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FDP Forum / Amp Mods, Repairs, and Projects / Mesa-Boogie amp/electrical gurus - help needed!

Surfinboy

USA

Practice!
Sep 22nd, 2016 10:27 PM   Edit   Profile   Print Topic   Search Topic

My Mesa-Boogie 5:25 Express Plus has been shutting down in the middle of playing, just like it was turned off almost instantly and the lights on the amp dim until they're both totally out and it goes completely quiet. It usually coincides with a sound that's similar to when another appliance on the same circuit kicks on. The first time it happened was when our dishwasher (same circuit) kicks into another cycle and there's a small brief pop/spark noise coming through the amp at the same time. I immediately checked the fuse which appeared fine - I put the same fuse back in, turned the amp on and off a couple times and then it came back on with no problem. It ran problem-free for a few days in my home studio, even after leaving it on and letting it run for an hour or so. Then it happened again during soundcheck at a gig, same small popping noise then shutdown and it didn't turn on again after a couple tries. That was a week ago and I haven't tried it since.

One difference after the first time it happened: there's always been a mechanical/light switching sound that happens when I shut if off, but after the first incident there was a little popping noise when I turned it off, similar to the sound I always heard when switching off my 2-channel Peavey Delta Blues amp I once had.

This is puzzling - something is failing intermittently, and I'd think if a fuse, tube or recifier tube had gone bad, it would never have turned on again after the first incident. Thoughts/advice?

pdf64

UK

Sep 23rd, 2016 08:02 AM   Edit   Profile   Print Topic   Search Topic

Fuses that appear fine may be blown. Basic DMM, to confirm the status of a fuse, are really cheap.
If the fuse really is ok and the tubes go cold when the fault exhibits then there is almost certainly a problem with the mains supply to the primary circuit of the power transformer.
So it's likely either a problem fuseholder, switch, wiring / connectors, PT itself.
That should be fairly simple to identify and fix.
Consider contacting Mesa as they should know their products / failure modes best, and should be able to advise of a suitable tech to resolves the issue, as necessary.

External link

rfrakes331K
Contributing Member
******

IL USA

RonHalen Jokingly He Says
Sep 25th, 2016 07:52 PM   Edit   Profile   Print Topic   Search Topic

I would also recommend Mesa. I accidentally got 2 mismatched EL84s in my LSS and it was going into runaway, popping the fuse and toasting the Rec tube. They were awesome in the help they gave. Also, the warranty lasts 5 years. If you have a service center near, take it to them.

(This message was last edited by rfrakes331K at 09:58 PM, Sep 25th, 2016)

Surfinboy

USA

Practice!
Sep 26th, 2016 09:10 AM   Edit   Profile   Print Topic   Search Topic

Thanks folks. I've talked to 2 local amp techs on the phone and they're stumped. Uh oh, better get Mesa!

Surfinboy

USA

Practice!
Sep 27th, 2016 10:31 AM   Edit   Profile   Print Topic   Search Topic

I talked to Mesa and they're sending me to a local authorized service place. They think it's a low-voltage power problem, either caused by intermittent surges (i.e. the dishwasher or something else on the same circuit as the amp at the gig) or a problem with the actual voltage supply. Since it's run fine on my home circuit for a couple years, I'd be hard-pressed to think there's an identical problem on both that circuit and the one at the venue that would cause the amp to cut out.

Peegoo
Contributing Member
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Eat. Sleep. Guitar.

Repeat
Sep 27th, 2016 10:45 AM   Edit   Profile   Print Topic   Search Topic

The "switching" sound at cutoff, when the amp is warm, tells me it's a heat-induced fault. Could be in the PT.

If that were my amp, I'd replicate the failure, and then blast the PT with insta-cool aerosol to see if the power kicks back on.

rfrakes331K
Contributing Member
******

IL USA

RonHalen Jokingly He Says
Sep 29th, 2016 03:40 PM   Edit   Profile   Print Topic   Search Topic

Freeze mist!

Surfinboy

USA

Practice!
Oct 3rd, 2016 07:47 AM   Edit   Profile   Print Topic   Search Topic

Tech figured it out: the fuse harness was loose, which would explain why it came back on after I re-inserted the same fuse. He replaced it with a sturdier one. Easy fix - I lucked out!

rfrakes331K
Contributing Member
******

IL USA

RonHalen Jokingly He Says
Oct 3rd, 2016 09:22 AM   Edit   Profile   Print Topic   Search Topic

This is Great Surfinboy! The Boogie lives on!

picnic
Contributing Member
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NJ/FLA

I like guitars and amps
Oct 3rd, 2016 09:26 AM   Edit   Profile   Print Topic   Search Topic

Good news! I had rectifier tube blow a fuse but no other damage.

I've had a bad EL84 go and take a grid resistor with it. Warranty was over and a local tech fixed it and did the MB recommended updates. 5 LSS's over 20 years and I can't complain.

FDP Forum / Amp Mods, Repairs, and Projects / Mesa-Boogie amp/electrical gurus - help needed!




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