Behringer BX1200 - no sound :(

[terrapin]

My Behringer BX1200 Ultrabass quit on me - power lights on but no sound. I tried to use the other inputs in the back to see if I could get any sound at all - but nothing!

I gave it to my father-in-law who is a electronics guy (I am not) and keeps asking me for a schematic - which I understand are only for the highly privileged

So, I'm turning to this board for any help or suggestions on where to start troubleshooting.

Thanks in advance!

[Enzo]

Tell dad to isolate the problem.

Unplug the speaker and connect the amp chassis to some other speaker. SOund come out of that? If so, the amp is working and your speaker is suspect. No sound that way? AMp dead. Plug the speaker cable into some other amp. Speaker working?

test the speaker itself, does it work? Pull the wires off the speaker and measure resistance between the terminals. You should get a few ohms. if it is open, then the speaker is dead. You could also just touch a 9volt battery to the speaker terminals for a second. You will hear a pop if the speaker works.

If both speaker and amp seem to work, remember, the speaker wiring between the two could have failed.

Does sound come out the headphones jack? Plug something into the powr amp in jack - your guitar for instance. Sound come out the speaker? Send the preamp out jack to anopther amp input. Anything coming out the preamp? Play a guitar into the input, does the tuner out jack have signal on it? How about the DI out? The tape out? Does feeding some signal into the tape in make sound? Using all those jacks for their purposes will tell you i0f parts of the amp are working.

Speaker OK? then look in the amp. Are all power sup[plies present? The preamp runs on +15 and -15VDC. Find an op amp IC and check if power is at its corners. The LEDs only tell us that one of them is working, we know nothing about the other. The power amp ICs run on higher voltage. On the power amp, you should be able to find the main filter caps - the largest ones. See if both have voltage across them. I don;t know what that voltage is, 30-40 volts probably. All I care about is if it is ther or not, and both polarity sides about the same. The amp would work with +/-25v or +/-50v. So the voltage is not the issue if the amp is dead, just a lack of it completely.

Got all voltages? Then we start chasing signals.

[jrfrond]

We've serviced countless BX1200's here, so I just looked over our database. Overwhelmingly, this problem was caused by a bad power amp IC, LM3886T, followed by an open speaker. Sometimes both. Like Enzo said, you still need to isolate the problem. If it was on my bench and the speaker tested good (i use a 9V battery as a quick test), then I would change the power amp IC immediately. Why? Because a low-cost part like this that has such a high failure rate is cheaper to replace than performing additional testing. In our line, time is money, and it's a game of percentages.

[terrapin]

Thanks Enzo and John!

You guys are fast! I'll pass those on to the FIL and post back with questions/results. I know the speaker works as I connected it up to another amp -but I'll get him to start from the beginning just to make sure we haven't missed anything obvious.

[terrapin]

Working now - but question

Well it seems my father-in-law fixed it - but he doesn't really know what he did. He took the heat sink from the circuit board and noticed a round green PTC? that he says changes resistance depending on the temperature. He said the leads for this seemed to have been caught under the heat sink - so maybe it was shorting out??

Well he put it back together and it started working. I brought my bass guitar over, and it seemed to be fine. But it leaves us both asking questions: When does the fan come on or is it supposed to be on right away? So far, the fan does not turn, regardless of how long the amp is on.

[dav]

I've got the same bx1200 amp but it makes constant tone/hum and doesnt output any of the guitar signal I put in. The speaker is good, the preamp works when I plug it into another amp, and if I plug in headphones the same tone plays through them.

Is this going to be the same LM3886T IC more than likely?

[Enzo]

Probably.

[zero cool]

When does the fan come on or is it supposed to be on right away? So far, the fan does not turn, regardless of how long the amp is on.

The fan only comes on when the amp warms up. stays off until warm.


Zc

[zero cool]

These amps use two LM3886 amp chips, one or both are probably bad.


Zc

[Bigred69]

JOHN I have the same problem one of the power amp IC's is burnt and when I tried to remove the board I didnt realize the little blue transistor with white wires was glued down and I pulled the wires off and dont know where they go. Can you tell me where to solder the two wire of the transistor to on the board? Thanks bigred

[beasleyiv]

I have the same problem as Dav. I bought a bx1200 refurbished. I pulled the amp board and it looks like it was repaired by a 3 year old. I'm thinking of ordering a new output board, but the pictures I see of the part makes it look like it doesn't come with the ICs. Also, on the existing board it looks like not all the pins of the LM3886T chips are soldered. There are also several wire leads soldered on the bottom of the board. (see pics) What I'm wondering is what is the original board look like? Are all the pins of the LM3886T supposed to be soldered? Are there supposed to be external wires soldered to points on the bottom of the board?