Behringer B1800 D.... dead

[drewl]

So, customer has a Eurolive B1800D Pro with a bad power supply, with a replacement board I'm to install and test.
You have to install the main diodes and mosfets.

Did that and supply works, but of course there's a problem with the power amp, hook it up to power supply and supply won't turn on.

Turns out T4 mosfet was shorted, replaced it and power supply comes up, but no output from power amp.

Preamp is fine, signal gets to the driver chips from IC4.
I replaced the driver chip for T4's side in case it may have been damaged, still no output.

No obvious fried components, driver chips seem to be getting proper voltages, but no output.


It has four output mosfets, two per side, one driver chip for each side.
Can one side work without the other?

What am I not seeing?

Thanks.

[Jazz P Bass]

If T4 output mosfet was indeed bad then I would suggest that you replace IC 3 (2095S)

[drewl]

I replaced IC3, voltages are about the same as the other driver chip.
Thanks.

The input to both driver chips is a clipped square wave, I'm guessing because it has no feedback from the power amp.

[Jon Snell]

The IR2095S is NOT just a driver chip!
It is the oscillator for the switching control of the FM output stage.
Are they both oscillating?
If one is not oscillating, there will be no drive and no load created across the speaker.

Always check for oscillation, pin 7 and drive on pins 10 &14.
Check for VCC and the 12volt supplyfor the Low drive output on pins 11 & 1.

[drewl]

No oscillation either chip.
pin 10 -65v on both chips
pin 14 9v on both

pin 1 12v both chips
pin 11 -54v both chips

Don't know what you mean by "no drive and no load across speaker" Right now I don't have a load or speaker attatched.

BUT: Every 30 seconds output of IC4 drops from a large square wave to a normal signal, and there is output from the power amps, and at speaker wires.
I'll check if the oscillator is trying to kick in on pin 7 of both chips.

Thanks

[Jon Snell]

This amp utilises a full bridge class D PWM power amplifier. If one half doesn't work (floats) then there will be half a signal on one side and none on the other meaning as one side pushes possitive, the other doesn't pull negative it just floats, so no speaker output.
Pin 2 is the Mute pin, if low the oscillator will stop. Take a close look around there.
If you slip with your oscilloscope probe, that could be fatal for both halves of the bridge!
If you don't have an oscilloscope send it to someone who has as you will have great difficulty finding the fault.

Sorry, Pin 2 should be high ... I think, long time since I've worked on one.