Mark IV problem - Where is this buzz coming from?

[ketiov]

Turned on my Mark IV combo today. It's from '91. It started buzzing as if it weren't grounded right. I checked the cables, guitar, even unplugged everything and left just the amp and speaker going. Did this on every channel.

Everything was the same as the last time I played it, so I don't know where this could have come from. The only thing that has changed is that I opened a window...

Did some switching of power options. Put it in Class A, and it buzzed less audibly, and only on the clean channel. Put it on tweed, same thing; buzz is only noticeable on the clean channel.

Switched it back to SimulClass and Tweed/Full power, and again, buzzing only occurs on the clean channel.

The tubes are all less than 4 months old, and haven't been pushed at all. I let them warm up four or five minutes before playing every time.

Turning gain knobs doesn't do anything to change the buzz. Neither do the treble and other knobs.

Nothing in the effects loop. It is bypassed and empty.

There are no nearby electronics. Moving the amp a few feet had no results whatsoever.

Am I right in assuming this could be a preamp tube? Since the problem seems to now be isolated to the clean channel, I mean.

Thanks for any help guys. I'm really concerned about this...

 

[MesaDCLP]

Well looking at the tube functions in the preamp ... It may be V2 that is bad, but that should be noticable on R2 and Lead. Swap with a spare to see if that helps.

Try an old set of power tubes just to be sure the problem isn't there.

The amp is old enough to have a cap job, so if it has never been done, that may be the problem. :idea:

 

[ketiov]

Well looking at the tube functions in the preamp ... It may be V2 that is bad, but that should be noticable on R2 and Lead. Swap with a spare to see if that helps.

Try an old set of power tubes just to be sure the problem isn't there.

The amp is old enough to have a cap job, so if it has never been done, that may be the problem. :idea:

Did the tube swaps, no change.

Does anyone know how much it costs to send one of these in for a total overhaul at Mesa, and expected turnaround time? I don't know anything about where the amp has been before January of this year, as I bought it off of eBay and the guy there didn't really know his stuff very well.

 

[MrMarkIII]

Throwing out some ideas:
Didja try flipping the ground switch?
It's got three positions.
Checked the footswitch plug/jack?
You may want to take the amp to a different location to see if it's the outlet it's plugged into.

 

[ketiov]

Throwing out some ideas:
Didja try flipping the ground switch?
It's got three positions.
Checked the footswitch plug/jack?
You may want to take the amp to a different location to see if it's the outlet it's plugged into.

I've tried plugging into various outlets in the house, but to no avail. Most of the rooms have their own separate circuit.

Tried the ground switch, also nothing.

I'll check the footswitch connection, that's the one thing I haven't done yet.

Thanks for the ideas! It doesn't seem like many people truly know or bother to even say what's on the top of their head.

 

[MrMarkIII]

I always say what's on top of my head.
It's usually the only thing there.