Behringer PMX2000 Right Side Not Working

[wilsonsk]

Hi,

I got a PMX2000 for free, left channel not working, I have signal at the outputs on the front of the mixer, and nothing when I feed the right side power amp in jack on the front. Also have no bridged output.

I checked the power transistors in circuit, ranged from 3k-8.5k.

I see nothing burned or visibly damaged.

What should I look for?

Thanks,

Steve

[The Dude]

Do you hear any slight hiss or anything that would indicate the power amp is even slightly operational? Is there voltage on the output transistors? Any blown fuses? I would normally check transistors using the diode check function, but since your readings did not show any shorts, I'm (possibly wrongly) assuming the outputs are ok.

Schematic is available here:

http://elektrotanya.com/behringer_pm.../download.html

Edit: Apologies. I just looked at that schematic and it does not contain the power amp section. Leaving the link anyway in case it's useful.

[Enzo]

The PMH2000 hs a conventional power amp, not one of the digital ones behringer used all over the other PMH series models. This circuit is a straight steal from the Peavey XR684

Service manual, eprom result list | ElektroTanya | Service manuals and repair tips for electronics experts


On this page below the thumbnail pix it will say processing for 30-40 seconds. Once that changes to "get manual" then click on those words to download. All the big green download buttons are for software and stuff.

[Enzo]

And like the Peavey, ther are crowbar triacs on the speakon jack board.

Make careful notes where the wires go, ther is some layers of disassembly on the power amp.

[wilsonsk]

Do you hear any slight hiss or anything that would indicate the power amp is even slightly operational? Is there voltage on the output transistors? Any blown fuses? I would normally check transistors using the diode check function, but since your readings did not show any shorts, I'm (possibly wrongly) assuming the outputs are ok.

Schematic is available here:

BEHRINGER PMX2000 Service Manual free download, schematics, eeprom, repair info for electronics

Edit: Apologies. I just looked at that schematic and it does not contain the power amp section. Leaving the link anyway in case it's useful.

I felt that the lack of a dead short reading at any of the power transitors, and the lack of blown fuses or any other of protection modes pretty much rules out power transistors as the culprit, does that sound right?

I don't hear any hiss or anything, nor any popping or typical transient noises you make plugging in to the 1/4" jack. Pretty much dead air on both the right and bridged outputs, regardless of LEFT/RIGHT or MAIN/MON mode selection. The LED VU meters do work for both sides, and the front panel outputs do as well.

So what is significant about the "crowbar triacs" on the Speakon output panel?

Thanks,

Steve

[g1]

You said the left channel is not working, then you said the right side power amp doesn't work?
I'm guessing both main outs have signal but one side power amp doesn't work, is this correct?

[Enzo]

The crowbar reference was to warn that even if the outputs seem OK, there still could be a dead short across the output, and when you pull the amp out of the chassis, those crowbars are left behind on the jack board.

[wilsonsk]

The crowbar reference was to warn that even if the outputs seem OK, there still could be a dead short across the output, and when you pull the amp out of the chassis, those crowbars are left behind on the jack board.

The jack board is still connected as it is hard soldered to the amp board. How would I correctly trouble shoot that?

Yes, both sides of the mixer output work, only one side of the power amp.

[The Dude]

You can measure across the speaker terminals to see if the triac is shorted. No need to even take anything apart, yet. A simple continuity check will tell you if the triac is shorted. (of course, with the amp off)

[wilsonsk]

I checked the speaker terminals, very high resistance, 3-5M, I'm assuming those are OK.

I checked the big resitors in circuit and get a short at each, not sure if that's not normal in circuit.

[The Dude]

We can assume then that triacs are ok. I would next check voltages on the output transistors. Make sure you have supply voltage and transistor bias. I'm not %100 sure which resistors you are speaking of, but I'm assuming emitter resistors, which will be extremely low value, so your reading are likely correct.

[wilsonsk]

OK, I'll check voltages next, i probably won't get time til saturday.

[Jazz P Bass]

Seeing as the power amp is not eating fuses, I would suggest that, first of all, you prove that the signal is even reaching the power amp.

After all, you may be 'checking' a working power amp.

[Mark Black]

You might also want to check across the power amp input jack looking for a possible short.

[The Dude]

In post #5 the OP says he hears no hiss, thump, or anything that would indicate output. Normally, if you listen closely, you can hear something come out of a working amp even with no input. That is why I suspect an output problem rather than an input problem.