Another Hot Rod Deluxe channel switching issue

[Randall]

90's American HRD in for the complaint of after being on for a hour or so, it will sometimes switch to drive mode and light the yellow LED with no footswitch plugged in. I haven't done anything to it yet except inspect the usual suspects. Nothing seems damaged or loose. The area around the cement resistors is a little discolored, but tugging on them has no effect. So far it has not exhibited the problem.

I wonder, could a dirty switch jack cause this? I can make it switch by probing the jack, so I have cleaned it, as well as the push switch. It seems to switch just fine when operating the push switch or foot switch. I must admit, I have always struggled with this circuit.

https://www.thetubestore.com/lib/the...-Schematic.pdf

[Enzo]

Randall, some day you will palm your forehead and this circuit will fall into place for you, and it will no longer be a mystery. It actually is not all that obscure.

A dirty panel switch could do this. You want to know if the FS jack can do it? Poke an unwired plug into the jack to lift the contact. What happens?

[Randall]

Plugging a dummy plug in the FS jack makes the More Drive red LED come on actually, not the yellow drive LED. So far it's been on for two hours and has been flawless.

[Enzo]

So opening the jack contact causes something different from your problem symptom. From that I assume the jack is not the problem. Leaves potentially the switch. Also, possible one of the op amps warms up and gets wooly. I might make a chart of TP voltages when the amp is cold, then after running a couple hours, remeasure to see if any of them are climbing (or sinking).

[Randall]

I did spray the switch, and after 4 hours it is still flawless. I will try the cold/hot voltage readings.

[Enzo]

Cool. If they are fairly stable, great. If they start to drift, it MIGHT be a clue. Hell, you may have already fixed it.