Behringer Ultrabass BX4500H wanted

[J M Fahey]

there seems to be no centre tap to the high power winding on the transformer, so cannot figure out how the voltage rails are maintained equally about 0v

Ok, thanks for checking.
In that case, the centrepoint of the main caps floats and becomes the "speaker out" rail.
And then, *how* does it stay there?
The amp itself, works as a DC servo, and keeps the centerpoint where it should ... if it's working properly.
In this case, it slams the speaker out hard against a rail.
First suspects are output transistors, but if not, check drivers and so on, going backwards, at the input there should be an Op Amp which might be bad, or lacking one or both +/-15V rails .
There may be other problems, but these are usually the most common.
Take a long hard look at this QSC schematic, the basic principle of operation is similar to what you have there.
http://www.qscaudio.com/support/libr...ies/USA400.pdf

[knutknut]

Hi JM,

Thanks for the suggested schematic. I looked at others on their website, and amazing, the ISA280 is even more similar to what I have. I have now flogged through the board and mapped out the whole schematic.
There are a few minor differences and a single error around IC1. I have now fixed the amp. Turned out to be short circuit C20.

For anyone interested, here are the schematics

P3000 power.pdf
P3000 amp.pdf

[TieRBuoY]

If you give us specific complaints or problems, we can probably give specific advice.

I have the schematic to look at, I am just forbidden to post it. I can answer questions about it and discuss it though.

One of these just came across my bench. No output and the limit LED is on solid. It doesn't look like a schematic is readily available. Having read through the various posts on this site regarding this amp, is this characteristic of the two main faults Soeren listed ( output transistor soldering and output cap's ).

Thank-you in advance for any replies..... there are a lot of very helpful and knowledgeable people in this forum.

[Dr Kaos]

I have just had one of these across my bench also. It had the same symptoms TieRBuoY listed.
In this case it was one of the two 150k resistors (R17 & R20) which centre the DC on the output.
They are thru hole types mounted on one edge of the board (fortunately).
Hope that helps.

Dr Kaos

[HDSarge]

I had a BXL3000 in for repair with the LPA 1300 power amp. Enzo directed me to the K3000 which uses the same power amp and is listed at Electrotanya. I posted the following in that schematic request thread.

To complete this thread, the problem in this LPA1300 power amp was a faulty D6 (15 volt zener). The bias string, and the op amp driving the power amp receive their supplies via zeners coming from the main Vcc supply. D6 is the minus side of that 15V supply. The smt op-amp is mounted on the backside of the board, so a schematic was very helpful finding the missing supply from the accessible front-side. There are also 15 Volt regulators for the preamp on this pcb, and they were running strong. Data like that can be misleading without a schematic.
JM Fahey gives a nice discussion of a very similar Behringer power amp(P3000) in his #16 response to Ultrabass BX4500H (next thread in this section). He also points out the QSC amps that use this grounded collector design. I had a QSC1400 schematic in hand when Enzo came through with the location of the actual Behringer schematic.
Again my thanks to all....this place is a great resource.

[TieRBuoY]

Thanks for all your help guys. The QSC schematic came in handy. A shorted D17 was the culprit.

[Pedalphernalia]

I have a behringer bx4500h i was playing it then it started humming so i turned it of then when i turned it back on it sounds nasty and the uv meter always goes all the way up, then i decided to open it and look around and tried it without tbd heat sink on and i guess it overheated and the 10 amp fuse popped i got a new one but it popped again but i got a fast acting fuse does it need to be a time delay fuse any ideas on what the problem might be off have i completely fried it

[Jazz P Bass]

NOT a sound practice, running an output section without the heatsink attached.

If the amp is popping a 10 amp fuse, it is irrelevant whether a fast blow or slow blow is used.

A working amp will not even pull 1 amp at idle.

Whatever was wrong initially, you now most likely have shorted output transistors.

[Pedalphernalia]

Damn I'm going to have to buy a new bass head unless i find someone selling just the power amp for this head

[ricpt]

Can someone help me?
capacitor c2 burst after its replacement

[Enzo]

C2, as in the 4700uf/80v filter cap?

Most likely reason is it is in backwards. Note C2 and C7 are wired opposite polarity from one another.

A shorted rectifier would put AC on the cap, but would also be blowing fuses.

[musictronics]

If anyone still needs this...

Behringer_BX-4500H Ultrabass Amplifier.pdf

Sorry... just noticed it's just the input stages & footswitch schematics!

[Enzo]

The power amp is the LPA1450.

[musictronics]

The power amp is the LPA1450.

I've got 426 Behringer service manuals/schematics... but not that one!
Do you have a schematic for the LPA1450?

[Enzo]

I do, but I cannot post it due to a signed NDA.

Unfortunately what I find online is only the preamp.

If you can find it, the LPA1450 is EXTREMELY similar to the P3000 power amp used in the BX3000. Right down to the part numbers for the most part. Very minor differences.

In case I am unclear, LPA1450 is the power amp module itself, not some other model of equipment.