On the DX 77HML:
The S-meter seems normal. The RX speaker volume is loud when you first turn the radio on after leaving it unplugged for a while. After a few seconds, the RX volume slowly drops to the point where you have to turn the volume control wide open just to hear anything. Once you unplug the radio and let it sit for a while, the RX volume will go back to normal levels for a few seconds. Talkback volume seems normal.
Other symptoms (may or may not be related);
1. NB seems to have no effect.
2. Squelch works, but does not completely cut the sound off, you can hear some faint audio still coming through. This may be because the volume is turned all the way up.
3. When you put the radio in PA mode, the S-meter jumps to about 3/4 of the way, and holds there.
4. There is a low pitched squeal on USB TX, but not on LSB TX. This happens even with the mic gain turned all the way off.
I've replaced D1,D2,D23,D24 and TR17 with parts from a junk radio that used the same board. No effect.
I'm going to start checking voltages on all the TRs and ICs and post results.
Also, C190 and C193 appear to be swollen, but I haven't replaced them yet. They appear to be part of the SSB circuit, and may be causing #4 above, but I wouldn't think they would affect AM. I am going to change them out and see what happens.