well that's a completely different story!
saying that the mic TX pin is shorted to the chassis is completely different from saying that it is shorted to PC board means something very different.
of course it is shorted to PC board ground, because the radio is in TX mode.
put the mic jack and squelch knob back in place, and move your attention back to the MB3756 voltage regulator.
i know you have checked it before, but just for fun, lets do it again.
radio on, no mic connected, radio is in TX mode right?
measure the voltages on pins 1-8 of the MB3756 and post them here.
you should measure 8 volts DC on pin 6 if the radio is in receive mode.
you should measure 8 volts DC on pin 8 if the radio is in transmit mode.
pin 5 is the pin that switches that voltage from pin 6 to pin 8.
my guess is that you will find 8 volts on pin 8, seeing as how the radio is stuck in TX mode.
if that's the case, unsolder pin 6 and pin 8 from the PC board and bend them up so that they are not making contact with the board.
now check the voltages again.
you should see 8 volts on pin 6 and when you key the mic, you should see that voltage move to pin 8.
please post the results of your experiment.
LC
