Behringer Ultrabass BX600 repair - low 60Hz tone

[Bajaguy]

I've been out on the road for the past couple of months, and this last weekend I ran into a guy that gave me a really nice looking Behringer bass amp that needed repair. I ended up in a hotel with WiFi for a few days, and thought I'd try to find some information on this amp. I have looked for the last couple of days for a schematic, but it's proving to be a tough on to find. I'm on the road, but I have the trusty old fluke multimeter and a bit of time so I thought I'd take a crack at this.

I got her opened up and found that it's powered by a LM3886T. I pulled the input from the power Trans off and checked it. It shows as a 28-0-28, and measures at 31-0-31VAC. There's no discernible burn marks or broken components on any of the boards, but when you fire the amp up it sends a medium volume 60Hz tone through the speaker. There's a couple of sets of wires that run from the amplifier/power supply board to the preamp boards, but not having a schematic is a bit tough on figuring this out.

Any chance someone has a schematic for this board or can give me a clue where to look next? I verified that the rectifier diodes are good, and none of the big caps are shorted, but I'm kind of lost from there.

Baja

[Roly]

Behringer circuits are not available on the web.  You could write requesting one but I suspect they will tell you to take it to an authorised repairer (to have your arms ripped off).

It is possible that this is being caused by a capacitor that has aged and lost its capacitance value, effectively gone open rather than shorted.

Disconnect the speaker and measure the DC output voltage with the Gain and Master volume at minimum.

Is the hum influenced by the volume, treble, or bass controls at all?

[Bajaguy]

Disconnect the speaker and measure the DC output voltage with the Gain and Master volume at minimum.

I measure 40.1VDC at the speaker terminals.

Is the hum influenced by the volume, treble, or bass controls at all?

No, volume and tone controls have no effect on the hum.

[Enzo]

DC on the output?  You have a bad 3886.


Make sure both +40 and -40 power supplies are present

[Bajaguy]

Pin 1 - +40.1VDC (checked against ground @ pin 7)

Pin 4 - -40.1VDC (checked against ground @ pin 7)

Looks like just a bad 3886. I'll drop an order in and get one on the way. I'll report back when I get it to make sure that was it. Thanks for the help guys!

Baja

[Bajaguy]

I just got on Mouser and TI (who bought out National in 2013) lists 2 different chips for the 3886T, one in a package that I don't recognize. I went ahead and ordered both, any idea what the difference is? They list a TO-220 11 and a PFM-11.

Baja

[Enzo]

Look at the data sheet.

The LM3886T is the plain old chip with the metal tab up top.   The LM3886TF is the same IC, but the metal tab is enclosed in plastic.  The plain T needs a mica washer and insulating washer on the mounting screw.  The TF can both right to the heat sink with a screw.

Did your old one have a metal tab on an insulated plastic one?

[Roly]

I measure 40.1VDC at the speaker terminals.

Ouch!  Well that is pretty definitive.

Check your speakers for health.

[Bajaguy]

The old chip had the isolation washer and the screw cover, so it sounds like either one will work.

I checked the speaker last night and all is well. Thanks again for all the help!

Baja

[J M Fahey]

Remember that on the plastic encased one you *still* need grease between case and heatsink.

[Bajaguy]

Ok, I got the new chip in this morning just before I had to leave for the job. I had time to solder it in and power it up before I left. I checked the voltage rails and the chip output and all looks good :dbtu:

I'll stuff it back together tonight and try it with the speaker and report back when I've got tunes....

Baja

[Bajaguy]

  Got it all stuffed together last night and it works like a champ! I'm realy suprised at the fantastic tone from this little amp, it is quite amazing considering it's size.
   I just wanted to take a quick second and thank everyone who helped out with the troubleshooting of this. I know it's quite an undertaking to fix an amplifier that we can't find a schematic for, but with all of your help this was a great sucess and I appriciate it greatly!

Baja

[Roly]

 :dbtu:

Actually for the vast majority of gear I've worked on having a circuit was a luxury (and if you did have one it was invariably for a different model or something), thus you learn to "technish".  This is why we tend to be wary of "techs" who say they can't fix something without a circuit, and any sort of amplifier in particular - if you know what you are doing then amps are generally a doddle (as you have just found; lots of volts on the speaker = dead OP stage). 

Thankfully this was pretty straight forward because a having a circuit as common ground when trying to do repairs by remote control is rather more important than when the beast is on the bench under your nose.