Behringer Europower PMX2000 Mixer/amp bad right channel

[mboessen]

Hello, All:

New guy. Old fart. Learned electronics in the Marines in 1970. Fixed 2-way radios for Motorola for 20 years and been fixing xray machines for the last 20 or so. I have an also "old fart" musician friend who gave me his Behringer Europower PMX2000 amp to fix. He's old and poor and can't afford a new one. I loaned him my old Shure 60w power head, which he finds very disappointing.

I paid something like 20 bucks for a schematic a while back, but had back surgery before I could get the project started. Started today and found out the darn thing doesn't have the pages for the power supply/pa section. If anyone can fix me up with that, it would sure be great. Now to the problem(s).

I am feeding a 1000 hz sinewave into channel 1 input. Mode switch set to left/right. Left channel seems fine. Right channel output is very low and garbled, coinciding with his description of the problem. Even running it at very low volume, the right channel gets hot enough to trip the thermal cutout after about 10 minutes. If I unplug the ribbon cable from the main section to the PA it cools off in a minute or two, so I don't expect any DC shorts. With the schematics I have I can trace the tone to pins 1 and 7 of IC34 and they seem fine there. The waveform seems good and uniform at the left and right preamp outputs. Just following my nose, I get the impression that T18 and T19 are possibly driver transistors for the PA. The waveform at T18 looks ok. The waveform at T19 is larger and very garbled. The ohmmeter says the PA transistors are all fine, for whatever that is worth. Eyeballing the circuit boards and wiring produced nothing remarkable. All the plating and solder work looks great.

Can anyone help me bail out an old, liberal democrat, no-fish-catching musician friend? Any help would be greatly appreciated!

Thanks.

Mike

[Jazz P Bass]

Welcome to the forum, Mike.
I must say, you most certainly picked an ungainly amplifier to cut your teeth on!
The basic rule in Audio repair is power supply.
Are the plus & minus rails of the output board the same voltage (sans polarity) & relatively free of Vac ripple.
Are the + & - 15 volt supplies up to snuff? (same voltage, NO ripple)
If so you can proceed to dividing the problem in half.
Preamp or Power amp.
You can use the jacks on the front.
PA in L/R will access just the power amp.
Inserting the signal here will bypass the preamp.
Pa L/R Out will be a preamp out signal.
Scoping these pins will give you the ability to test the pre signal, before the power amp.
Note: I have attached the PMH2000 schematic.
I believe it to be the same as the PMX2000.
The power supply is on Pg 13.

[mboessen]

Hi, Jazz P Bass

Thanks for being on the forum on Labor Day weekend. Thanks also for the schematic! That should help.

Power supplies are great. -15 is -14.85. +15, +5 and +48 are all dead on. I have good waveforms coming into the PA, so problem has to be there. With your schematic I was able to locate an open transistor T32. I robbed T 25 from a circuit that I don't know what it does. Didn't fix the problem, but T32 is right in the thick of things. I have clean signal at the input to IC 6A and it is garbage at T19, which sure looks like a driver transistor for the PA. I am currently trying to figure out how to configure all the pieces on my workbench so I can keep the amps cool and still get at the electronics. I think I'm good except I need to make a heatsink for the 7815 and 7915, and figure out some way to safely hook up the line filter off the metal case.

You are correct that this is a very complicated circuit. It looks like IC 5 A and B are input preamps for the right channel. The output feeds out to T21 and T19 which look like the driver ckt for the outputs. IC 5B output is also fed to T28, which feeds T32. T32 output hits a complex voltage divider that seems to feed IC 8 on at least 2 pins. IC8 is not looking like your average op amp. Trying to figure out what should be on what pin is making my head spin. Has the feel of some kind of feedback loop that controls the input level to the PA. Nothing harder to troubleshoot than a feedback loop. Sure seems awfully overcomplicated. I think Digikey can fix me up with all the surface mount to replace every active component in the suspect area for less than 10 bucks, but, of course, I run the risk of creating work-on-it-itis, and it could just be a bad cap somewhere in there. However, I don't like my (or anyone's) chances of logically troubleshooting that circuit, so I might have to take that route.

I was dearly hoping someone would jump on and say "aw yeah, I had that problem, replace IC8" or something like that. Never know. It might still happen.

Thanks again for the schematic. BIG help. Hope the rest of the labor day weekend is good for you. 'Way too hot and dry around here.

Regards

Mike

[mboessen]

Hey, Jazz P Bass

Looks like it was that Little T32 after all. Apparently T 25 didn't survive the transplant. After 3 hours of laying all the pieces out and extending the wires I troubleshot to the exact same transistor. Pulled out T 24 and put it in there and it seems fine now. (Or some intermittent cleared itself and is waiting for me to get the parts in it and get it reassembled to occur again.) Now to order 80c worth of transistors and pay 8 bucks shipping. (Grr) Still don't understand the function of T24 and T 25, but I guess I'll order them and replace them.

Thanks a ton for the schematic. It did the trick (if, in fact, the trick is done.)

Mike

[Enzo]

It may look like an X, but the model name is PMH2000. The later versions of the PMH stuff were all PMP