Jackson JG-2 50 Watt - Intermittent Problem

[PepeSilvia]

Hey guys, got an 80s Jackson JG-2. Been trying to find a schematic for her but have come up empty so far. The problem I'm running into is this: sometimes it'll sound great, sometime it sounds like someone put an open wah in front of it, absolutely no low end at all and really lacking in volume. I've tried swapping out preamp and power amp tubes. If I plug into the effects loop return, I get a nice full tone, so I'm guessing the problem is in the preamp somewhere. Any of you experts have further troubleshooting advice?

Thanks!

[Enzo]

Yes, next time it happens,plug a spare cord from FX send to FX return. ANy help? If that restores the sound, then the return jack needs service or replacement.