Behringer PMX2000 Mixer to PA connection issues

[Resh]

Hello,

I was recently given a Behringer PMX2000 that the previous owner said was "having issues". He basically said that one day the PA portion stopped working but that the mixer section continued to worked fine. I plugged it in and when I turned it on there was loud pop through the speakers and then a very loud constant hum. I took it apart to look for any obvious burnt out components. I checked over all of the boards (both on the mixer and PA side) and the only thing I could see was what seemed to be burn marks on the 12 wire ribbon cable connection on the mixer side. I reattached the cable, wiped down the board, and hit a few other solder joints on various components that looked a little iffy. While it was taken apart I turned on the PA section without the mixer board connected. The speakers had the pop but no hum.

I put it back together and now the pop is not nearly as bad, though still present, and the hum has come down. Unfortunately, the unit still does not quite work. When I connect something to one of the 10 inputs, it appears that the mixer works but something is reducing the signal to the PA portion. Turning up the level on the channel sends the signal directly to the speakers, seemingly bypassing the volume/EQ/FX controls. The master volume and EQ have no effect on the signal and the meters don't register anything. Even with the level turned all the way up the speakers are very quiet. Also, the meters don't register anything. Turning up the effects registers on the effects meters as expected, however, the effects dont make it to the speaker (at least not that I can hear).

Taking the signal from the pre-amp jacks to another PA works perfectly, the EQ's work, the volume knobs work, and the effects all work sending a very solid signal.

Plugging an ipod into the PA input jacks has a similar issue as using one of the channels. The music comes through the speakers, just very quietly. The meters and the EQ's work just as expected with no issues. (even when peaking the meters the music is barely heard through the speakers).

I'm hoping someone can help me out. Even where about to look on the boards would help me out greatly.

Thanks!

[soundguruman]

Hello,

I was recently given a Behringer PMX2000 that the previous owner said was "having issues". He basically said that one day the PA portion stopped working but that the mixer section continued to worked fine. I plugged it in and when I turned it on there was loud pop through the speakers and then a very loud constant hum. I took it apart to look for any obvious burnt out components. I checked over all of the boards (both on the mixer and PA side) and the only thing I could see was what seemed to be burn marks on the 12 wire ribbon cable connection on the mixer side. I reattached the cable, wiped down the board, and hit a few other solder joints on various components that looked a little iffy. While it was taken apart I turned on the PA section without the mixer board connected. The speakers had the pop but no hum.

I put it back together and now the pop is not nearly as bad, though still present, and the hum has come down. Unfortunately, the unit still does not quite work. When I connect something to one of the 10 inputs, it appears that the mixer works but something is reducing the signal to the PA portion. Turning up the level on the channel sends the signal directly to the speakers, seemingly bypassing the volume/EQ/FX controls. The master volume and EQ have no effect on the signal and the meters don't register anything. Even with the level turned all the way up the speakers are very quiet. Also, the meters don't register anything. Turning up the effects registers on the effects meters as expected, however, the effects dont make it to the speaker (at least not that I can hear).

Taking the signal from the pre-amp jacks to another PA works perfectly, the EQ's work, the volume knobs work, and the effects all work sending a very solid signal.

Plugging an ipod into the PA input jacks has a similar issue as using one of the channels. The music comes through the speakers, just very quietly. The meters and the EQ's work just as expected with no issues. (even when peaking the meters the music is barely heard through the speakers).

I'm hoping someone can help me out. Even where about to look on the boards would help me out greatly.

Thanks!

It's probably on the power amp module. The module gets so hot under normal operating conditions that it destroys it's own components. Sometimes you can resolder it and it will work again, but in your case I think you have a blown transistor or IC component. Hence the HUM which is common when you have a shorted part in the power amp.
The units I have seen were basically made to operate in Japan at 100 volts. They are labeled "115 or 120 volts" and sold in the USA. IMHO, these units are being overvoltaged, which causes them to run very hot, even at idle. The heat sinks are designed with the thought that heat travels DOWN, which of course is entirely incorrect. In otherwords, the heatsink does very little or nothing to cool the components.
(there has been a big tendency to operate 100 volt equipment at 120 volts, by manufacturers who make stuff in Korea, China, Vietnam, etc...)
You turn the unit on, allow it to idle, and the heatsink becomes hot enough to burn your fingers. It works for a while, I guess until the warrantee is expired.
The heat breaks down the components, eventually. We have added fans, and other mods to cool off the products so they can continue to operate at 120 volts.
You may have picked the wrong product simply because the manufacturer won't supply schematics or any other help to customers. They want you to send it directly to them for repairs. Many years ago, they did send me schematics, but not anymore.
And soooo, if I repair them, I am entirely on my own. I have to find cross references, and add fan(s), substitutions if needed, add my own modifications, to cool off the unit...to the point where it will continue to operate reliably.

