Behringer PMP4000 half working

[nickb]

These are good results. The PWM chip should be operating so it narrows the problem to after that point.

I suggest check you have continuity from the junction of C4/C5 (sorry I meant C4/C6) to pin 2 of TR2 i.e. the transformers are not open. Also check that you have around 160V at the same point pin 2 of TR2. All that being OK it's time to pull the FETS T3 and T4 and test them. Care need to be exercised when handling FETs as they are very sensitive to static. Also, make sure the main filter caps C4 & C6 are discharged before touching.

This is a difficult PSU to work on since it requires power from the second supply to operate.

[DaedalusWren]

These are good results. The PWM chip should be operating so it narrows the problem to after that point.

I suggest check you have continuity from the junction of C4/C5 to pin 2 of TR2 i.e. the transformers are not open. Also check that you have around 160V at the same point pin 2 of TR2. All that being OK it's time to pull the FETS T3 and T4 and test them. Care need to be exercised when handling FETs as they are very sensitive to static. Also, make sure the main filter caps C4 & C5 are discharged before touching.

This is a difficult PSU to work on since it requires power from the second supply to operate.

Thanks for the next step. That was junction C4/C6 I presume?

[nickb]

Yes, sorry C4/C6.

[DaedalusWren]

C4/C6 junction to pin 2 of TR2 ring through fine. Pin 2 of TR2 carries 169vdc. Unless there is another test beforehand, the next step I will be taking is testing the mosfets.

[DaedalusWren]

Will be pulling the mosfets to test in the next couple of days, are there any other probes I should perform before removing these components?

Thanks

[nickb]

No.

[DaedalusWren]

Ok, it seems that both fets are burned out. The gates fail to open and the continuity between source and drain always remain infinite even when the gate has supposedly been charged. Now, is this common for both fets to go, and is there something I should be looking for that would cause this condition to happen?

Off to source out the fets in the meantime.

Thanks for the help, this seems to be moving along nicely.

[nickb]

Yes, it's very common for them to fail. There may or may not be another cause, especially as there is no overcurrent protection that I can see in this design. It would be prudent to check the the output rectifiers are not shorted. Also, after replacing the FETs, power up the unit with no load on the output in case the power amp is drawing too much current.

[DaedalusWren]

Fets on order. Couldn't find an electronic component retailer around me that had the part listed, so they should be here in the new year. Checked the output rectifiers and they seem fine. Guess it will just be a matter of installing them and bringing the power up slow to see what happens. I know a current limiter by way of a 75watt bulb in series with the line voltage was used with the tube amps for service. Would there be any benefit to using this when powering up this unit for the first time?

[nickb]

Fets on order. Couldn't find an electronic component retailer around me that had the part listed, so they should be here in the new year. Checked the output rectifiers and they seem fine. Guess it will just be a matter of installing them and bringing the power up slow to see what happens. I know a current limiter by way of a 75watt bulb in series with the line voltage was used with the tube amps for service. Would there be any benefit to using this when powering up this unit for the first time?

Limiters on switchers often cause problems and are best avoided.

[DaedalusWren]

Ok, Here is where things sit after replacing the Mosfets T3 and T4. Upon power-up, the unit comes to life. A small audible click is heard from the Amplifier board, The fans still fail to energize, Amplification of input from any source is weak and distorted. The amplification of the audio is moderately clear for a split second when volume is initially turned up, and degrades quickly, with a fading of the source audio moments after, to a point of basically static and distortion. Seeming almost like the amplifier board is not receiving the power it needs to cleanly and clearly output an amplified signal. The diagnosis continues....

[nickb]

Well, it's progress.

The fans are temperature controlled by R69 so I would not worry about them, at least as this stage. What is the voltage to the power amp at V+ and V- (X5 and X3) wrt to 0v (X4)?

[DaedalusWren]

Alright, with the Amp board connected to the PSU via the ribbon cables and to X3, X4 and X5:
the voltage from X5 to X3=1.3vdc,
the voltage from X5 to X4=1.13vdc,
the voltage from X4 to X3=0.16vdc.

When power is disconnected:
the power from X5 to X3 slowly rises to 5.5vdc and drops off again,
the power from X5 to X4 slowly rises to 2.7vdc and drops off again,
the power from X4 to X3 slowly rises to 3.1vdc and drops off again.

[nickb]

Not good voltages.

What is the shutdown pin 10 of IC4 doing now? If it's low then it's probably an overcurrent trip. The current is sensed across R29. If too high it triggers the 556 shutdown timer. In that case keep the amp disconnected and remove the rectifiers D24, then D27-D30 to see if that solves the overcurrent problem - monitor the shutdown pin to see what is happening as you won't get any output. Remember that IC4 is on the HV live side of thing so probe with care. Call me a scaredy cat but I will often solder a wire to the point of interest to avoid probing live if there is any chance that I might slip.

[DaedalusWren]

Pin 10 on IC4 is still high at 3.7vdc, Dropped D30 and tested the secondary windings TR4-B and TR4-C which show continuity. Checked some other points,
D3 output = 330vdc,
Vref = 5vdc.
D24 tests out at ∞ Cathode > Anode and 253 A > C
D27 through D30 tests out at ∞ Cathode > Anode and 369 - 373 A > C
D1 & D2 tests out at ∞ Cathode > Anode and 498 - 501 A > C

With Diodes 24 and 27 to 30 removed, I plugged in the ribbon cable between the PSU and AMP. Powered up and tested the secondary side of TR4 and found there was no voltage at all in the secondary windings. (this was tested before I removed D1 and D2)