Collins VHF 251 not transmitting?

RyanB

RyanB

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Having an issue that I can't seem to figure out. There's a Collins VHF 251 (an oldie I know) but it's been solid as a rock. In the past month or two, it's began developing an issue where it won't let you broadcast over it. The 'XMIT" comes on when I press the PTT, but it won't transmit when I speak into the mic. If I press on the radio a little and monkey with it a time or two, it will usually come alive and work as typical. It was sent off to an avionics guy at annual a week ago, and he claimed that a couple of transistors were going bad after he bench tested it. Supposedly he had the parts and fixed it, but alas, it's still having the same issue now. Any idea what's going on? I'm planning to have my IA send it back off to the avionics guy to 'fix' again.
 
Those are great radios though heavy and long.
I may have a spare if it comes to replacement.
 
RyanB said:
Man if you do, we’d be interested!
Click to expand...

I just looked. I do have one. I think it works but it's been a while and I don't have a means to test it.
There are a bunch on ebay too. We're in east TN but I'll likely be going to Gatlinburg sometime the first week of April. If you decide you want it make an offer. I don't have any paperwork on it.IMG20230422072427.jpg IMG20230422072434.jpg IMG20230422072442.jpg IMG20230422072457.jpg
 
Collins made first class equipment. If the radio starts working after you twiddle the knobs, it might be more of a mechanical issue. Caig deoxit is excellent for connectors and switches. Make sure the ground connection is good. Some shops connected the ground wire to the tray instead of to the airframe..the mounting screws carried the ground connection which becomes a poor connection over time. The transmitter requires more current than the receiver. Hope this helps...good luck.
 
RyanB said:
If I press on the radio a little and monkey with it a time or two, it will usually come alive and work as typical
Click to expand...
Given it works after you mess with the radio check the rack backplate for security and double check the pin fitment is still good. Common problem on some older radios.
 
Bell206 said:
Given it works after you mess with the radio check the rack backplate for security and double check the pin fitment is still good. Common problem on some older radios.
Click to expand...
Thanks! Oddly enough, it worked just fine tonight for the whole flight, up until I was on the downwind leg, then it decided to stop letting me broadcast. Really strange problem, the XMIT comes up, but I couldn’t broadcast my voice. Any idea what would suddenly cause it to act up like that?
 
I would suspect an intermittent short on pin 5 or 6 would be consistent with your issue.

Also an intermittent short on your antenna cable could do it.

vhf-251 pinout.png
 
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RyanB said:
Any idea what would suddenly cause it to act up like that?
Click to expand...
Did you try pushing the radio into the rack again? If your backplate is loose or there is slop in the pins the vibration of the flight could cause it to suddenly disconnect. Since the radio does work I'd start at the rack and work from there.
 
Intermittent failures are always a pain to troubleshoot, at least for me...
I'm not convinced the radio itself has been ruled out as the cause. As such, I would think sliding another radio in the tray to be the simplest means of isolating the failure to the plane vs the radio.
 
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