Mark IV Power Issues?

I have a Mark IV Widebody Combo amp that I bought used about 3 years ago. It's worked great up until last week...during the first set of a gig it would be fine for a while, and then all sound would cut out and the channel light, EQ light, and FX loop light would dim/shut off for a few seconds, and then the sound would come back. It would make a loud popping sound when the sound came back. The blue master power light always remains on. It was almost like someone pulled the plug and plugged it back in. I was hoping it was just bad power at the venue, but it happened again the next night at a different place and is now happening at home. It seems to randomly happen. Any ideas? I've never changed the power or preamp tubes so maybe it's as easy as that...

I've managed to catch it on video as well: https://streamable.com/8jfbv

Any ideas are appreciated!!

Interesting video and issue. I have the same amp and the issue I had was almost identical except the channels would randomly switch.

I'm guessing of course, but maybe there is a connection between power section that temporarily cuts after the amp gets to a certain temp? The heat could be causing the connection to separate.

I would pull the plug, pull the power tubes and clean the tube sockets with contact cleaner. If you have a set of new tubes, I would replace/swap out pairs and see if the problem continues.

*Actually, I would try this first:*
Cleaning the connector where the footswitch connects to the amp. Then check the connector that attaches to your FU-3. The outside collar can come loose, and you might need to tighten with your hand a little. The wires kept pulling/pushing on the circuit board inside the footswitch which was causing the random channel anomalies. Once I tightened the outside collar, the problems stopped.

It seemed that when I would put the FU-3 on the floor, the loose connector would push against it a certain way and cause shorting. For me it was a matter of tightening the collar.

Recap it. I'll bet that the power supply capacitors are original which means that by now they're definitely ready to be replaced.

When the caps fail, power supplies don't work right. That means NOTHING works right that's fed by that power supply.


All tube amp owners need to KNOW this: Recap your amp every 10 years, or 15 at most. Beyond that point you're gambling with even more expensive repairs and more damage done if the caps should actually completely fail.

I would take the amp to a tech right away. There is a separate low voltage power supply for the switching matrix which has voltage regulators that can overheat and fail. If this is the case the longer you let it go you can get heat damage to the circuit board which is very difficult to repair. Also check that your ft.sw. cable is not cut or loose in the xlr plug. Try unplugging the foot switch and set the mode selector to a channel and see if the problem keeps happening.

Boogertone said:

There is a separate low voltage power supply for the switching matrix which has voltage regulators that can overheat and fail. If this is the case the longer you let it go you can get heat damage to the circuit board which is very difficult to repair.

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+1

I had a similar problem years ago.