Behringer PMH2000 schematic

[Jazz P Bass]

I just got off a PMH2000 repair.
What a POS.
Well designed amps do not go up in smoke like these amps.
Components blown in half or torched, copper traces vaporized.
Nice job Behringer.
At final testing the amp burst into flames at idle.
Fine for ten minutes. Drawing 15 watts from the wall.Then POOF! My shop was on fire!
Should have used an inline lamp.
Good riddance, piece of crap.
Sold the lady a Samson.
Anyway, after that seemingly endless rant, for all those brave enough to try to repair one of these crap amps.....
Here is the schematic.
Good luck!

[Enzo]

I have a Crown Microtech that erupted in flames here. Blew apart all the transistors that were not welded metal cases. Charcoaled the resistors and large holes in the board. I have a nice Audio Centron powered mixer with piles and piles of burnt out resistors to go with the rows of bad transistors. Ever face a large QSC with shorted transistors, burnt resistors, bad MOSFET commutators, and burnt little surface mount step controller boards? Ever see a Marshall MF350 (Mode 4) that worked? or fixed a blown up one on the first try? HAd a Crest something (8002? 3002? I forget, nice big amp) One of the two power modules burnt up its switcher. Had to replace the whole unit. Not enough board left to repair. Got a Mackie SWA1501 subwoofer amp in front of me, blew up most of the transistors on it, and the board under the main filters basically burnt away. I replaced that board as well.

When the power supply is ready to provide hundreds and hundreds of watts of power, and an output transistor fails, a whole lot of stuff can burn up before the mains fuse even thinks of blowing. behringer has no exclusive on burning circuitry.

[Jazz P Bass]

Pmh2000

Enzo you are absolutley correct.
I may have a run of bad luck with these amps.
It just seems that everyone I get is poofed.
The blue board really makes it hard to visually trace faults.
Is there a middle layer to these boards?

[Enzo]

Yes, once they blow, they tend to take a lot of parts. I forget if the boards are multilayer. I have found open traces between two points before and just tacked a wire on in place of them.

[wilecoyote]

me too

I had a Ramsa (still kicking around in the shop) with a bunch of blown output transistors. Replaced them all, and noticed that while it was working, it was still only swinging one way, while I was poking with my Oscilloscope probe I managed to slip and short a leg to ground, I literally watched it blow each output transistor one at a time, and then end with a nice little resistor fire.

Anyhow, I've got the PMH2000 open on the bench right now, I just wanted to say thanks for the schematic, it just fried a test speaker I had here in the shop, I've got 65VDC on the speaker, should have checked that before connecting a speaker. Lucky the speaker was worthless to begin with.


Cheers,

Chris

[Jazz P Bass]

65 Vdc on output

You really should use a "dummy" load.
As to the dc on the output, you might want to check the following parts along the edge of the board.(I don't know which channel you are working on, so I'll post both)
T18 (T19) 243 transistor
T20 (T21) 5400 transistor
If these check fine, IC4 (IC5) may be at fault.
Good Luck.
John G

[Enzo]

You should start with no load at all, and remember that Behringer puts crowbar triacs across the output like on many PV amps. SO always check for a shorted triac on the speakon board before applying power to a repaired circuit.

[wilecoyote]

Gee Thanks,

I wasn't planning on getting any troubleshooting help, but it's much appreciated. (especially the bit about the triacs on the o/p board) I haven't checked everything because it's pretty much beyond economical repair for the client, but pretty much the entire R power amp section is fried. R51 and R41 kind of look like the epicenter of the disaster, but for sure T13 and T14 are cooked, and a bunch of current went through the path from T14 through D30 D31 etc. I'm not sure how bad the board is around the burn area but the only possible repair is to pretty much replace the entire section. The power transistors appear to be ok, but really if I even do the repair it'll be in my spare time if the client decides they don't want it back.

As for my "bad" shop practices, I'm a University AV tech so I do everything, and as such I tend to get a bit lazy, and my dummy load was across the hall, and I had a junk driver in front of me. Sad excuse I know, but I was expecting it to either be passing signal, or nothing at all, it's the first faulty amp I've ever seen with DC sitting on the output. I was frankly rather surprised. I've learned my lesson. BTW, whenever I'm testing a power amp I've just repaired or built, I always bring it up slowly with a variac.

FWIW I switched careers 4 years ago (18 years as an Avionics tech for Air Canada) and my amp knowledge comes from hobby audio for the most part. From my limited experience, most power amps I've dealt with usually have some form of either active or passive speaker protection. From what I can see on the schematic, there's nothing, am I missing something?

Thanks for the input,


Cheers,

[Twist]

Thanks so much for having the nerve to post a Behringer schematic. I used to think they were trying to protect the product line but who would want to build copies of anything they put out. I've thrown away a dump truck load of Behringer gear. "Bewaringer"

[wilecoyote]

Really?

Since you've thrown out so much, do you see a trend as to any particular product type or all across the board? So far I had a prof kill a wireless body pack within the first hour of use, and I've got a Europower 2500 and the above Mixer/Amp both dead.

I've got a small collection of stuff spread across campus and at our loan counter, and for the most part it all seems to be working out ok, but for anything that I need to depend on day in and day out, I only have one piece (a crossover that runs 24/7) and it's a test. So far it's been running for a year.

Cheers,

Chris

[wilecoyote]

EP2500

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

[Enzo]

If it is drawing a lot at idle, then readjust the bias.

[wilecoyote]

Bias Adjust

Thanks Enzo,

The thought did occur to me, but frankly with only the schematic in hand,(no service manual and no values on the schematic) I wasn't confident that I would do it correctly. There are 4 pots in that circuit to adjust and I could guess at what I should do with them, but not with any kind of confidence.

In case you're curious, I've attached the schematic, R61,62,64,65 are always too hot to touch. And if you're really bored, perhaps you could explain the point of the Mosfets. My guess is they modulate the rail voltage in order to make the Amp more efficient? Class H? Am I close?

Cheers

Chris

[Enzo]

I forget the class designations. The MOSFETs are commutators for switching the higher voltage rails in and out to accomodate audio peaks.

I'd expect those resistors to be warm, they are dropping resistors for the low voltage supplies. That is why they are doubled in parallel, for better dissipation.


VR6 and VR3 are maked on the schematic as the bias adjustments. Monitor the mains current draw. Adjust each channel's bias control to reduce the mains draw. As you adjust each channel, starting from the cold end of the pot travel, you should note no change in mains current at first, but eventually you will hit a point in the adjustment wher the mains current starts to climb with the adjustment. Run the pot up just to that point, where that increase starts, then back off just a hair. That is where the bias wants to be. Do the other channel.

[Mark Black]

Behringer cracks me up -

Check out the striking similarity of the QSC RMX2450 schematic to that of the EP2500.

I bet other than part designators you could use the QSC service manual to work on the Behringer. Certainly the circuit description & service adjustments would apply.

BTW the Behringer EP1500 seems to pretty much be a QSC RMX1450...