Boogie MarkIIB going sour

[zizybalooba]

I bought a 1981 Mesa Boogie MarkIIB last year, a Wood&Wicker Hunree, and I've just gotten around to using it in loud applications. I'm the oddball Boogie owner that uses one for a clean sound, relying on external stuff for distortion. I'm having a problem with the amp now that it's being turned up passed bedroom volume. The amp will produce a nice super clean Fender sound for about an hour then degrades to a soft overdrive that kills all the clarity in the sound. It has been fitted with all new tubes since this problem occurred. Any ideas on what could cause this problem?

[Chuck H]

There are a couple of things I would check. First Q for me would be "What could cause an amp to change tone due to hard use?". Polyester caps can be sensitive to temperature. I don't know which "orange drop" caps MB used, but a swap to 715 or 716 orange drops, which are polypropylene, may help. The second thing would be the heater voltage. I had a Studio Preamp once that sounded pretty good for about an hour. Then not so good. I never did find out why. But I checked and rechecked alot of the circuit. I never changed out the caps. This was before I knew that polypropylene was alot more temperature stable than polyester. But at some point after I got rid of that amp I wondered if the heater voltage may have had something to do with it. My brain cooked up notions of cathode poisoning or too much happening in the space charge, etc. I really thought it had something to do with the tubes because I could swap them out and the amp was fine again...for awhile.

Good luck

Chuck

[Amp Kat]

They use orange drops in those amps but it doesn't mean they still wouldn't benefit from being changed. I would be willing to bet a good Electrolytic cap change would make a major difference in it's performance since they are only 28 years old. If they have been changed before I'd look to things like cold solder joints mainly on the tube sockets or corroded loop jacks or any jacks for that matter and possible corroded or loose tube socket pins. When it drops out try gently wiggling the tubes and see if it comes back. If those are the original filter caps then you would be crazy not to do a total recap on it. If you see a 220uf 350 volt orange cap on the right side on a board with two other caps it is most likely the originals.

[Steve Conner]

You said you fitted all new tubes. Did you actually try reproducing the problem afterwards to see whether the new tubes cured it?

I wouldn't care to try changing all the plastic film capacitors in one of those things. The electrolytics, maybe, but plastic film caps don't wear out. If I remember right, they are PCB amps and working on them is really fiddly.

The wood and wicker Boogie was always my favourite combo amp, as far as looks go anyway. I've never played any of the Mark series and wouldn't know what to expect.

[zizybalooba]

You said you fitted all new tubes. Did you actually try reproducing the problem afterwards to see whether the new tubes cured it?

The problem existed before and after the tube change. Swapping out the tubes was the easiest thing to try first but it didn't yield any cure. I have inspected all the solder joints on the PCB and they all look fine.

It does look like a recapping job is in the cards for now. This has been a difficult problem to diagnose because the change in tone is not that drastic. Just enough to make things like digital reverb and 12 string guitars sound gooey rather than chimey.

[Wally]

I would lean toward the electrolytics caps going weak. Give a listen to the A, 5th fret, 6th string. Is it a bit 'flabby' and weak sounding with someut-fo-tune sub-harmonics? Does the high end lack sparkle? Is there a lack of 'dimension'. Weak caps exhibit these symptoms long before they start humming or fail outright, ime. I would worry about the tone caps, but I do repalce all of the board electrlyitcs and the bias caps when replacing the filter caps. After the electrolytics are freshened, then a good analysis of the amp for minor problems should be done.