My C+ is powering itself off

[zebpedersen]

I was playing last night and after about an hour, my C+ powered itself off. No pops / bangs / extra noise / smells / smoke / drama, just a fade out in volume (same as the sound of the caps draining when you switch to standby while playing). I've only ever had a single tube go bad in the past (an EL84 on my Subway Rocket) and the amp stayed on, it just sounded like garbage all of a sudden and one of the output tubes was glowing red hot; an easy diagnosis.

I'm going to do a bit more diagnosis later this evening but would welcome any and all thoughts the wise denizens of the Boogie Board have to offer.

Observations:

• First thing I checked was the fuse, which is both intact and the correct rating

• I left the amp overnight and powered up this morning, and the same thing happened (this time after only about 4-5 mins).

• The first time it happened I was playing the lead channel, the second time on the clean channel (with a TS808)

• The rest of the rig stayed powered just fine both times

Pertinent rig details

• It's a US voltage amp, I'm running it in the UK with a 110V step down (I have used this for 5+ years without issue)

• The step-down is the same as this one https://www.tlc-direct.co.uk/Products/SC5476.html, 500VA / 5A @ 110V rated

• Amp is DRG spec

• I have recently (in the last 6 weeks) added a UA OX Box attenuator to the rig; I run the master volume on 5 in simul-class mode

• I have recently (in the last 6 weeks) added a Furman PL8-CE power conditioner to the rig; the step-down, attenuator, and pedal board PSU are each connected to separate outlets

Things I'm going to try later [suggestions very very welcome]:

• Power up and play again (for as long as possible) and watch the tubes for any inconsistency

• Do the 'tap test' on each tube while powered up

• Remove the attenuator from the situation and play (necessarily) at lower volume through the speaker direct

• Attach a second device to the step-down to see if it's tripping something in the transformer

• Remove the power conditioner from the rig

[/list]

 

[woodbutcher65]

Powering off? Without blowing fuses....hmm.

Is the indicator light staying on when that happens?

Is the effects loop being used at the time? (Signal path issue involving the loop)

Has it ever been recapped?

Have you verified that you have no problems with the speaker jacks?

If a coupling capacitor shorts, depending on where it is in the circuit, it can mute the amp. If the cap is intermittently shorting, and in the right place in the circuit, it could explain what's happening.

And it could be a connection in the power supply that's loose and loses contact but comes back into contact after the amp cools or you bump it.

Check wires and connections, check solder, check for clean tube sockets, clean every socket, jack, and pot with Deoxit or other high quality electronics dleaners. Start with what's easy to do first. If that doesn't take care of it, then it's probably a capacitor related problem.

 

[zebpedersen]

@woodbutcher65
Yup, totally powering off, not just muting or speaker disconnect (it's as if you unplugged it from the mains – tubes power down, signal dies, power light off…). The amp is just sat in one corner of my flat and never moves so I'd be surprised if it's managed to work any connections loose, but nothing's impossible.

I did a bit of further testing today:

• The amp powered down this time after around 25 minutes

• I had the amp turned round to watch the tubes, absolutely zero drama, just faded out with the signal as if the power had been pulled

• I did a tap test on each tube (which are all seated well); no microphonics or noise, even on a high gain setting and the master north of 5

• I had an iPhone plugged into the second outlet on the step-down and it stayed powered on the whole time

• This time, I didn't power the whole rig down, just turned the amp switches to the off position for a minute. After one minute, I switched back on and the amp came back to life, although I didn't do a play test afterwards.

Next things I'm going to test (please do keep suggestions & thoughts coming!)

• Try just wiggling or knocking the amp when it powers off again

• Try wiggling / knocking the power switch when it powers off again

• Power back up after a power-down and see if the amp plays as normal

• Look in the service history for the last re-cap and re-tube

 

[zebpedersen]

Another thing I'll be trying is class-A operation rather than Simul to see if that makes any difference.

 

[xdg999]

I just bought a Mark IIc combo for a great price because it wouldn't power up. Turns out the on/off toggle switch is bad and works intermittently. Got a new one on the way.

 

[woodbutcher65]

I've seen those power switches fail before. Sounds like that's what's happening. Get a proper replacement from Mesa. That way you can be sure you get exactly the right one. But it's made by Carling. Short bat SPST type.

 

[zebpedersen]

A small update on this – the behaviour has now completely gone…after a 36hr period of being easily reproducible. Aside from playing on the class-A setting for a while (what a huge difference, and not in a good way) before switching back to simul-class, I haven't changed anything about the rig or used it in any different way.

For now, a mystery. Thanks for the suggestions and help.