Help following SX-828 preamp schematic - troubleshooting dropped channel

[MBuras]

Backstory. Fully rebuilt this SX-828 about 3 years ago. Been fine until recently. Started dropping the left channel. Usually within the first 5 minutes of power on and doesn't matter phono, aux, etc.

-injected sound directly into amp section and both channels were fine for 30 minutes. Eliminated the amp section.

-took apart and cleaned the high and low filter switches as there was some spark of life when I manipulated them. Seemed to work for a little while but now back to same old problem.

-Focused on preamp board. Transistors were all replaced during rebuild but checked anyway. Fine.

-2SK30's were swapped between channels just to be sure they weren't an issue. Still dropped on left side.

-I'm now injecting a 1kHz signal into the AUX and probing it with my BK1477 scope (that I'm not an expert with). I can follow the waveform through the balance and volume controls. R3 and C1 have waveform as well. And the last one I get is going into the loudness switch. Couldn't quite figure out the schematic from there but skipped over to R9, R11, Base of Q3. Nothing but a tiny ripple.

My current theory is that I should have signal coming out of the loudness switch even with it disengaged, which means maybe the switch is dirty.

Any suggestions? Am I on the right track?

 

[Lowetlx]

Did you try to reflow connections on the top and bottom of the AF board?

 

[MBuras]

I’ve reflowed the entire board and used a magnifier to check the topside. Nothing off that I can see. Might have to do continuity checks on each trace just to make sure everything is connected properly.

Thinking out loud. Can’t help but wonder why it takes a song or two until it cuts out. Must be something heating up. If not a transistor then it has to be a solder connection. Hmmm....

 

[Lowetlx]

This isn’t the proper way but I had a similar problem with that board. I tipped the unit up on its side and played some music. I tapped the af board with the plastic handle of a screw driver to help narrow down where on the board I had a poor connection. I found that the holes in the board that have no parts had to be soldered on both sides. Maybe that will help you if not hope someone with more experience using the scope can help you

 

[MBuras]

Thanks for the tip @Lowetlx Definitely gives me something to check that I hadn’t done before.

 

[MBuras]

Just to close this out...I hope. Been playing fine for 30 minutes despite vigorous poking with a stick. Haha!! Who woulda thought low tech wins the day?

Last night I tried playing music with the bottom cover off and it wouldn't cut out. Go figure. But sure enough after pushing on the preamp board with a wooden chopstick it cut out when I reached the longer section behind the bass/treble controls. I thoroughly reflowed all the bottom connections in that section of the board and then went over the top as well. Put the board back in position and we're good so far.