Eurodesk PSU

[BalancedLineOut]

Hi there...

It's a long story you probably don't want to hear, but I am the owner of a Behringer MX3282. Last night while sitting in the other room we heard...well, nothing. That is to say we WERE hearing music, then we weren't. MY MX3282 should have had 3 or 4 PFL's lit up, but they weren't. I could see audio indication on the last strip, but all audio indicators were lit up, solid - except the adjacent channel to the active one. I have a funny feeling my PSU went south. No audio coming out of the board. I heard a hiss when I railed the stereo main, but nothing else. I think metering was working. I'll have to check again.

I know these units are not revered, I get that. Having admitted that, is there anyone who can offer me some guidance in troubleshooting the PSU? A schematic would be an ideal place to start, I would think. I'm just starting to lo9ok into this now, and this site was my first Google hit..so I thought I'd ask around here... I've done A LOT of repairs for myself and friends, this is a bigger monster than I've tackled before though. After I post this I'll be pulling the PSU out of the rack and taking a peek inside. I'd like to know what voltages are supposed to be on which pin, that kind of thing.

I found a link to a schematic for the PSU in anothyer thread, but when I try to download it it comes up as CLEAR.GIF

Thanks for the read.

Steve

UPDATE 1 - feel really dumb for asking, tHe voltages are on the top of the PSU case.
UPDATE 2 - this doesn't look anything like a switch-mode PSU, so I should be safe to power it up and test voltages on test points without a load?
- there are 4 test points: +17, -17, +5, +48. I don't see a test point for +12 or a GND point.
UPDATE 3 - appears there is no -17 at the test point.

[Enzo]

Yes, on just about any mixer, when all the peak lights are on, it generally means you are missing one of the analog voltages, -17 in this case.

The PS unit is just a plain old linear supply.

Elektrotanya has your schematic:
BEHRINGER MX3282 SCH Service Manual free download, schematics, eeprom, repair info for electronics

Look below the image on the page, wait 30-40 seconds until you see "get manual" then click there. All the other large colorful DOWNLOAD buttons download other things.

If the power supply is not in that file, get back to us.


No need to apologize for the mixer, I have one myself.

[Jazz P Bass]

Yeah, it is a plain old linear supply.

The voltage regulators are LM317.( Edit LM377 )

Although it may simply be a failed regulator, you have to be sure something is not 'pulling down' the -17Vdc supply.

[BalancedLineOut]

After I cracked the PSU open, I could see just how "basic" it was.

As for something in the mixer pulling the voltage down. Unit I replcea the LM337T, I don't know (wont know) how to find that out. I guess I'll be buying a couple V regs...

[BalancedLineOut]

Yeah, it is a plain old linear supply.

The voltage regulators are LM317.

LM377T is what it says on PCB, and VReg.

[Jazz P Bass]

LM377 it is.

(fat fingers)

Remove the regulator & measure the output pad to ground.

That may help in deciding whether or not there is a downstream issue or not.

[bkahuna]

LM377T is what it says on PCB, and VReg.

[ATTACH=CONFIG]28101[/ATTACH]

You might want to look at that again...LM377 is an audio amplifier.
Your picture shows LM337T which is a negative regulator.

If you can come up with a suitable power resistor, you could test whether the power supply is capable of outputting it's rated amperage.
A 15-20 ohm 20watt resistor would probably get you there. That would tell you if the regulator can hold up under load. If it does, then you know there's a short in the console.

If the supply won't stay up driving a 20 ohm load, then the regulator is probably bad.

My experience with these mixers has been that a short in the console is typically a shorted .1uf bypass capacitor that can be nearly impossible to find.
You can figure out which pcb the short is on, and then disconnect the others. Sometimes the additional current available by not having the other pcbs hooked up will cook the shorted cap and allow you to identify it. Another trick is to power the offending pcb from a bench supply with several amps of output available (the caveman in me loves this one). This will usually cause the shorted cap to start smoking (or bursting into flame!) so you can identify it.

Good luck!

[The Dude]

^^^^^^Agreed. I've had the same experience a multitude of times. Sometimes you have to force the smoke out to see where it's hiding. It's either that or unsolder every bypass cap until the voltage comes up......NOPE!

[BalancedLineOut]

V reg repalcement

Well the replacement LM377T came in the mail the other day.
-17 all good now.

When I plugged the PSU into the mixer all the signal lights lit up AS WELL AS all the SOLO/PFL LEDs.

Is this some kind of service diagnostic built into the board?

Needless to say i unplugged the PSU and need to do further testing.

[Enzo]

Just because you repaired the power supply doesn;t mean the mixer itself doesn't have a short on the -17 line, or a break in its distribution.

All the LEDs on simply means the other power supply voltage is missing. They rin on +17 so -17 isn;t getting there.

[BalancedLineOut]

Understood.

But, first things first... the -17 V reg was toast, so it had to be replaced. I just benched the PSU again and checked outputs at the DIN connector and I get +17, -17, +5 and +48, so I'm guessing PSU issues are corrected.

This is becoming a very large job. I haven't opened one of these before. I guess I had better get a bucket for screws and knobs.

[diydidi]

Check those voltages with the PSU connected to the mixer. Write us your findings.

[BalancedLineOut]

All right. We're getting somewhere.

With the back off the mixer I can see where the DC is supplied to the first PCB. I can measure 48, -17, and +17 VDC across that connector, when it's unplugged from the PCB. When I plug the connector back onto the PCB I lose the +17VDC. Nothing else is visible component wise on the the PCB, so this means I'm going to have to disconnect all the jumper bundles, remove all the knob for the mains section and try and remove this PCB from the chassis.

Now to print that schematic and try and trace the +17...

S~

[diydidi]

Remove the jumpers between the larger boards, one-by-one first, each time checking to see if the voltage comes up.

[diydidi]

OH yes, there is another way to track down a shorted component you may try.
With everything plugged in, feel around the board with the back of your hand for areas/ components that are hotter than normal/ rest of the board. Do this with PSU relatively cool, so as to prevent LM xxx reg chips from going into shutdown. If this doesn't show up any results, Power the +17V rail from another power supply.
Remember to current limit this supply using its onboard current limit adj or using a series resistor.
This should "smoke-out" the faulty component(s)