peter(at)sportingaero.com
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 Posted: Mon Jun 07, 2010 9:57 am Post subject: Strange radio transmit problem - any suggestions? |
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Guys,
Thanks for your suggestions regarding this problem.
It turned out the radio was transmitting carrier wave through the frequency range, but only transmitting voice below about 120MHz, and above 130MHz. When no voice was going out there was no side tone either. Becker said if its transmitting on any frequency then there's nothing wrong with the radio as there is only one transmitter, etc.
I believe I have solved it by replacing the antenna feeder. I'm not sure if the problem was a degraded piece of coax or a poor termination. The previous feeder was a piece of RG58 of unknown pedigree and is now a piece of RG400 that happened to be in my spares box and was 6" too long  (Makes a change - everything is usually 6" too short). Crimp on terminations from Stein using Bob's method note.
I still don't really understand what was happening - I guess the radio was smart enough to only provide a side tone when it was actually radiating speech on the carrier wave, and it realised that no speech was going out on the frequencies in question.
Peter
On 12/05/2010 21:17, Peter Pengilly wrote: [quote] Hi,
I have a Becker AR4201 radio in my One Design that I have just started flying (2 hours on the tacho). Initially the radio worked great (after I got the mike jack wiring correct - there's always one bozo moment in any project). Halfway through the 2nd flight the transmit quit - receive was still good. Plugging in another headset on the ground and the transmit worked OK.
A couple of days later I pushed the aeroplane out for another flight and no transmit at all. Carrier wave is being transmitted but no modulation (and so side tone in my headset). After trying to figure it out for an hour I put it back together and went flying, still with carrier wave only. After a while I called a local tower (119.0) - loud and clear. Returned to my field (124.1) to the same problem - carrier wave only. What is going on!!! 
I have not been able to test further - but I need to step through the frequency range and find out which freqs work and which do not. This is a very simple airplane with no intercom, headset connected directly to the radio. Receive is good all the time, on all freqs tried so far. Radio shows the transmit arrow whenever the PTT is pushed, and puts out carrier wave on 124.1. Why is it modulating on some freqs and not others? Headset works in other aircraft (have tried 3 so far with same results).
Does anyone have any idea about what is going on?
Any suggestions gratefully received.
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