Mesa Express 5:25 amp, warms up, then volume fluctuates

[leadfingers]

Folks,

A guy I jam with has a Mesa Express 5:25 amp. He bought it used from another person that took meticulous care of his equipment. I don't think this thing ever left the house. Anyway, it has been working fine the last few times he used it during our jams. Never got very loud. Used probably once every 5 weeks. So, it's about 4 years old? (longer, maybe since the first owner preferred new gear to used) This guy has had it maybe 2 months. He was trying to play something through it yesterday and now informs us of his problem. He turns it on, waits the usual time, (No idea exactly how long but when we are all together, he turns it on and we start playing within minutes.) and it sounds fine, then it starts fluctuating.

I have NOT seen it yet, so I have no first hand info or experience with it.
(we are planning on jamming this Sunday, so he was hoping for any info)
I've looked for Schematics, to try to get an idea of the circuit, but no luck in that.

Any idea where to start?
Power section, pre-amp, power supply?

As usual, thanks in advance

Andy

[sumgai]

Andy,


My first place to look would be the power supply filter caps. Even if they're not decades old, they can still become leaky, and exhibit the described symptoms. Simply hold a DMM's lead to the main B+ point (before any choke, if one is used), and look for the readings to be unsteady as it idles along. If they are steady, then use an alligator clip to secure the probe in place, and play something through the amp, while observing the meter. At non-stage volumes, the meter should dip and climb a little bit, like 5% of the idle reading. If it's much more than this, then the filter caps are likely bad.

One way to double-check is simply temporarily connect another cap across the suspected bad one. (Do this one at a time if you wish, but I prefer to "double up" on all of the filter caps at once.) If the readings get much more steady, then you've found the problem, and should be able to take it from here.

Second, and easier to do, but doesn't actually cause the problem nearly so often, is to simply replace the tubes. I can't be sure of your particular model if any one pre-amp tube is a "favorite" to go out, so again like with the caps, simply shot-gun the whole set (of pre-amp) tubes, and see what happens. If the problem is solved, then go back and do a substitution one-by-one until the culprit is sorted out.

Third, power tubes are the least likely to be the cause, but we'll have to eliminate them all the same, just to be thorough.  The same method applies - substitute with a known good component, that's the name of the game.

If all of that fails, then start looking at solder joints. I once had a Fender solid-state Bassman come in with an intermittent crackling sound, really loud. The customer demonstrated by playing it at stage volume, and it sure did crackle. But interestingly enough, when I moved it from the floor to my bench, it made the same noise. Hmmm. A few applications of percussive maintenance later, I found the filter caps had effectively come unsoldered from the printed circuit board. It looked good, but (with an orange stick!), I could move the caps, and receive a nice reward in the ears for my efforts. Two minutes with a soldering iron, and all was copacetic in Owner-ville.

After all of that, we'll need the exact schematic to start delving into other possibilities. But my money is on something simple, even if it's costly in terms of new tubes.

HTH




sumgai

[leadfingers]

Sumgai,

Thank you sir, for the quick response. Hopefully, he can get it here before Sunday when we use it. I'll update as to what I find.
Thanks Again!

[leadfingers]

Folks,

Well, after having it on the bench for a couple of hours, here is what happened.
Plugged it in, started it up. After about 3 minutes, it made a crackling noise, volume dropped about half for not more than a couple of seconds. Before I could even position myself behind it to see if there was anything going on by watching the tubes, it returned to normal. Played for another half hour, nothing out of the ordinary.

Powered down. I unplugged each tube and looked for any discoloring or questionable marks on the pins, or if they were loose. Nothing.

Next morning I plugged in a signal from my computer from Spotify, continuous music, at a relatively high level. For over two hours, I was working on a couple of other items and had this going the whole time. Not once did it misbehave.
I returned it to the owner that Sunday and after church he plugged it in and after about 2 minutes it started acting up. This time, however I was behind it already when it started. I did see the preamp tubes changing brightness as he played. And it dod NOT seem to correspond to the loudness of his input. Once, it did return to normal, briefly, and all the tubes returned to one steady brightness, then just as fast , it lost power again and the tubes started changing brightness again.
He is taking it to the local certified Mesa repair center and let them handle it.

The pcb is so loaded with components, it is almost impossible to follow any path and try to find the power legs. And no schematics makes it mission impossible.

Thanks for all the guidance even though I withdrew on this one. Your help and info is always appreciated.

Andy