This is why I mentioned the Remove TR19 part, a floating voltage indicates the transistor may be bad - and the line is active all the time.
But too, the VR4 Carrier Balance also feeds a error correction offset voltage to Pin 1 - and TR19 is supposed to keep that voltage at ZERO when in AM - your offset is a different beast in AM mode so VR4 is not functional in AM mode - only in SSB modes.
It (TR or Q 19) turns off the AN612 Audio side (Pin 1) in AM mode - so the AN612 Pin 1 should (it needs to) go to 0. Base of TR19 goes active and shunts all the audio off of AN612 so the AM regulator and its' amp can pick it up.
But, in SSB mode, TR 19 goes off AND only in the SSB modes. That means you should see a voltage on that pin 1. (Remember Carrier Balance VR4) But in AM? AN612 - Pin 1 should be 0 volts. TR19 stays on - all the time, TX or RX, it's pulled high by the MODE switch - you just said the voltage goes up - did you replace the cap feeding Pin 1? C104 a 563 disc cap, by Pin 1 of the AN612. look back at my previous post from here, that long leg lead labeled Audio in the photo - look that up - see if audio can return. (long shot don't expect much)
It doesn't mean the AM regulator is perfect either, you need to look for any voltage on the line that goes back to the AM regulator. There is a cap back there, C166 - it is by the 8 Volt regulator in the photo I posted - that green mylar Chiclet cap 0.022uf or 223. One side will have the mic amp power, the other side has the AM power trimpot - that feeds R185 back at the AM regulator.
- VR10 the AM PWR pot, can fail - part of the substrate can go open - especially if the user wanted to get more power out of it and tried changing it out and trying a different part. When that happens 8 volts is sent all the time - only it's impedance is affected - good luck eh? So you can lose audio because the AM PWR and MIC sides are even more unequal. Too much voltage from one side swaps out the signal the other side of the cap is trying to send.
- C166 will have a LOWER voltage on the MIC amp side than on the AM PWR side - and the AM PWR side would rise and fall as you adjusted the AM POWER pot.
If it's not sending a voltage up to that cap (like it's Zero) or negative, that is different than the Mic amp side, then we have a bad foil lead somewhere - AM mode has an 8 Volt line - it has to turn off the AN612 in AM mode else you be sending out FM too. May have a broken foil trace in there.
This may mean TR 19 might be bad, or the line that feeds TR19 - is not working (its' open no voltage from AM mode gets to it)
Only way to know this is to remove TR19 - if audio returns check/replace TR 19 - Test that transistor, see if the transistor has shorted out - it's possible that the base voltage is flowing backwards into the Collector - when this happens the line to the AN612 gets saturated and there's nowhere to "dump" the voltage so it just sits on the line, pinches off audio - this also affects the Carrier Balance control VR4.
OR the AM mode switch is also damaged by that impact, and you'll need to go back to the Front panel and look at those switches all over again. TR 19 needs to be ON in AM mode, but OFF in SSB mode - so it'll need a voltage from the MODE switch AM on the front panel.
