Amp won't power up, all fuses are good. What now?

[CA_Dan]

I have a Behringer PMX2000 PA head that won't power up when the power switch is turned on. It was working fine until turned on one day and it made a loud "POP" through the speakers and everything went dark.  All fuses are ok. Power cord, power switch, power supply caps, and thermal protection all test good. No obvious burnt traces or comonents. Any ideas? 

Also, Behinger doesn't supply schematics or allow them to be posted on the Internet. Anyone with a schematic for the power section of this amp please feel free to email me the schematic anonymously at dgushue(at)att(dot)net. It would really help me in testing voltages at various points. Thanks.

[joecool85]

Are your speakers tested and in working order?

[CA_Dan]

The speakers are in good working order and work fine with other amps...

[Enzo]

Fisrt, keep searching the internet, and indeed this forum has its own search.  I think the 2000 may have been posted here.  There are a bunch of Behr schematics floating online.  Usually page 4 or 5 of google before you get to them. Seems to me there is a site in Hungary or somewhere in that part of Europe with a bunch of them. Also, though it looks like an X, I think you may have a PMH2000.  SO search both PMX and PMH.

However, Most Behringer schematics do not include the mains wiring anyway, including this one.  But this is a very conventional amp.  The mains come in, got your standard things like power switch, inrush thingie, fuse, and power transformer primary.  Oh I think the primary side also runs through one of those thermal breakers on the side of the power amp cover.  The wiring is accessible, so see if your mains is getting to the primary wires of the transformer.  If not, simple tracing will find where the mains is being interrupted.

[J M Fahey]

Adding to what Enzo said, ***unplug*** the amplifier from mains, open chassis, put your multimeter on the buzzer (continuity) position, or in diode test; touch one power plug (still unplugged from mains) with one multimeter probe, momentarily touch it with the other one to hear the buzz (or read almost 0 on the screen) to confirm it works, and leaving one pin in the power plug pin, visually follow the cable inside the amplifier until you see metal again, it will be soldered to *something* (power switch, fuse, thermal breaker, thermistor, etc.), it should still beep when you touch it with the free probe.
Continue along the path until you reach the transformer.
If you don't get a beep on "the other side" of someting, there you are losing continuity.
Do the same with the other power plug leg.
The 3rd leg should go to the green/yellow wire which is connected to the metallic chassis, that's the ground terminal.
If you can reach the transformer successfully, measure from one power plug leg to the other (not the ground one); turning the power switch on and off you should see that on your multimeter.
Post your findings.

[CA_Dan]

Thanks for all the tips folks, I actually found the problem and was returning to post the answer. The IEC electrical cord socket mounts directly to the PCB on this amp (actually a small PCB dedicated to the mains switch and such). The board mounted IEC receptical must have been knocked around some, no doubt from strain on the end of the electrical cord plugged into the amp while the amp was transported and gigged for years. One of the solders or traces lifted on the IEC socket and blew a trace right there at the socket. I couldn't see it until I started removing all of the boards. J M Fahey's suggestion of tracing out the path starting from the electrical cord pin would have worked perfect to diagnose this.

[joecool85]

Glad to hear you got it sorted.   :tu: