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Cobra 25LTD - No Modulation but PA works
- Thread starter Mikey D.
- Start date
First try tightening the board mounting screws. Then you may have a bad solder joint.
Hi!
Thanks for re-checking My work - never know these days...
When you are able, this thing (radio) when you take the covers off - you get modulation back?
Ok, that means the foil traces, you have got a cold or broken solder joint.
Ok, there may be hope for it.
When you lose modulation - does power drop as well?
C94 was the one I had questions about, but when you said that the PA worked - it meant that the MIC amp circuit works in this case - so C94 would not have been the problem.
Ok, you did a little more research - and yes, it looks like something in the limiter section has failed. The best case scenario is to remove the Limiter transistor (TR14) - if all modulation is restored, then the limiter circuit was the problem - only now we have to look for the bad part.
But also - you have a two fold problem - you may have inadvertently located the audio cut-out problem too. Both TR14, TR13 (Limiter and Mic Amp - Respectively) and the Audio Amp are in this same area.
I would reheat, add and reflow and soldering joints especially around the Audio Amp chip, in an effort to re-establish the audio modulation and if this works - it's ok.! Meaning, the Limiter may be ok - it (TR 14 Limiter) doesn't fail normally, but the Audio amp - being where it is - along a side panel and oftentimes Torqued with knobs to mount it firmly - does distort the cases' side mounting rails and hence - pull the audio chip away and off of the soldering pads used to connect it to the radio...it does happen - a lot in mountings like in Trucks and Semis.
Do the above, reheat stuff and check for audio - if it returns you're ok, if not - remove TR14 and if the audio returns - and PA works without a hitch - then you need to make a choice.
Either you; continue to repair the limiter section, or choose to run the radio with you riding the mic gain controls all the time while you talk, or do you want the limiter to help you keep the radios' TX audio down to a level that others can understand you - a power mike can easily sound distorted if you run one without the limiter - and you'll have the "Test" the mic gain and adjust the power mike level so the background audio in the vehicle doesn't climb in with your voice - and cover you up.
:+> Andy <+:
Thanks for re-checking My work - never know these days...
When you are able, this thing (radio) when you take the covers off - you get modulation back?
Ok, that means the foil traces, you have got a cold or broken solder joint.
Ok, there may be hope for it.
When you lose modulation - does power drop as well?
C94 was the one I had questions about, but when you said that the PA worked - it meant that the MIC amp circuit works in this case - so C94 would not have been the problem.
Ok, you did a little more research - and yes, it looks like something in the limiter section has failed. The best case scenario is to remove the Limiter transistor (TR14) - if all modulation is restored, then the limiter circuit was the problem - only now we have to look for the bad part.
But also - you have a two fold problem - you may have inadvertently located the audio cut-out problem too. Both TR14, TR13 (Limiter and Mic Amp - Respectively) and the Audio Amp are in this same area.
I would reheat, add and reflow and soldering joints especially around the Audio Amp chip, in an effort to re-establish the audio modulation and if this works - it's ok.! Meaning, the Limiter may be ok - it (TR 14 Limiter) doesn't fail normally, but the Audio amp - being where it is - along a side panel and oftentimes Torqued with knobs to mount it firmly - does distort the cases' side mounting rails and hence - pull the audio chip away and off of the soldering pads used to connect it to the radio...it does happen - a lot in mountings like in Trucks and Semis.
Do the above, reheat stuff and check for audio - if it returns you're ok, if not - remove TR14 and if the audio returns - and PA works without a hitch - then you need to make a choice.
Either you; continue to repair the limiter section, or choose to run the radio with you riding the mic gain controls all the time while you talk, or do you want the limiter to help you keep the radios' TX audio down to a level that others can understand you - a power mike can easily sound distorted if you run one without the limiter - and you'll have the "Test" the mic gain and adjust the power mike level so the background audio in the vehicle doesn't climb in with your voice - and cover you up.
:+> Andy <+:
Thanks everyone. I will check the board more carefully this time on the solder side, also plan on paying more attention to the audio amp joints. I'm gonna check the screws that attach the board to the chassis and re-tighten if loose and add solder to them as well. Also, the points where the disc caps attach from the chassis to the can components will be gone over for 'cold' joints as well.
Mike
Mike
MikeD, seeing as how your radio went out after being installed in a vehicle, i would look at how it was mounted for clues as to what might have been stretched or pulled, or shaken loose.
if you used a mounting bracket that was not properly sized for this radio, then you probably pulled the sides of the radio away from the PC board when you tightened the screw.
you may also find that there are bundles of wire attached to the sides of the radio with one end going to the PC board. look for any of these wires that could stretch tight if the sides of the radio were pulled away.
just some places to start looking.
LC
if you used a mounting bracket that was not properly sized for this radio, then you probably pulled the sides of the radio away from the PC board when you tightened the screw.
you may also find that there are bundles of wire attached to the sides of the radio with one end going to the PC board. look for any of these wires that could stretch tight if the sides of the radio were pulled away.
just some places to start looking.
LC