Behringer Ultracoustic ACX 1000 has stopped working

[nufctoonarmy]

Both channels were working, then while a microphone was plugged in and being used, the amp stopped working. Both channels no longer work.
Now when you switch it on there's a loud humming noise.
The power lamp no longer illuminates but the led display for the effects comes on.
The VU meter needle sits over to the right when switched on.
Does anyone have any ideas what the problem might be or have a schematic for this amplifier?
Any advice would be greatly appreciated.

[Mick Bailey]

I've not come across a schematic for this amp, though there are some basic checks to narrow down the problem with any amp. The first thing though is to establish whether you're confident to dismantle and work on an amp and whether you have a DMM. In the meantime, don't power up the amp as this could cause more damage if there's any DC offset on either or both of the speaker outputs.

[nufctoonarmy]

Thanks for your reply Mick,
Yes the amp is currently dismantled and yes I have a DMM. I too have struggled to find a schematic and as I have very limited knowledge would welcome any suggestions from anyone.

[sugeda]

...

[Mick Bailey]

Disconnect the speakers and check for any DC on the speaker outputs - there may be a few 10s of mV, but anything much greater points to a more serious problem. Make sure that you don't short the speaker leads together. If those reading are fine, leave the speakers disconnected for now and measure the amp's DC voltages.

I think the amp uses a pair of LM3886 chips as power amplifiers, so the power amp voltage may be +/- 28v or +/-35v. There will also be something like +/-15v to power the preamp ICs etc. I would start at the power supply itself - does your amp have a toroidal transformer? If so it's a linear power supply and more straightforward to work on than a switch-mode PSU. The power amp voltages can be checked at the chip pins, but this is hazardous due to the proximity of pins and a short there could cause plenty of damage. Instead I would take a look where the main filter caps are located and pinpoint where it's convenient to measure the voltage. The same with the +/- 15v supplies. You may see a pair of voltage regulators bolted to a heatsink. as a clue. You could measure the voltages at one of the preamp ICs, but being Behringer these are maybe SMD and not so easy unless you have a steady hand and slim, sharp probes.

At this stage my suspicion is that you have one of the supply voltages missing.

Post some pictures of the PSU and rest of the amp if you can - it may help in narrowing things down.

EDIT: A big thanks to sugeda for the schematics. Preamp voltage is +/-17v and can be checked at x12. Main supply rails to the power amps can be checked between ground and one end of R5 and R6. Looking at the schematic and I see the display comes off a 5v regulator connected to the +17v side, so that's why it's working. The power on LED comes off the -17v supply (v-). I think that's where the problem lies.

[nufctoonarmy]

Thanks Sugeda and Mick for your help so far, it's much appreciated. I will update you with my progress.

[nufctoonarmy]

For information my amp appears to be the version listed as REVF on the schematics kindly supplied by Sugeda
The amp does have a toroidal transformer.
A friend of mine removed C3,C5,C7 and C11 I believe and tested them using a DMM and they appeared to be ok with no visible signs of leakage.
It does appear to have two LM3886 chips.
I hope this helps. I will attempt to upload a photo but have been unsuccessful to date.

[Mick Bailey]

You can't test a capacitor with a DMM - it will read the value, but that doesn't tell you whether the cap is good. Check the power supply voltages.

[nufctoonarmy]

Ok thanks Mick.....will do.

[nufctoonarmy]

Sorry for the delay, I have been away for over a week.
C3, C5, C7, C11 have now been refitted.
At R5 and R6 we are getting +38v and -38v At IC1 pin 3 a constant 15.5v
At IC2 pin 3 it started at -19v and climbed to -31v before it was switched off.
I hope this makes sense.

[The Dude]

If you have more than -17V on IC pin 3, most likely- either the regulator is shorted, or you've lost your ground reference on pin 1. Check solder connections, check regulator, check D6.

[Mick Bailey]

Looks like IC2 isn't regulating. The excessive negative voltage may have caused damage to some of the ICs it supplies, but you won't know until you replace IC2 to reinstate the correct voltage.

Edit: Take The Dude's advice first!

[nufctoonarmy]

Thanks to both The Dude and Mick for your advice, I will follow it and let you know how I get on!

[nufctoonarmy]

I replaced IC2 and the amplifier is now working!
Many thanks to Sugeda for the schematics and to The Dude and Mick Bailey for your advice.....much appreciated.

[Mick Bailey]

That's a nice happy ending - thanks for the update.