Mesa Mark IV suddenly cutting out.

[Les Paul]

So last night I was playing a gig and my Mesa Mark IV suddenly cut-out mid-song. The blue power light was still on but the yellow light over standby was off. Fuse looked okay. All tubes were glowing, apparently normal. This morning I plugged it in to troubleshoot and it seems to be fine. But I worry about a repeat in future. Any guesses out there as to WTH happened?

[Chuck H]

Intermittent problems that are limited to the symptom of "amp cuts out" are impossible to diagnose at face value. The only thing to do is find a way to induce the problem so that more clues can be gathered. Like is it just the guitar you can't hear but the amp is still making white background noise. Do any of the amp controls affect the background noise and then further along, are there any voltage changes analogous with the problem. Any flashing in tubes, funny smells, etc. You get the idea.

All I can offer at this point is that amps, especially gigging amps, often suffer intermittent problems due to oxide, dirt or some other contaminant on unsoldered contact point in the signal path. This would include jacks (especially effects loop jacks) switches, pots, ribbon connectors and such. Try this:

Turn the amp on and get it into play mode. Now womp your fist down upon it and see if it makes a crackle, crack or short buzz type noise. This can be a good way to identify an intermittent connection. If that happens try cleaning every non soldered contact. There are special cleaner/lubricant products for potentiometers. There are often switch contacts on jacks that cannot be cleaned without removing the amp from the chassis. If you do decide to clean all the non fixed contacts, put the amp back together and get it back into play mode and whomp it again to see if your efforts improved the intermittent contact.

[Leo_Gnardo]

Along with everything else, your yellow standby/operate pilot is back on now? If that's the case then for a while you had no hi voltage, that would explain no audio. I'd suspect the standby toggle switch. I've had to replace a few in Mesa & Fender amps, a cheap and fast enough thing to do. Of course since it's an intermittent problem the only way to prove the solution is that it doesn't happen again.

Of course if you have the amp out of its box, have a look around, especially in the hi voltage department, you may find something's come loose.

[Chuck H]

I thought about that, but I'm not familiar with the amps. I didn't know if the switch was illuminated one color and then another or if it was only on when it was flipped, etc. I went with the first go to for audible drop out. But good call on not ignoring it as I did. Yepper. They do go bad. Very few Fender products (I think the actual number is none) made back in the day had a proper voltage rating on the standby switches. The Mesa Mark amps were some of the earlier designs fashioned after their original modded Fenders, so...

[Les Paul]

Thanks guys - Chuck for the tips on the tubes. Leo - Your comment that the standby light/switch might have something to do with insufficient voltage is interesting. I didn't realize there was a connection with that particular switch. Good clue. I'll try to follow it down. I was wondering if there was some kind of circuit in the Mesa that would, you know, sorta' shut things down, if you got over/under voltaged? 😎