Behringer MX8000 eurodesk faulty

[alsaut]

My desk suddenly started to light up like a Xmas tree....... all the channel peak lights lit up, and there is a low buzzing sound. Any signal touted through the desk is totally distorted and soft. Checked the external power supply unit..... all seem s fine. Has been told the fault is 'somewhere on the desk'...... Any tips on where the most likely trouble spot would be...... any advice will be appreciated.

[soundguruman]

It's pretty hard to troubleshoot something when the manufacturer won't support the techs or customers with parts or schematics.
Finding the schematic (somewhere) would be helpful, if you can...and post it here. Good luck with that.
Otherwise, send it back to Behringer. It may turn out that this is the only option.

[Enzo]

You have a power supply failure. On any large mixer, when all the peak lights come on and stay, and sometimes the VU meters max themselves evem, it almost always means one of the power rails to the op amps has failed. So check your power supply. But it must be checked under load, you can;t just plug in the power supply alone and measure its output.

If the desk is indeed loading the supply, inside ther are large boards making up the whole deal, each can be disconnected from the system to try to isolate which board has the problem. Once that far, a schematic won;t help anyway, each IC has a small bypass cap by it, one of those shorts and you have to track it down, the schematic won;t tell you which one of hundreds is the short. Look at them closely, any have black marks on them?

Google for the schematic, and as always, try Elektrotanya. I see MX8000 schematic listed there. Free downloads.

Not all manufacturers distribute schematics to the public, but some of us stand there and curse the darkness, while others light a candle.

[Jazz P Bass]

Behringer MX8000 Schematic

And the candle was lit.
The light banished the darkness.
And the light was good.

[soundguruman]

Does this desk run on propane? I think I'll light the candle.

Seriously,
+18 volts
-18 volts
+48
+5
+12
So you think the 18 volts failed? Or the -18 I am thinking?
Probably one of the LM 350 adjustable regulators failed?
Do you have a shorted op amp that took it out, or did The 350 just roast until it failed?
The low buzz may be a shorted op amp...I am imagining.
Sometimes you can find a shorted op amp because it gets hot, but don't burn you fingie.

I think I would try the power supply, and see if those 5 voltages are there when it not connected to the desk...
If the power supply still works, you can try disconnecting boards in the desk one at a time, until you narrow it down to one board.
Find the board that when connected, loads the voltages down.

Or one more possibility is a cracked connection...take a good look and see if anything may have cracked a board, or cracked the connections where a connector is plugging into a board.

[jrfrond]

Usually, it's one or both of the LM350T's. However, as Doug pointed out, you could very well have a shorted bypass cap on one of the mixer PC boards. If the IC is bad, it will usually be bad unloaded as well. If you replace them, make sure you use heat sink grease and torque them down snuggly. What killed most of these IC's was poor thermal transfer due to improper factory assembly.

If you have a shorted cap in the board, the easiest way to find it is to "smoke it out". I learned this technique right from Behringer's lead tech when I was in Germany for training in 2001. We modded our test PSU with switches for each bipolar rail to supply the unregulated +/- 24V or so BEFORE the regulator, to drive the board. The increased voltage and current delivery snaps the shorted cap right open. If done in a dark room, you can see the flash from under the PCB, then desolder the affected cap, shake it out of the board, and install a new one on the reverse side. Like I said, FACTORY SERVICE TECHNIQUE.

[g1]

Thanks for that John, but I thought most of the chips were +/-18V max.? Is there concern about turning off quickly after the cap "pops" to avoid blowing chips?

[Martin Pedder]

From memory, there are some little caps between the faders that go short and pull down a rail. First, work out which board has the problem. I would then power the audio rails of that board with a bench supply that can shove 3 Amps through the shorted cap. I do play safe at around +/- 18V as you suggest. Just wait for the smoke and crackling sound. This is fun electronics.

I have used this technique on a number of these desks. It used to be a common fault. I am disheartened to find it was factory procedure. I thought I had invented it.

I do not have a thermal camera, but if you do, you should be able to see which component is sinking the current without resorting to such violence/ entertainment.

[synthdood]

And the candle was lit.
The light banished the darkness.
And the light was good.

Sorry to revive this old thread but the attached pdf documents don't seem to download anymore. Would it be possible for anybody to post them again? My trusten MX8000 is starting to develop some weird issues and I am wanting to dive in and schematics would make life a whole lot easier.

[Martin Pedder]

Hi,

You can find them here:
BEHRINGER MX8000 SCH Service Manual free download, schematics, eeprom, repair info for electronics

Martin

[synthdood]

Hi,

You can find them here:
BEHRINGER MX8000 SCH Service Manual free download, schematics, eeprom, repair info for electronics

Martin

Thank you so much. You are a hero!

[rdtguitar]

Behringer MX8000 service manual needed!

Thank you so much. You are a hero!

Hi Synthdood
If you still have the Behringer MX8000 service manual, can I get a copy.
It is no longer available from the web addrs that you got to from. THNX!

[rdtguitar]

Hi John
I have the original EURODESK 48/24 DUAL INPUT 8-BUS i think this the MX8000 but not sure.
The LED for +18v on the top right is out . Most channel signals lights are lit Green
The Power Supply is reading that +44.8v +16v -15.8 the other 3 pins are 0.0v .
I am not sure of the pin outs . I am assuming the center pin is GND.
Any thoughts?
I can do some repair work (BEE -jetplanes- mostly digital design) Can dBm tackle?

UPDATE: IF I measure Pins 6 & 4 using Pin 5 as GND I get -6.6v & -11.6v respectively on PS cable end , un-Loaded..
Since Digital would prob have a separate GND then audio, MY voltages are probably fine. But the Pin Outs would definitely help confirm before bringing it in.
Any thoughts

[Martin Pedder]

The power supply provides +18, -18, +48 ref the analogue ground and +5 and +12 ref the digital ground.

You may have a failure in the power supply. You may have a failure in the desk pulling a power rail down. You may have both if a desk fault has caused the PSU to fail.

Check and fix the PSU first. Then if you have a split rail bench power supply power the +/- rails of the desk with that to see if you have a shorted rail.

This thread has excellent advice on fixing common faults in both the power supply and the desk in earlier posts.

Good luck.

[rdtguitar]

Thanks Martin
I'm seeing negative voltages referenced to digital GND which I'm guessing is Pin 5 , that are close to what you and others have defined as Vout on this supply, only Negative ... Any idea what that means? The tolerance is dodgey too-- but I designed mil-spec back in my EE days so +18 was +18v not +16.6v but this is budget audio.
Anyway I've read thru the thread and not really sure how to accurately test the supply w/o knowing what its specs are for current delivery. Any thoughts? Thanks!