Ok, so we replaced...
Audio Chip,
Audio Output cap from audio chip (C57 - c550?),
Input cap C150,
Changed C70,
Receive Good,
Squelch Ok,
PA audio low...
Ok, then we also need to look as less probable, but still likely - that even the Modulation Transformer could have been damaged and has several shorted winds - caused by arcing or heat failure from that "short"...
So tomorrow, look at voltages to the Final and Driver - should be close to supply when unkeyed, but drop about 2 to as much as 3 volts when the radio keys up - that's the power draw thru the winds of the transformer pulling down the voltage. That is a normal result - it takes power so it draws it thru those winds to make the carrier and audio together. IF that doesn't occur or it's a lot less power draw - even a current (Ammeter on the DVM can help here) meter to see if the radio draws current like another radio you know that works. ..
IF the Pre-driver didn't work, I'm looking for ways that it can still provide power to the DRIVER to even make it key up...
Else even the Mic amp - and right now, I'm kinda leaning on that section as possible - for you say you have a great RX - if there were problems on the Modulation Transformer, it would affect the QUALITY of audio for both RX and TX - we only have weak TX - it may even mean we have a weak driver too. Again back to the DVM to probe for voltages...
But this also goes back to the SQUELCH and TR17 - TR17 has to hold up and supply 8 volts to the Pre-driver to make it work. If the regulator diode went bad, that TX voltage would be way way low - like below 6 volts - and that usually indicates a failed regulator or it's support parts - you mentioned TR17 earlier and you replaced or checked parts in it.
Well, the radio still has an RX/TX light - so look for voltages going whacky between keyup and unkeyed. Even if the "red" is actually "Red" and green is actually "green" not yellow...
I would like to think this is a simple fix but you may never know - Captain Obvious may be watching over us right now - laughing...taunting us...
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