#1
Just bought a used 3120. The foot switch changes channels properly, but won't turn the effects loop on. I changed the LED but that wasn't the problem. Any ideas?
#2
No experience with that amp but check the easy things. Make sure the cable pins are all good and you connect everything before powering on the amp.

Looks like the switch logic is tube driven from/after the clean channel so maybe try a new/good 12ax7 and roll it through the tube positions and see if it comes alive.

Also check your send and receive aren't all the way off on the back.

Check the footswitch for the loop is actually working with a multimeter.

If no one here can help try the Peavey forum. Maybe this is common?
Guitars:
Jackson Kelly KE3 - MIJ (Distortion/Jazz)
Jackson DKMGT Dinky (EMG 81/85)
ESP E-II Eclipse Custom (JB/'59)
ESP LTD EC-1001FR (EMG 81/60)
Fender MIM Strat

Amps:
Mesa/Boogie Dual Rectifier Roadster 212
Laney IronHeart IRT-Studio
Peavey Vypyr 30
Peavey ReValver Amp Sims
TOOOO many T.C. Electronic Pedals. . .