Mark IV, weird fx loop issue

[ochrisl]

Mark IVB. Works great. Once it get warmed up maybe 15 minutes the loop cuts out, no sound at all. If I disengage the loop via the switch, sound passes and it sounds normal, just no loop. Let it cool off completely, it works fine for another 15 minutes than does the same thing.Changed all tubes, no difference. Sprayed deoxit in the loop jacks and run plug in and out like 20 times, seemed to fix it. Put amp away for a few weeks. Come back and it's doing it again. Anybody have this before?

 

[swbo101]

Never had that issue, but you can look up which pre-amp tubes power the FX loop and try replacing them.
That would be my next test.
Good luck.

 

[user 21540]

After locating the preamps tube(s) that drive the loop (looks to be the b side of V2) and testing with a known good tube I would also try using the Deoxit cleaner you have on the preamp tube pins and then run the tube in and out of the slots several times. Worth a go.
https://mesa-boogie.imgix.net/media/User Manuals/Mark 4.pdf