Behringer PMP 980S no output

[Bishop]

Hey guys, I have a Behringer PMP 980S that has no output from the power amp.

The amp powers on, shows signal on the meters, and the mixer out actually works, just not the powered outs.

I opened it up and took out the power amp. I can see a burnt resistor, unfortunately I have not found a full schematic, but I did find a part of one that does match up with what I have (and has that burnt resistor on it).

The burnt resistor is R52. (top section of schematic)

All the locations on the partial schematic that have the triangle with the waveform in them measure at -164V. 3 of 5 pins on U2 measure -164V as well, the other two are around -150V.

I assume I have a short somewhere. The other side of the burnt resistor which measures at about 28K ohms instead of 47K), the side with D21, measures 500VDC!!!

Is there something obvious that would cause R52 to burn up, or am I looking for a needle in a haystack?

Looking for a little guidance as I'm kinda stuck. Any insight is greatly appreciated.

[52 Bill]

I assume I have a short somewhere. The other side of the burnt resistor which measures at about 28K ohms instead of 47K), the side with D21, measures 500VDC!!!

Try reading the resistor and then reverse the two meter leads. Are the two readings different?

Your schematic is part of the SMPS. So I would assume that the problem is a shorted transistor in the power amp section of the mixer. The fact that it powers up and doesn't blow fuses is a good sign.

Meter test the output transistors and look for problems like shorts or opens.

[Enzo]

Is D21 shorted?

Your modules all have model numbers, the power supply might be SPS1000, the power amp HCA2400. Those might be the ones, or not, I just picked random models. The power supply of the PMP980 is used in other models, so if we lack the schematic for the 980, we may well find it under a different product. Same with the power amps, Behringer only made so many power amps, they selected the one they wanted for each product.

[Bishop]

Thanks guys. The board says PSU18 SMPS. It seems like it's the board for the power amp AND the power supply.

D21 doesn't appear to be shorted according to my meter. It has 165V on one side and 500V on the other. That is the only node in the whole unit that I can find 500V.

I measured the burnt resistor r52 again both ways and it's actually 47K.

Can't find anything else that seems unusual, nothing odd on on the transistors when measured with my meter. .

Can someone quick explain exactly what the triangle with the waveform means? I should know that, but I'm not sure. Is it just AC signal, or signal ground?

[Bishop]

Found the schematic. Thanks so much Enzo. I will study it and hopefully find something, but any other insight is still appreciated.

[Jazz P Bass]

Please be careful where you ground your probe when working on the SMPS.

The ground reference is at the bottom of the input cap C3. (triangle with an ac squiggle)

Behriner+SMPSU18 Ground Reference.pdf

The output side of the SMPS is a separate entity, usually chassis ground.

[Enzo]

Yes, be extremely careful, these SMPS can be deadly.

The PSU18 is ONLY a power supply, the amplifier unit in this amp is the LPA2400.

The triangle squiggle is the symbol for common on the primary side. The SMPS DIRECTLY rectifies the mains coming in, NO ISOLATION. That creates roughly +170 and -170v with respect to earth. The control circuits are NOT referenced to ground/earth, they are referenced to the -170v rail. See upper left, that symbol is the negative corner of the main bridge rectifier as well as the neg end of the negative side filter cap. DO not measure primary side voltages to earth/ground, they are not the true picture. ALl primary side voltages are taken with respect to that -170v rail, the rail with the squiggle symbol.

Note the right side of the drawing, the secondary circuits, NONE of them use that symbol. Those are all referenced to chassis/earth/ground.

So U2 is running with -170v as its "ground", so those -150v pins are really +15 as far as the IC is concerned.

Find your power amp, I suspect the problerm is there.

Is the PSU18 making the high voltage rails for the power amp? Upper right, the 70v and 40v rails? Should be heavier wires, follow them to the power amp. Or look at the speaker connectors, and follow those wires to find the power amp.

[Bishop]

Thanks for clearing that up. That makes sense.

I checked the jumper that runs from the power supply to the power amp board and I am NOT getting the +/-80V or +/-40V rails. I pulled the jumper and still nothing. I am getting the +/-15V rail though.

So now I am looking at the top right of the power supply schematic starting from T1-B.

D1-3 all measure short going from anode to anode or cathode to cathode respectively, I'm having trouble telling if this is normal. I'm getting ground potential from these points and just a few mV from the 'middle' of these diodes. Are they shorted?

[Enzo]

Look at the schematic at D1-3. They are dual diodes. The dual ends are either a pair of anodes or a pair of cathodes. WHile in circuit, the ends are connected across the transformer winding, which will have a low resistance. it is unlikely all of them are shorted. You are probably measuring that.Work with the power supply disconnected from the power amp. If those rails are still absent, we will find them. Note, once working, they can hold a charge a long time, so I always discharge the 70 and 40 rails with a resistor once repaired.

I suspect your circuit is not switching for those supplies. Note those main rails come from transformer T1. WHile the lower voltage supplies below come from T3. The primary for T1 is over to the left in the main rectifier circuit. The main switching transistors, Q1 Q2 are driven by T2. Those transistors switch the main rail supply. And next to it is L1, which is a current sensor, detecting exces current through T1 primary. Note it has a signal out SC down to the shut down circuit, lower left.

The drive for T2 is the IC top center. U5. That chip runs on the +16v supply. Straight below, bottom center, D27 rectifies T3d, a dedicated little supply just for that 16v for the IC above. Remember, the low voltage supplies come from T3 as well.

Earlier today, we looked at U2. U2 actually powers itself from T3, but to get it started there is a kick start circuit. T3b does this, but the kick start is the two 51k resistors coming down from the 120v incoming mains top left, at 120-1. So very top left, not the plain 120 below the bridge. They let enough current trickle in to start U2, which then takes over for itself. Much like your car uses a battery to start the systems, but once the engine is running, the alternator runs everything.

SO...

Until U2 wakes up and runs, ther will be no +16v for U5 to run on, and so no main rails. So start there. Seems to me you said the low voltages were present. The preamp worked. That means T3 is running. SO verify that 16v supply is being made.

Also top left is the soft start resistor, Thermistor TH1. It is in series with the mains, and starts at a few ohms then drops to close to zero as it warms up over a few seconds. Parallel to it is relay contact K1b, which would short across it. The coil for K1 is below the main switchers in the top center. It is controlled by the main rails coming alive. Is that relay clicking on?

Back to those main rail rectifiers, D1-3. The thing to check is if any are shorted anode to cathode, we know they will seems to be shorted together on the transformer end. That is enough to shut the T1 system down.

[Bishop]

My +16V supply is at +13.4V. Is that within tolerance?

I checked the two 51K resistors and they seem to OK. The point where they go into U2 (pin 3) is at +18.4VDC.

The relay is clicking, and I measured the thermal resistor and it seems to be acting as you say it should.


I did a quick measurement of D2 (I'm late for work), with my diode setting I am getting the same .5V going from cathode to anode or anode to cathode. Also measuring from cathode to cathode I do seem to be getting a dead short 0.00ohms on my meter.