Help walking through behringer ep4000 repair

[GreenBlood]

Hi all. I am trying to repair 2 Behringer EP4000 which died in my loft from over heating I suspect.

I have the schematic and a repair guide from a very similar amp made by QSC.

I have bought a signal generator and an oscilloscope and have some electronics experience building simple circuits.

But I have made no progress with these amps and yesterday I removed and replaced the out put transistors in one amp, that was actually working but was getting very hot with no output. When I turned it back on, BANG. And now I’m very depressed!
I have 230v over the class H switching mosfet:-(

So. Basically I’ve messed up and need someone to walk me through it. Advise me if I need to buy anything else to help prepare not to blow up another! (A variable ac power supply I don’t yet have)

Here are the schematics.
Thanks in advance chaps
Steve

[bob91343]

First step is look for burned components.  Once found, replace.  Then, with a VOM, measure all semiconductors for shorts and opens.  Replace.

Measure with Ohmmeter from all power rails to ground.  If there are not too many power supply voltages, apply from external dc supply and see how much current is drawn.  Bring up power gradually and watch the current.  With no signal the current should not be large enough to allow any components to get very warm.

These are the first steps.  You must start with all components good, to the best of your testing ability.

Study the diagram to see if the power output semiconductors have bias circuitry.  If so, inspect to verify good components.  Look for adjustments in the bias network.

[CaptDon]

I hate those old school comp-sym amplifiers. They are so sensitive to drifted values and then suddenly you have D.C. offset at the output. Since there is no output transformer it is safe to run with the outputs open circuited. Remove the output load while troubleshooting, this will reduce the chances of blowing things up. All I see is BJT in the output. What switched MOSFETS do you refer to?

[bob91343]

Steve, your mistake was in turning on the amplifier after replacing output transistors.  What happens is the bias has to be adjusted to specs as power is brought up gradually with a variac.  Since you didn't do that, you probably started off with too much current and then destroyed the transistors.

I have repaired many such amplifiers and the first thing is to adjust the idling current with the bias adjust control, whether it be a pot or selected resistors.  In fact, often the bias network needs parts replaced as well.

[stephenOZ]

Hi Steven, I have two good audio boards but my main power board has failed. I know you have 2 of these ep4000 amps, would you consider sell 1 of the power boards? of course I will pay for the postage. thanks Stephen

[Uunoctium]

here comes the appropriate servicemanual
https://www.diyaudio.com/community/attachments/behringer_service-manual_ep4000_anotated-pdf.972433/

[GreenBlood]

Bob, I will be looking at this again soon as the winter forces me indoors… what kit do I need to start a repaired board up with out blowing it up? You mentioned a variac…?