[Enzo]

What utter baloney.

Behringer is a German company with their factory in CHina. The power amp in this model is a straight circuit lift from a Peavey XR684. It was never designed by, for, or in Japan. It was never designed to work at 100 volts, the mains voltage of Japan.


If we are talking about a pop when you throw the power switch, I never worry about that. MAny amps pop then. If you are talking about popping that continues, that is something else.

You have two power amp in jacks, one per channel. If using them to inout a signal results in both chanel working the same, very reduced in volume, AND they do this even when the graphic EQ is switched off, then I;d look for ssomething they have in common. For example, on the PA, T30 and T33 are mute transistors at the input of each channel. They are JFETs and must have a voltage at their gates to turn them OFF. No voltage at gate means stuck in mute. Check that out.

If you do a search for the schematic, you should find it. I think we have posted it here before. And for reference, get the Peavey XR684 PA drawing. Part designators will be different numbers, but the circuit is the same.

By the way, it really is PMH2000 not PMX. It looks like an X , I know. But searching for PMH2000 might give better results.

[Techknowman]

I have done a couple of those units. they have 1n4148 diodes that connect the audio signal lines - channels to the +/- 15V rails. If too much signal is shoved into one of these diodes, they can short, I have found that this will cause a loud pop on power on and off and if the upper and lower diode are both shorted you will get a noticable hum and the signal will be very very faint.
check these diodes for shorts.
D3,4,5,6
C59 and C136 was blown out on one unit i did.

[soundguruman]

I disagree, not only that I don't believe you.
I think several of these amps, including "Kustom," Behringer, and Hartke, and a host of others*, are designed to run on 100 volts AC.
If you have the schematic, and you power them up on 100 volts, all the voltage readings written on the schematic are correct.

If you power them on 120 volts, they are all exceeding the written voltages by 20%, and run hotter than Hades.

A number of effects units suffer from the same obvious problem, and a number of "wall wort" power supplies as well.

There has been a swarm of amps imported from China, Vietnam, etc...that run hot as heck, they don't last. If you look at the primary of the power transformer, they are labeled: 100-120 volts AC. There is no dedicated 120 volt winding for the primary.
I think that these are designed for Japan, 100 volts AC. When they are shipped to the USA, the same 100 volt primary is used for 120 volts.

The end result is that the design limits are exceeded, they run pathetically hot, and have a very short lifespan. A check of the voltages reveals that the voltages, designated on the schematic, are being exceeded.
When the voltage is lowered to 100 volts, they cool off, and run almost normally. Then the voltages on the schematic are correct.

I think these manufacturers are fully aware of this problem, and don't give a crap about whether the unit keeps working or not. The units last long enough to surpass the warrantee period, that's about it.
And as you noticed, we get very little schematics, no parts, no tech support, no help, the majority of the time, when we want to fix them.

So when you buy this junk, expect limited service life. That's the reality.

* except, as you can see, that Peavey has a REAL 120 volt primary on the transformer. If it is the exact same design, Peavey is not over-voltaging the power supply. Behringer IS over voltaged..

[Jazz P Bass]

The blather that you come up with never ceases to amaze me!

[Resh]

Thanks for the info! I found the schematics for the pmH2000, its going to take some time for me to go through them, its been years since I've done anything with circuit diagrams but definitely a huge help.

I haven't tracked down T30 and T33 yet, but I think the issue, at least part of it, is on the mixer side. When I plug the ipod into the PA inputs, the sound comes goes through the LED meters as expected and the music is definitely quieter than it should be but able to be heard. When I plug it into a channel I can't even get it to register on the LED meters. Its almost as if the signal is leaking through somewhere and bypassing the master volume control; the EQ does work when plugging it into a channel or using the PA inputs.

I checked the diodes and they all read fine, showing about 14.5 volts across each one. Interestingly I get a static clicking sound through the speaker whenever I attach the probes to D3, I don't think it means anything but its interesting that it only happens with that one. I also visually checked C59 and C136 and there's no obvious sign of damage.

I also figured out that the hum through the speakers is from the electrical field of the transformer. When the EQ/Master portion of the mixer board is close to the transformer (as it is when installed in the frame) it hums. When you take it out and move it away from the transformer the hum gets quieter and quieter. Another interesting thing is that it no longer pops when I turn it on. As of this morning, after doing nothing but unplug it for the night, the amp simply turns on with the quietest of clicks through the speakers.

