Mesa Mark III

[AtomicMassUnit]

Hey guys. I know some of you work on a lot of Boogies, so I was wondering if there are any common problems with the Mark III amps I should look out for. I have one that the bias circuit is all burnt up, the guy I got it from said a bad filter cap caused it. It's an '89 model. I'm just going to change the filter caps and redo the bias supply, but I definitely want to verify the cause of the problem if I can. Is there something common with these amps that makes the bias section burn up? I did repair another one of these that had the same problem, but that was caused by the aluminum shielding from inside the cab flaking off and shorting out something, taking out the bias supply. Anyway, tell me some Mark III stories!

[Amp Kat]

I just did one and the coupling caps were bad causing one of the El-34's in Class A mode (which it's really not) to go cherry. I changed the wrong coupling cap and it solved the problem but the bias at idle was jumpy. After changing the other coupler it was rock solid. The bias on the EL-34's and 6l6's are different with a dropping resistor from each side of the EL-34 to 6L6 and each side has a wire from the supply on the power board to the socket board attached so each side can be a tad off from the other side. It's not adjustable so if you want to raise it or lower it you'll have to change the resistor on the terminal strip to the octal tube socket. Nice sounding amps and quite loud with good cleans.

[Enzo]

Any amp can fail, especially when it comes to filter caps.

As it turns out, the MArk 2 I just serviced had a shorted coupling cap in the preamp somewhere making noise and putting 100VDC on the master volume control. But I would say it is a coincidence that KB and I ran into the same problem. No reason to think the orange drops in the MEsa amps are any less reliable than they are elsewhere.