I had an old black face President Jackson given to me. The radio powers up and receives fine, but appears to have no transmit audio. This radio has been tinkered with. A previous owner has opened it up and added a knob on the back of the radio to adjust output power. On AM and FM modes I do see decent output power on my watt meter, but absolutely no transmit audio is going out. I have ruled out the microphone by trying a known good mic with the same results. Any ideas on what to look for?