Behringer PMP4000

[lowell]

This unit when turned on all the leds blink, dimly, but they blink at about a 3 times/second rate. There is also a "beating" sound on all outputs that is related to the blinking. Not sure if I should even bother attempting a repair with it. These units tend to be a waste of time when it's not something I'm immediately familiar with. But if someone's repaired such an issue and has a clue about it let me know. Can't post a schem but will send one.

[Jazz P Bass]

About all that I can suggest is 'check the power supply'

[lowell]

Yeah that's what I'm thinking. I'm bad w/ troubleshooting switching supplies.

[Enzo]

It may not be the power supply's fault though.


Switching power supplies - SMPS - try very hard to proptect themselves. If the output is shorted, then they will try to turn on and instantly turn back off, then try again, and so they sit there pulsing, trying to start. A shorted amplifier channel can load down an SMPS. Disconnect the power amps from the SMPS, including the low voltage cable, typically 6 pins.

Does the mixer part light up properly now? Easy enough to verify if the output devices are shorted, even if you don;t plan to repair it.

My test is usually the fan. I run the SMPS by itself, no connection to power amp or mixer. If the fanruns, it might be OK, and certainly check the power output voltages with a meter.

If the primary side of an SMPS is faulty, it usually blows fuses or burns up. If the SMPS has failed and cycles, I usually expect to find a shorted rectifier or two on the secondary side.


SO the process is: isolate the problem. Is it the SMPS itself or is it a shorted load? If it is the SMPS, are any of the diodes shorted?

[lowell]

Enzo thanks. I have just the SMPS running with fans connected. The fans aren't running... however I'm curious, do they only run when the heatsink temp goes up? I'll check those other suggestions you mentioned too.

[Enzo]

I guess it depends upon the SMPS design as regards the fans. Just look to see if it is producing the supply voltages then, and forget the fan.


Which power supply board is in it?

[lowell]

SMPSU29 is the board.

There is no fan voltage, no high voltage rails either. I checked all diodes on the secondary and none seem shorted or open. I can HEAR the beating if I put my ear close to where the mains come into the supply board.

[lowell]

Enzo have you any suggestions at this point? I've come to this point with these on a few previous occasions an d have had to abandon the repair. I really don't understand SMPS. I've "read" about em... But it doesn't really help with troubleshooting.

[lowell]

Okay I did just read some troubleshooting page on SMPS. THere was speak of some SMPS relying on the correct load to operate. Is disconnecting the PA and mixer changing the load? I doubt It's enough of a change to make the thing stop working right. I mean all the filter caps and voltage regulation is still intact I think. Sounds from the turorial that you're right enzo... As usual. . It also stated that certain diodes could be removed to see if they're the problem... Regardless of whether they measure correctly or not. But also that some cannot be removed and could potentially lead to meltdown. ?? I'll have to reread it with the schematic in front of me tomorrow.

[Enzo]

If you still get it cycling with all the loads disconnected, then the SMPS has a problem.

SO many things could be wrong, but USUALLY cycling means a problem on the secondary side. I could be wrong, but in my experience, primary side problems are usually fuse blowers.

You can't ground your scope to the primary side. But the secondary side you can. One trick I use if I find no obvious shorts is to scope each output voltage. With it cycling, I expect to see each supply spike up for an instant and then quickly drain away. Well, actually they can drain away at their own pace, depending upon bleeder value.

But if you find one supply hardly budging, that might be a clue.


This is a complex power supply, and parts of it power other parts of it, so if the first part doesn;t wake up, then neither will the rest of it. My schematic covers two pages, yours too?

The low voltage supplies are on the second of two pages, and are made by rectifiers D19-23. That is +/-15, +5, +48. Those ought to be the small sqaure surfacec mount diodes if I recall.

Then on the high voltage section, the power amp supplies, we have four MUR1520 rectifiers. D27-30. I forget them too, but I bet they are standing up screwed to a heat sink. SO don;t overlook them while checking the little flat guys on the board. The IC that controls those power amp supplies gets its power from the low voltage section. That is Vcc coming from D6.

[lowell]

Okay yeah I wasn't aware of the 2nd page. In any case all diodes check out w/ a meter at least. I probed around on all secondaries and supplies. One the main supply D27-10 both +/- supplies have some DC and pulsing. On page 2 all supplies have some DC and a pulse as well. The fan supply however looks DEAD. I don't see ANY voltage and there's a VERY slight pulse with my scope at 200mv. I lifted 1 leg of L8 to see if the pulsing stopped (by ear) and it did not stop. All other voltage supplies on page 1 (+15, -15, and +V) have some DC and pulsing.

[lowell]

Again I'm stuck here Enzo. Any ideas from my last post regarding scope results?

[Enzo]

Um....


Well first get the second page going, the low voltage stuff. Note the controlling IC gets its power at pin 3 from VS. Back to page 1 up top, see VS comes from the main V+, the rectified mains.

REMEMBER all the control side circuits are refernced to V- NOT to ground.

On second page, VS comes in to power the thing uper left, and next to that is VH, a power supply output that runs back to the first page. And below that is VCC, which also runs to page 1, and that powers the controller ICs there. So if page 2 doesn't run, then page 1 won;t either. And the fan supply is on page 1. SO that is why the fan is dead, no power from page 2.


Each low voltage supply has a small load resistor, R70, R71 for example. I see 5 supplies. Note they do not all reference to the same grounds. I would measure resustance across each and see if you get something remotely like their value. A short indicates a problem.

Look to the right, see the 10-legged 5-way inductor? Measure resistance from supply to supply, in case two supplies have been shorted together. Like +15 and -15 might not be shorted to ground, but if shorted to each other, it will surely shut you down. The inductor is one place they might get shorted together.

If motivated, I might start removing rectifiers to see if doing that to any supply allowed the controller to come up and stop cycling.

Of course that is just me still thinking we have a load side problem. It is possible we have a controller side problem. Or even a bad TL431 telling the thing to shut down.

Or the controller could be defective, or even T1 could be a vad MOSFET and can;t drive the system.

[lowell]

I can't find the VS output on pg1. I see VS input on pg 1 near D9 on low left. I only see VH by the mains rectifier.

[Enzo]

Sorry, I didn't look close enough. On page 1 the lower left IC, IC3, look above it to D9, and to the right is where VS comes in. VAC comes from the left of the main bridge up top, a bit left of where VH originates. I think I was tired. I didn;t describe that well, and I made some typos.


ANyway, VAC comes through D9 making supply VS, which goes over to page 2 to kick start the controler IC. Note that kick start comes through three 150k resistors in series. Make sure none of those has opened. That kick start SHOULD get the page 2 controller IC going, which pulses the main MOSFET T1. Now on page 2, both VS and VH are near each other. SO the main high voltage, VH, comes from page 1, and is on the top end of the transformer winding TR1A. The bottom of that winding is the MOSFET. The MOSFET switches current on and off, driving that transformer. Once it gets going, then look just below at TR1B winding, That winding powers D8, which in turn powers the controller IC. SO in essence, it is like your car. At first it works off the battery - enough to turn the starter and make all the systems work. But once it is running the alternator takes over and runs the car, you could even remove the battery and the car still runs.

SO VS kick starts the controller, which starts switching TR1, whose windings then make power for the IC through D8 which keep it running. That same winding also powers D6, which makes VCC for the first page controller.

ANything that prevents the page 2 controller from setting up a steady switching means that D8 supply never gets going.