Behringer PMH2000 schematic

[Enzo]

Over on the DIYAudio board a while ago, someone posted interior photos of the QSC, the Behringer and another, MAckie maybe? SHowed contrasting transformer sizes, board differences. yes, they are very similar.

[wilecoyote]

EP2500/QSC2450

Thanks for the input guys. Yeah I knew that those resistors were there for just that (dropping a whack of voltage) but I really had to talk myself into it. Really how big a deal would it be to add another winding?

Thanks much for the tutorial on bias adjustment Enzo.

Wow, I've heard the accusations that Behringer is just a copy of something else many times before, but this is the first real example I've ever seen. I get the distinct impression they copy some things and design others.

Thanks for the post Mark, I can set the bias now, as well as pat myself on the back for getting the class H thing right thanks to the QSC manual.

Cheers,

Chris

[Gtr_tech]

I just thought I'd drop a quick note to all to comment on a Behringer EP2500 I just finished with.

A bunch of bits on the channel 1 power section were fried. For what it cost, I figured I'd just replace all of the active bits even though only 3 had failed.

What was interesting though, was that all the big caps were completely dead as well. They had puffed up so bad that they had broken the glue holding them to the PCB. I notice with this amp that there's a fair amount of current being drawn, even at idle with no signal. I had it in a spot where it was on 24/7 (which I've replaced with a crown) and I'm guessing that this idle current just slowly cooked the caps.

Anyhow, for anyone else who ends up finding this on a search in the future, if you find one of the MUR1520 diodes cooked, check for a shorted power transistor and shorted 50n06 mosfet.

Cheers,

Chris

What kind of cap was it that that was "puffed up"? Jamicon 12,000u maybe? I had an import amp in here some time back that one of those main filter caps suffered a dramatic failure. Failed and vented spewing hot electrolyte all over the pcb. Burned a nice size hole in the board and generally made a huge mess all over that side of the chassis.

[wilecoyote]

These ones were Xunda's, 12000uf, no damage to the pcb, they just puffed up and dried out. All of them, including the ones on the good channel. I replaced them all, and one actually fell off the pcb when I desoldered one leg as the other wasn't even attached to the cap any more. When you shake them it feels like theres just a hunk of wax loose inside the can.

I've got the amp back on the bench today and I'm going to set the bias as per the QSC manual. Maybe that will yield some info on what killed the caps.

Cheers

Chris

[Gtr_tech]

Cut one of those open and have a look at whats in there. Could be fakes with something else inside.

[wilecoyote]

Too late, they're all gone to the land fill. I hadn't considered counterfeits, I've only heard of fake transistors, I suppose you could stick a 12000uf 25V cap in a 63V case, I've not heard of this though.

I just did the bias this morning, no real revelations there, it was a bit low on both the side I repaired and the other side, so I doubt that was an issue. Idle current is 600mA so that's about 50mA high according to the QSC manual(500mA +/- 10%) but there's no guarantee the transformers etc are exactly the same so I'm not worried. I can't adjust the current limit because I don't have a 1200W 2 ohm load. (I only see an amp a year or so on my bench)

I have another EP2500 that I'm going to open up just to get a look at the caps and measure the bias.

I've already closed it up and put it back in the gator case, but I'm thinking I should have done Enzo's procedure and compared the level to the voltage recommended by QSC. Oh well, I'll do it on the other one.

Cheers

Chris