T

teaguy

Member
I am attempting to troubleshoot an intermittent transmit issue and I am very confused and in need of a radio wiring expert. For starters the problem usually happens just after run up and awaiting clearance for take off, no kidding, I can communicate no problem taxiing out to the run up area but it quits after I run up and sit waiting for clearance to take off. To be honest my ability to transmit has stopped working just after takeoff but usually just before take off when I try and accept clearance. I have an Icom A200 and a Flightcom 403 intercom. The intercom works flawlessly, also the passenger transmit also is flawless as a matter of fact when the pilot transmit fails I have the pax communicate for me or I plug into the pax jacks and talk my way back to the airport/hangar. Next, the control tower can hear me keying the mic but no audio. So how can the pax transmit work, the intercom work, the PTT open the channel but the audio not work on the radio but on the intercom? Next, I replaced the intercom, no joy. I replaced the mic jack, no joy. I rewired the PTT, no joy. Any guesses which wires I should change next? The audio line to the intercom obviously works as the intercom never fails (I know because this happened with pax and they can hear me), the connection between the intercom and the radio obviously works because the pax transmit works fine....anyone have a guess what can fail to cause this to happen? I invite any suggestions.
Mark Mercier RV-7A C-GQRV
Vancouver, BC
 
Suggestion

Try failure first. Since this is happening at the same time when you are getting ready to take off or just after take off, my money is on the radio. You could transmit on a different frequency so you won't interfer with tower to a friend with another radio and see if you can induce failure with about the same amount of transmit time as you do just prior to takeoff. If so, I think you know where the problem lies.

If you induce failure, see if you can do a radio swap. Next fastest and easiest thing to test. Does anyone have an Icom you can borrow? Instead of spending hours re-wiring, spend 1 minute sliding yours out and another in.

The fact that you lose your xmit audio but intercom works suggests anything intercom to pilot is not the problem. The PTT is activating xmit but audio not coming out but is reaching the intercom (which you replaced) suggests to me it is the radio or the connection from the intercom to the radio (pilot side). Last wires to check are audio from intercom to radio. Check wiring last because it all works before you heat the radio up during transmit.

Otherwise, fly right seat. Good luck.
 
Could it be the gain from the Pilot microphone (through the intercom) is slightly low when it gets to the radio and the squelch feature of the radio cuts it out entirely? Just a wild guess.
 
Mark, I had a similar problem and found the mike jack was grounding. Opened up the jack hole and aded some plastic so the jack would not touch any metal. No problem since. Hope this helps.
 
Antenna Masking?

One last thought is antenna masking. At my home airfield, right on the hold short line for one runway, the tower often cannot hear me transmit on my number one radio. If I move a few feet, change the heading of the airplane, or use the number two radio, they can hear me fine. It's never been an issue at another airport or any other runway at KGPM.

Pat
 
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