Behringer PMH 660m

[kalvinb]

One of our audio engineers (TV station) tossed a Behringer PMH660m into the dumpster (some would say that's where it belonged in the first place) and I pulled it out to see if it was repairable. I was told the problem with it was a power supply cap, and sure enough found one of the 4700mF/100v caps blown (C11). AND two of the circuit board traces on the back side of the power supply board burned! That stuff is easy enough to fix but I'm sure there's more to the problem than a blown cap with that kind of damage and tried to disassemble the rest of the system. I took as many screws out of the box as I could find but can't separate the mixer section from the case or remove the power amp from the back of the case. I'm missing something (no comments about missing screws in my head please!)--hidden screws, latches, tabs? I don't want to cause more damage trying to get the thing apart. Any info would be useful. Thanks.

Kevin

[Jazz P Bass]

These amps where designed by a friggin insane person.
Throw it back in the bin, Now!

Seriously, there are 4 sheet metal tangs that hold in the 'box'.
Remove the 4 nuts, push in the screws (they are glued tight & they come loose when you reassemble it), lift out the box.
The speaker connections must be removed from the zobel board.
The IEC connector ground wire must be removed.
And finally, there is a 16 pin header that connects the power amp to the front panel.
That must be removed.
Reassembly will earn you a 16 pin connector that is a PITA to get back on, cuts from the sheet metal & an unreliable POS of a powered mixer.

I do not post this out of vengeance or malice but as a caution.
(I just repaired one & I swore, "not again")

[kalvinb]

Thanks for the information and the warning. If it's not a relatively cheap and easy repair it WILL go back in the dumpster, but every once in a while I need a challenge and this could be it. Then again I may say "not again" myself after this! Thank you.

[kalvinb]

Thanks to your instructions I got the thing apart. The nuts were painted and so were the lock washers under them but the box finally came apart. Now comes the real work-trying to figure out what blew the PS cap and why the "failsafe" circuits didn't protect it. Thank you again.

[Jazz P Bass]

If the cap simply went belly up, there is no protection for that.

[kalvinb]

My thought was that something else in the beast may have gone first (and maybe an over current protection circuit failed?) that's why the cap blew. The other boards look clean with no burned components (for what that's worth!) and I haven't got the time or patience to go through the thing with our Huntron tester to check the components in place. We didn't have any caps close to the right value in the shop so I ordered 2 (one backup) and I'll drop one in when they arrive. I guess the sequence will be check the PS voltages first with nothing connected, connect the low power mixer section and see it that functions, and lastly hook up the power amp board. Smoke test!! Thanks for all your help and I'll post the results when I have 'em.

[Jazz P Bass]

I am not positive but I think the PSU needs the small connector hooked up to the power amplifer.
And make sure you drain the caps before you hook up the power amps.
The supply does not bleed down.

As to the left & right amps, you can check them one at a time.
Measure across the outputs of each to be sure there is not a blown protect triac.
If you get a low ohms reading further inspection of the amp and/ or the triac board is called for.

[kalvinb]

Yes, I think the small connector to the power amp is part of the protection circuitry and has to be connected to bring everything on the PSU up. A 10 pin header from the mixer section may serve the same purpose so I'll try them both, one at a time. I just want to keep the high current stuff offline till I'm sure all is right with the PSU--no sparks! In your initial response you mentioned a 16 pin header that connects to the front panel--mine must be a different version because the header to the mixer has 10 small wires (power to the mixer) and 2 shielded cables which I think are audio out to the power amp. I just left the 14 pin header and wires on the back of the mixer in place since it was covered in glue (not meant to be serviced-thank you Behringer!) since the other ends on the PSU and power amp were already loose and could be passed through the hole in the sheet metal.

The PA outputs both show 9.35 KOhms. This thing may be more trouble than it's worth but if I get it running for the $20 and the few hours of work I have into it I'll pass it on to some poor, struggling band in the area. Thanks for your help.

What was the problem with the one you worked on?

[Jazz P Bass]

Sorry, the '16 pin header' statement was from memory.
Whatever the pin count is, it is.
This last one had a blown left channel.

[kalvinb]

14 pin or 16 pin, whatever the count it does look like it would be a pain in the butt to get back in the mixer panel intact. We'll see what happens when the caps arrive sometime next week. Thanks again.


Adding: When I put the power amp back in the storage box I noticed a glint of light off the back panel of the unit--a peeling circuit trace. Looking closer at the Speakon output board (still attached to the metal back) I saw 2 burned circuit traces there running from a triac 2N6348A mounted on that board. I'll have to check that out too--could have caused the problem if it's bad.

[Jazz P Bass]

The triac is used as a fusable link to save the speaker if & when the output section has a serious fault. (Vdc on the output)
It shunts the output to ground on a failure.
As it usually shorts out in this situation, it gives itself to save the speaker.

[kalvinb]

Looks like triacs on both sides survived (although the burned traces were just on one channel). Looking at the overall damage I suspect the cap blew first (it's showing a short rather than open between it's terminals), that caused a huge DC flow in the system, the triacs worked as they were supposed to and shorted the speaker outputs, and the main fuse took it's sweet time in blowing while the current flow burned the circuit board traces. I repaired the burned traces with copper tape with conductive adhesive (but soldered them anyway to be sure) and once the new cap is installed I'll see what the voltages look like. I might also replace the triac on the "burned" side just to be safe.

[kalvinb]

The replacement cap arrived today. I installed it and have a working power supply. I laid everything out on a piece of particle board and connected the small wires but left the main PSU power amp connectors off. The mixer worked at power up and I have +91 volts and -91 volts referenced to the center pin on both PSU power amp connectors. I'm going to let the PSU caps discharge for a day or two and use a resistor to fully discharge them before I connect the cables to the power amp--no sparks, please. At least I've got a mixer if not a powered mixer yet.

Jazz P Bass thanks for all your help.

[Jazz P Bass]

You may as well go with the drain resistor.
Those supplies will hold up for days.

If you want to give the power amps a quick glance:
Check along the heatsink for any shorts on the mosfets.
There are a few zener diodes that blow when the amp goes.
Check those for opens.
Some diodes parallel a resistor, so don't let that fool you.
If diode check 'beeps' I usually flip the meter to read resistance.
Then compare it to the other channel.
If there are no obvious opens or shorts you may very well have dodged the bullet.

[kalvinb]

I'll have to check the power amp modules. Drained the caps, plugged in the power amp to the PSU and when I turned the whole thing on the large wire wound resistors (10W, 20 ohms) on the PSU started smoking. I shut it down, removed the power amp supply wires , and tried with just the power to the mixer and small wire harness to power amp but the resistor still smokes so something on the PSU was taken out too. I'm going to put it on the side for a while until I have more time to trouble shoot the PSU and power amp boards. If I find the problems I'll post 'em here. Thanks for your help.