I have a high VHF MT500 that has basically no transmit deviation. I don’t have a service monitor for it so I’ve been testing by just transmitting and talking into it with a nearby receiver. There is basically no deviation unless I turn it up all the way on R211/R212 (the only thing to do with transmit deviation I could find in the service manual) and even then it is barely audible. The carrier it puts out is nice and clean, and it also receives fine. I have swapped out the microphone in the radio to no avail. Are there other adjustments? I don’t know if the PL deviates/transmits properly, this is a DPL model it seems like and I don’t know the code. I suspect that there’s a dead microphone preamp somewhere, any tips on how to replace that and where it is? I may seem dumb, this is my first time with a crystal controlled former part 90 HT. Thanks.
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MT500 no transmit audio
- Thread starter knockoffham
- Start date
Is it possible that it could be the mic element have you tried a speaker mic?
Then again, the age of that radio it could be a capacitor.
Then again, the age of that radio it could be a capacitor.
I will try a speaker mic. I have tried front cases/mic elements from 2 radios.
Speaker mic works fine. So I might have 2 bad mic elements? Does the speaker mic follow the same path electrically as the internal mic element or are there some other components along the way that could be giving me trouble?
The audio amp circuit should be the same for both internal and speaker microphone.
Thanks. I guess I’ve got 2 bad mic elements then.
It's been a very long time since I have worked on one, but I recall the side block had both mechanical switches for both RF and audio for use in the ConvertaCom. You might want to consider that as well being the speaker mic worked but the internal mic doesn't.
Ok. I will check that out. The internal mic does work, but at a basically 0% audio level. The only way I can hear anything is tapping on it. Thanks!
What you hear when you tap on it can be microphonics and not actual audio being picked up by the microphone. Hope you figure it out!
Ok. I was thinking it could also be moving the antenna a little bit by tapping it. Thanks.
Check the audio out of both the stock mic and speaker mic with an oscilloscope to see if the levels are similar. If so start tracing with the oscilloscope and a radio manual which should be online somewhere.
Ok. Will do.
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