Triple X channel lights/switching not working, no sound

[king_tuck]

Hi all, first post here though a longtime peavey user. I just acquired an older Triple X head in a trade the other day, and unfortunately am already having some issues. First was that the mains fuse was missing, simple fix. After working fine for a day or so, the clean channel volume dropped immensely but only after switching channels, seemed odd but I figured I'd look into it in a couple days and just play around with that awesome crunch channel in the meantime. Went to play it yesterday, and now the real issue starts: amp powers on no issue, but all three channel lights are dark and switching makes no difference and no sound other than a low level hum is coming through regardless of cable or guitar used. Here's the rundown of my troubleshooting so far:
- No loud pops, noises, etc before issue started
- Cleaned FX loop and input jacks, turned up and down volumes and send/return levels, no difference
- Plugged into normal input and effects loop, no sound from either
- Master volume control CAN change hum volume
- Reseated tubes, all look to be powering up as they should, except the 12ax7 directly in front of the leftmost power tube if you're facing the back of the amp, though it might just be a bit dim, doesn't seem like it would affect channel switching though
- tested with a different tube, same dimness in this socket, may have to look into this once the channel issue is resolved
- Cleaned front panel channel switch using non-residue contact cleaner, not using a footswitch as I don't have one for the amp yet
- Tested switch, continuity tests showed some weirdness, so I resoldered the switch and it seems to be making connections as it should
- Resoldered mains fuse holder, as it felt loose and the connections had continuity issues when tested
- Tested all internal and the new external fuse just to be sure, all passed continuity test without issue, all look to be the correct type/amperage for each spot

After all this, no change in nature of the issue. To reiterate, the amp turns on (jewel light lights up), the channel lights do not turn on and channels do not change, no sound except for a low level hum that can be turned down with the master volume.

I've scoured the forums and internet in general looking for answers or stories of similar issues, but have not been able to find much of anything, any and all help or suggestions would be appreciated. I know my way around a soldering iron and electrical, but amp circuits have never really been my forte, I'm fully aware of the dangers involved in working on amps and I am taking the proper precautions, for anyone concerned.

[JamesPaul]

Usually no sound when plugging into the effects return indicates power amp section issues. Any sound from the effects send? How about the line out?

You might quickly check the +17V supply (junction of R93 & C61) as it is prevalent in the effects loop and channel switching.

For anything more than that, I would need to spend some time reviewing the schematic in detail.