Behringer PMP6000 DC on output

[Delta362]

The Behringer PMP6000 came in with a very vague description of something was plugged in wrong and now it doesn't work. While doing initial checks, I'm finding 5.1VDC on both Channel A and Channel B outputs. I have no DC on any of the 1/4" output jacks on the top of the unit. I'm trying to determine what could cause both A and B to have a 5.1VDC on their terminals.

I have good +/-87VDC coming in, plus good +/-15VDC and +/-12VDC. I'm not finding a logical source of 5V on the schematic. D31K and D32K both have 1.1VDC and the outputs of U16 (pins 1 and 7) are both around -10V.

Any guidance would be greatly appreciated. Thank you!

[nosaj]

What voltage do you have when using a dummy load?
nosaj

[Delta362]

I didn't even think to try a dummy load with DC on the output. I only have one correct adapter so put an 8 ohm dummy load on Channel A and left Channel B alone. A had 0V across it now, while B still has the 5V.

Looks like I'm ok to figure out what they did on the input side of things next. Thank you!

[The Dude]

Also, make sure the amp is not in bridge mode for testing or comparing side to side.

[Delta362]

I checked the preamp section and I have all inputs and functions making it to the MAIN 1 and 2 L/R, MON 1 and 2 and MONO out. Signal is solid going to the power amp board.

Both of the rear outputs A and B will get a brief burst of my signal every 5 seconds, then no signal, repeating the sequence. I found C102 fried (looks like it had a lot of dust and failed open). I'll replace C102 and C74 as they are parallel on the back of the board together. D9 also reads lower DC drop when comparing the diode function of it to D7, D8 and D10. Before I order new caps and the diodes, I'd like to understand the circuit more as I suspect those aren't the only issues.

My V+ rail is 86.2, V- rail is -86.2.

I removed C102 and cleaned up the board for the below measurements. For both IC4 and IC10, There's a blip every 5 seconds.

Pin and voltage of IC4 and IC10 (measured the same) are as follows:
1 VDD 10.2
2 CSD has charging sequence (approximately 3.5V then up to 7.5V) every 5 seconds
3 IN 0, then about -0.4 during the blip
4 VSS 0V
5 n/c
6 VREF -80.6
7 OCSET -80.6
8 DT -85.8
9 COM -85.8VDC
10 LO -85.8, with slightly less negative during the blip
11 VCC -73.9V (Seems high?)
12 n/c
13 VS 0 to roughly -0.6 during blip
14 HI 0 to 1.2 during the blip
15 VB 14.7 with slight drop during blip
16 CSH 2.2V with slip increase during blip

Any guidance?

[Delta362]

Datasheet of the IRS20955S attached. Seems perhaps Vref and Vcc are way out of line for both IC4 and IC10 when looking at the chip's datasheet.

[Jazz P Bass]

It seems that the IRS20955S is 'obsolete'.
Nice!
I believe the IRS20957S is a valid substitute.
Pin to pin compatible. Higher voltage rating.

[Delta362]

Thanks Jazz P Bass! Do the voltage readings lean towards bad IC4 and IC10, or do they look more like they are responding to external stimuli instead? I don't want to just throw parts at it, but am trying to understand the circuit better as well.

Looking at the datasheet for the IRS20955S, it seems Vref should be 5.1V or so. If I interpret that as 5.1V above common (-86V), then I get -81V, which would make sense then, perhaps. Yes? No?

To me, the 5 second timing seems to indicate a protection scheme that briefly passes the signal before shutting down again. The four output MOSFETs are not shorted. None of the diodes tested as a short, though D1 and D5 initially threw me off until I saw they were paralled with 10 ohm resistors. The electrolytic caps do not show signs of failure, though I did not pull them off the board. Could a shorted cap of another variety in the circuit explain the behavior.

Switching over to the MOSFETs, T1 drain has 86V, with the gate and source being near 0.
T2 Source and gate both have roughly -86 on them, with the drain near 0. I don't measure a short for T2 gate to source, though.

[Delta362]

With the IRS20955S being obsolete and its replacement IRS20957S being about a year lead time, I'm hoping those aren't the issue. I did reach out to Audio Labs GA, where they told me the power amp board is a 60-90 day lead time. If I can find the problem, I'm sure it'll be less than the new power amp, but this one is baffling me.

[Jazz P Bass]

The CSD (Pin #2) activity looks like the IC is going into protect mode.

[Delta362]

That definitely sounds like what is happening. I'll remove the four output MOSFETs and retry, hoping to see if I can isolate the issue.