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Strange transmit problem - any ideas?

P

penguin

Well Known Member
Patron
Hi, I have a Becker AR4201 radio in my airplane and have just started flying (2 hours on the tacho, experimental but not an RV). Initially the radio worked great (after I got the mike jack wiring correct). 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 airplane 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!!!:confused:

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. 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.

Pete
 
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Might have nothing to do with it...............but initially I was having periodic transmitting problems. Turns out part of my antenna cable's outer shield was shorting with the center antenna wire on the BNC connector. I found the problem with a continuity check after pulling the radio from the tray. Note: was a different brand radio.

L.Adamson --- RV6A
 
Hey Peter

penguin said:
Initially the radio worked great (after I got the mike jack wiring correct). Halfway through the 2nd flight the transmit quit - receive was still good.
Click to expand...

Mike jack wiring still sounds suspect since you have a carrier but can't talk, maybe loose ground or something. Might be worth a recheck.

How have you been? How much is fuel over there?
 
Check the BNC connector. The wire in this cable must be long enough to go into connector. I had similar problem with Garmin radio. Found antenna cable had short wire protusion on both ends. Cutting cable and extending the wire solved problem. Has work well for 100s of hours. :D
 
Mic Line

If you're certain that the headset is not at fault, I would next carefully check the mic line from the jack to the radio. Is there a bad connection? Plug the mic and wiggle it while looking at the jack. Does the jack touch ground when pushed a certain way? I suspect that it is a wiring problem rather than a radio problem. Because you have carrier, I would not suspect it is frequency related, the antenna or the coax at this point.

Hope this helps
 
Push on the..

fmarino1976 said:
If you're certain that the headset is not at fault, I would next carefully check the mic line from the jack to the radio. Is there a bad connection? Plug the mic and wiggle it while looking at the jack. Does the jack touch ground when pushed a certain way? I suspect that it is a wiring problem rather than a radio problem. Because you have carrier, I would not suspect it is frequency related, the antenna or the coax at this point.

Hope this helps
Click to expand...

....pins/sockets on all of the connections in any D-Sub or Molex type connectors in the mic to radio circuit.

As stated above, sounds like a loose connection, and I've seen several cases were pins were not snapped into their connector shells, and pushed out when the connectors halves were mated.
 
Thanks for all of your suggestions. More testing has revealed that the TX works from 118MHz to (about) 122 MHz and from (about) 130 MHz to 136 MHz. In fact I was talking to ATC from at least 20nm at the weekend on 119.0. Becker report that there is only one modulator, so if it works on some frequencies, then it is unlikely the radio is at fault. That leaves the antenna or the coax. The antenna has been there for several years and worked with the previous Terra comm radio, so it may be the coax is the culprit. I will swap that over at the weekend and re-test.

Pete
 
penguin said:
Thanks for all of your suggestions. More testing has revealed that the TX works from 118MHz to (about) 122 MHz and from (about) 130 MHz to 136 MHz. In fact I was talking to ATC from at least 20nm at the weekend on 119.0. Becker report that there is only one modulator, so if it works on some frequencies, then it is unlikely the radio is at fault. That leaves the antenna or the coax. The antenna has been there for several years and worked with the previous Terra comm radio, so it may be the coax is the culprit. I will swap that over at the weekend and re-test.

Pete
Click to expand...

I swapped the coax this weekend and it seems to work :). Not sure if it was the coax or terminations - probably the latter according to 'lectric Bob - and probably a poor shield connection. Amazing symptoms from a basic fault.

Pete
 
Strange stuff

Sometimes Xmit problems can be traced to RF energy getting into the mike circuit and garbling or wiping out modulation at certain frequencies. This can take place from a high SWR transmission line due to connection problems, antenna currents on the coax shield from having the coax in the antenna pattern, an unshielded mike lead, or having the mike lead bundled tightly with the output radio coax. Unless you use a high quality, low-loss coax such as Andrew FSJ1-50 with a solid outer shield, common coax, such as RG-50U, has some leakage through the shield which can get into other low level signals, causing some meters on the panel to jump when transmitting. This is especially common in non-metallic aircraft construction.
 
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