluctant to Elininate them because they have to use them to access their water damaged radios. Truly a self-eating watermelon. 

Aborher possibility is water under the antennas. Screw then down with PRC type sealer under them and they will stay dry underneath. 

[wburger1]

Thank you for the suggestions. I will try them this weekend during the Harvey Storm here in Houston.

[jamesm]

Another possibility  is the either a antenna(s) that started go  bad or/and  a bad antenna  ground plane.  On my Dad's first Mooney ('63C)   which had  one  Narco  com Radio. The strangest thing would  happen my Dad's  partner flew plane no problems  the radio  worked fine but when my Dad flew the plane  com radio would be intermittent  with the com transmissions. How they  discovered the problem is that they both went flying one day that is when the antenna  failed hard.  The antenna look fine  from the outside it was when they removed the antenna  that  they discovered a corroded antenna  ground plane.   They had the  com antenna tested  and found  signal strength  was questionable and had the antenna replaced. 

[takair]

Does TX light up on the radio (I think it has one) indicating PTT is working? Can you hear it trying to transmit in a handheld?  Do you get side tone?  Could be PTT line, mic modulation, or transmit. Narrowing it down to which area is failed will help zoom in on fix.   With all due respect to Byron, I personally prefer to keep the antenna ground plain clean (metal to metal) and seal around the edges or use conductive gasket.  The problem with sealing under the antenna is that you now rely on the screws and nuts or nut plates for a ground plain connection.  Will typically work for some time, but I find this method degrades faster.  Definitely need to keep water out. A variation may be to just PRC around the perimeter.  

[wburger1]

Yes the T light comes on but it flickers and does not stay solid until after the rain stops and the COMS start working again.  Same thing happens on both transmit button, both headsets and with the intercom turned on and off.

[takair]

20 minutes ago, wburger1 said:

Yes the T light comes on but it flickers and does not stay solid until after the rain stops and the COMS start working again.  Same thing happens on both transmit button, both headsets and with the intercom turned on and off.

That seems to indicate a PTT circuit problem.  You could be losing the ground or it could simply be corrosion on the pins in the audio panel compounded by water.  Would pull audio panel and check pins on panel and in rack.

[jetdriven]

Let me rephrase and clarify my recommendation. Clean the aircraft and the antenna to bright metal on the bosses where the screws go through, for the ground plane. Use stainless screws with star washers for good bonding.  All this is from Comant.

Only use the PRC around the outer perimeter of the antenna, ~1/8" or so. Just to keep water out. 

[Piloto]

The radios are probably getting water through the external cooling fan intake. Check the intake has a screen to limit water in.

José

[Mooneymite]

Just don't fly in the rain!  

[N601RX]

The older planes use a hose off the passenger side fresh air vent down by their right knee to cool the avionics. There is a lever under the panel to turn it off if flying in the rain.  Older stuff may overheat with it turned off.