Behringer PMP1280s bad connection to channels?

[MickDuprez]

Greetings All,

I have acquired a second hand PMP1280s PA system that had a blown speaker which I re-coned successfully, now I have an intermittent issue with the channels.

I have narrowed the problem down to what seems to be a bad connection (ground?) and I can reproduce the problem by fiddling with the AUX in volume control.
My question is then would this pot cause a problem with all the channels going dead? The rest of the front panel lights up fine but all channels go dead but can be coerced back to life with a fiddle of the pot (I think I even got it back once by pressing the front panel).

I have the unit apart and can't seem to see anything obvious and everything is on the PCB with the only other connection being the 10 pin cable going to the back section off the unit. Any help on what else to look for or on how to test the pot or other parts for problems would be great. I'm learning electronics at the moment (hence this 'project') and have some test equipment but I don't mind taking it to a pro and a bit of a clue may save us both a lot of time, thanks in advance,

Mick

[Enzo]

It won't likely be the pot, but fiddling with it applies pressure to the board under it, as you found by pushing on the panel. Some connection is loose, start with interboard things like cables. DO ALL lights go off? Do the lights stay on but the sound quits? Just what happens?

[MickDuprez]

Thanks Enzo for your fast reply!

It's hard to say, when it's working and you power it up the clip lights all flash, other wise they don't and there's no sound. All the effects and eq. lights are on. No response on master levels etc.

I've attached a couple of pics, one showing the PCB and some scratched onto the board caused by one of the brass offset pole to the main body frame, I'd imagine this should be isolated given the pcb print out isolating the screw hole.
It looks like the screw is snapped off or cut leaving the thread behind. Someone has been here before too, one of the B output wires was off (another problem fixed ) and there are broken screw seal etc.

[Enzo]

Wait, are all the clip lights lit? Even with nothing feeding the channels? That usually means one of the 15v rails is missing.

[MickDuprez]

Wait, are all the clip lights lit? Even with nothing feeding the channels? That usually means one of the 15v rails is missing.

Not quite, they just flash once quickly when I hit the power button (when it's working). I figured this was an initialisation type thing.

[MickDuprez]

After a good look around the unit, checking plugs/connections etc I isolated that stand off peg from the PCB and everything works

A few other controls are working properly now as well (the cd/tape in volume control at the equalizer wasn't responding at all, it now works as expected).

A prod and a fiddle with the above mentioned knob has no effect now so I'm hoping that the PCB short was the problem and I can use the PA with some confidence.
There was a screw in that hole but it wasn't going into the thread of the post and it pushed the post downwards slightly causing the scratches and perhaps a ground/short.
I'm just glad it didn't blow anything up in that state.

Thanks again for your help Enzo.

[cheez]

I have a Behringer pmp1280s powered mixer both channels have a high-frequency oscillation on the output anyone have any ideas? I can't remember the oscillation frequency but it's like 250 k Hertz or higher

[Jazz P Bass]

That 250K sounds like the switching speed of the SMPS.