Hello 9c1Driver,
The AM regulator is not relevant when in the PA mode. If the PA mode is functioning properly, that will establish that your microphone, mic. amp section and audio output IC is working. Yours does not, so I would be leaning more towards your mic. amp circuitry and switching. It is highly unlikely that the audio output IC is causing the problem because you said you have receive audio that is normal. One unanswered question that is important to the situation at hand is the history of how this problem came to be. Was the radio working properly and just quit, or have you obtained the radio in this condition. I have had radios brought in where someone had tried doing a "Hi-Fi" mod or direct inject mod, that they did not perform successfully, which resulted in your exact condition. One missing SMD component or damaged PCB trace can cause this. It is never fun trying to figure out what someone else did wrong on a radio like this. Signal tracing on a multilayered PCB is not a job for the weak at heart and requires a higher level of technical ability and equipment to get the job done. I don't have a service manual on the Lincoln II +, but I do for the Lincoln II, which I am attaching here for you. I don't know how much it differs from your actual radio, but it has PCB diagrams that you can compare to your radio. They may be very similar. Without having your radio in front of me, I can't say. Someone on here may know and can chime in. Before tracing the microphone signal path, I would suggest checking the ribbon cable and make sure it is seated properly with the pins lined up correctly. I have also seen this scenario when a mic. gain control was bad or totally oxidized. If you can get at the control to clean it, you might get lucky. If you have a signal tracer, that would be good place to start checking for mic. audio. You will find the mic. audio circuit on page 25 of the attached manual. The major players there are U500 and U501, which are NJM4558M op amps. I have had to replace these before in radios, but I don't find them to fail very often. One quick check you can do, is to leave the radio on for a few minutes and take your finger and touch each one briefly to see if they are abnormally hot. If so, it is probably bad. If you got this radio in this condition, I would also suggest looking in areas where people would be inclined to do modifications to the radio, for evidence of soldering having been done.
