Behringer BX1200 power stage burnout

[stevekendal]

Just got a nice little second hand Behringer BX1200 bass combo and after just one quiet session it developed a serious hum after I plugged in the footswitch and then no sound at all, with the pong of something burned out. Nothing looks melted inside and I have the schematic, although I am much more comfortable with valve stuff, so can anyone help me out please? There are two 3886 chips , should I just replace those and see how I get on? Steve

[Mooly]

Use a bulb tester (100watt filament bulb in series with mains) for testing.

Probably one of the IC's and they are cheap enough to just replace both (make sure you get the right package for eg LM3886TF which is a plastic insulated package).

Always nice to be sure though. If you have the circuit then check the all the rails and do a quick voltage check on the output pin of the LM3886 as thats a good clue as to IC failure.

[stevekendal]

One 3886 shows 42 volts on pins 1 and 5, zero on pin 3 output? Other shows 42 on pin 1, zero on pin 5 and 42 on pin 3 output.

[Mooly]

Sounds like the one with 42 volts on pin 3 is duff.

What voltage is on pin 4 ? That's the "minus" rail and should be a mirror of the positive one at minus 42 volts.

[JMFahey]

Agree and add: Pin 5 is directly connected to Pin 1, so "having 0 Volts" there can only come from some catastrophic failure, such as a blown track or something.
Check that.

[stevekendal]

Well, I got a pair of chips now, but they are the insulated type and the ones in the amp arent. Also, I rechecked the voltage readings and I must have touched the wrong pins on the second chip, because they are all ok now. Question, is, can the chips be faulty even though the voltages look ok and will these insulated chips do to replace the metal ones? Steve.

[Mooly]

The insulated package are fine. Use the bulb tester again though when powering up. In your first post you mention something having burned, and the LM3886 is favourite. Voltages can sometimes read more or less OK and the chip be faulty.

[stevekendal]

Well I got a pair of power chips but, with so little spare time, I passed it on to another repairer, for him to fit. He just reported back that there are several tracks on the pcb burned through, which is borne out by the pong it made when it died. Says its a considerable job making this good. How likely is it that other components have caused this? Might just get it back and fix it at my leisure.

[Mooly]

An LM3886 amp hasn't got many parts and shouldn't hold too many fears for an experieced tech.

Maybe post a picture of the damage to the PCB.

[stevekendal]

Well, I just got the power module back off the other guy and he's made a lovely job of fitting the new chips, but said it still wasnt working and had him baffled. I plugged it back in and hey presto, we're back to the deafening hum, but with no signal output, and no frying either. Filter cap perhaps?