[J M Fahey]

The units I have seen were basically made to operate in Japan at 100 volts. They are labeled "115 or 120 volts" and sold in the USA. IMHO, these units are being overvoltaged, which causes them to run very hot, even at idle. The heat sinks are designed with the thought that heat travels DOWN, which of course is entirely incorrect. In otherwords, the heatsink does very little or nothing to cool the components.
(there has been a big tendency to operate 100 volt equipment at 120 volts, by manufacturers who make stuff in Korea, China, Vietnam, etc...)

Another truckload of cr4p, as usual.
I think mental institutions should continue making Computers available to interns, as a therapeutic tool, but with an "information diode" fitted, so outside world info can get in, but interns crazy ideas can't get out, to avoid contaminating the Internet even more.

[Techknowman]

I checked the diodes ....Interestingly I get a static clicking sound through the speaker whenever I attach the probes to D3, I don't think it means anything but its interesting that it only happens with that one. I also visually checked C59 and C136 and there's no obvious sign of damage.

the meter or scope has a small amount of diferential charge that discharges into the circuit, thats he pop you hear.
the damage to those diodes and caps happens when someone tries to pipe in another amplified signal into the Power amp inputs, which happened to both units I had.
I actually still have one here. everything works on it except channels 8 and 9 are very noisey. Mic inputs have some kind of issue.
Check and make sure you are getting +/- 15V on the mixer. on any of the 8 pin 4580 chips you should get +15 on pin 8 and -15 on pin 4 a loss of one or both of these voltages would cause issues. if there is a loss of one or both. feel the chips and see if one is hot. if you find a hot one, you probably found a shorted one.

[Resh]

I just checked the 4580 chips, I'm getting ~30V across pins 8 and 4 on all of them. I tried the tape out outputs and they seemed to be functioning normally. I re-looked at the pre-amp outputs and now I'm second guessing if they are functioning. I can get sound from them but I need to turn the levels on the channels up farther than I should to get anything. This would lead me to believe that the issue, or at least the effect of it, is in between the tape out and the pre-amp out. This is just a guess at this point though.

[Jazz P Bass]

It would be best if the voltage measurement was from the test pin to ground.
What you will see is a + voltage on pin 8 & a - voltage on pin 4.
Hopefully both are 15 volts.
Anyway, if you study the schematic you will see that inputs 1-6 all go to the mixing circuit.
(Schematic page attached for reference)
Also on this page is the L & R buss and the monitor buss.
At the bottom left of the page is a circuit labeled "Rumble Filter".
Ch 1-6 all go through this circuit and emerge as L & R.
If Ch 1-6 are all acting up, this is where I would look.
The signal emerging from the rumble filter (as L & R) must be good.

[Resh]

ok, I have made some headway. After just about checking everything on the mixer board I moved over to the PA side and found a low +15 voltage. On pg. 13 of the attached file is where I traced the issue back to. On R27 I'm seeing -61.9V on one side of the resistor and -15.8V on the other side (everything compared to the chassis ground). On R26, however, I see 61.8V on one side but only 9.6V on the other. I'm guessing this is my issue. I'm not sure what is causing this however. The resistor itself looks fine and looks a little tight as far as taking it out to check so I wanted to see if there were other possibilities before I go poking around with a soldering iron. I'm not sure what the V+ and V- symbols are for instance. I also noticed a small burn mark on the transformer but all of the voltages coming off of it seem to check out.
behringer_pmh2000.pdf

[Resh]

I pulled out R26 and the resistor is fine. Does anyone know what might cause a voltage drop like this on the +15V on the PA board?

[Steve Conner]

Probably a bad capacitor or op-amp loading the 15V rail down. Or maybe the op-amps are fine, but something else is blown, causing a DC imbalance that makes them pull too much current trying to rebalance it.

Another truckload of cr4p, as usual.
I think mental institutions should continue making Computers available to interns, as a therapeutic tool, but with an "information diode" fitted, so outside world info can get in, but interns crazy ideas can't get out, to avoid contaminating the Internet even more.

I would miss all those troubleshooting threads on MEF. "Plz help, just got a new jacket, but the arms appear to be tied round the back"

[Jazz P Bass]

My guess is that something is shorted on the +15 part of that circuit.
Is there Vdc on the unloaded output?
If you look at the schematic for the +15, you will see that is goes through R 26 & stops.
D19, C7, & the collector of T3 should not be conducting at idle.
Unless T3 base is on.
Measure T3 (diode check) for a failed junction.
Check D19, C7 for shorts.
Measure the base voltages of T3 & T4.
The +15 & -15 are basically the same, except for polarity.