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lexzyz 13th September 2012 04:40 AM

Behringer Eurodesk MX9000 mixer humming
Hi.I recently bought an mx 9000 eurodesk mixer from my friend.He had used it for about 2 years for home studio,and I use it for a small sound system rental.The problem is this mixer produce humming sound that high enough.When all group selector and main mix disengaged(i.e. channels assisted to nothing) with main fader opened to the max.there's no humming on mains out,but when one button of the selector engaged for example channel 1 to main mix there's humming on out mains,and it grows with every channel and every button(5 buttons per channel)and even harder when the subgroups 1- 8 assigned to main mix.For total it's sound like hell,and it's gone when the main mix fader closed.It's all happen even with mute button engaged.Can anybody suggest what should be done?Is it the ribbon cable or sockets?or poor quality bypass capacitors at the 4580s?Main reserve caps at psu?I have changed lm350s to lm338s at the psu,but no caps that doesn't help at all.Thanks.

Enzo 13th September 2012 05:38 AM

Have you verified ALL power supply voltages are what they should be and are free from ripple?

And do not only check power supply voltages at the power supply, also check them directly on the ICs on the boards.

You may have already changed oput the voltage regulator ICs, but if other parts in those circuits have failed, you still may not have correct voltages.

You may have a missing ground connection within the mixer.

Your hum is caused by something wrong in the mixer, NOT poor quality parts or poor design. If those were at fault, it would have always acted this way.

Does it make a difference if phantom power is on or off?

lexzyz 14th September 2012 02:18 AM

Thanks Enzo,I'll check it.Maybe it's a missing ground connection in the mixer.And if it's right it must be the ribbon cable socket since it's and old mixer.I'll change it and I've borrowed my friend's mx9000 psu and still the same.Voltage reading is the same with my friend's psu.It's about 15 v at + 17v and -17v,but it's a cheap digital voltmeter,maybe it's not accurate.At 5V it's 4,9V and at 12V about 11,8.I've never used phantom power yet but I'll check it.Houw could I check that it's clean from ripple?I don't have an oscilloscope.Thank you very much for your help.

lexzyz 14th September 2012 02:23 AM

Thank you Enzo.So if it's missing ground i'll check the sockets,especially the ribbon cable socket.I've borrowed my friend's psu to compare it with mine and it's still the same.

Enzo 14th September 2012 03:01 AM

You could measure the ripple on the phantom, but really, just click the phantom on and off and see if it changes the hum. if not, then phantom is not involved. And I forget, but is this mixer one that has a phantom power push switch for groups of channels, like maybe 8 at a time, on the rear panel?

Those power supplies are nom,inally 17v, but 15 is fine, just a litle less headroom you'd never miss. What matters more is that whatever voltage it is, BOTH positive and negative are at about the same voltage

geraldfryjr 14th September 2012 08:47 AM

Do have anything connected to the inputs?
My Mackie 32-8 does this as well.
Although in my case it is not enough to cause any issues sometimes it just annoy's me that it is there.

If I disconnect my MSR16 tape deck then it is dead quiet.
It is an unbalanced in/out machine and I could have a ground loop issue.
It is the last thing I have not sorted out yet.

If yours is built anything like the Mackie those ribbon cables and connectors are notorious for making bad connections and noises as well as things not working properly or even not at all.

For years I went without the solo system until I took the whole thing apart and re-seated them all and they still act up every once in a while.

When I do a FFT scan with my Gina24 card straight through the board my noise level is down about -112db to -120db where it should be except for a hump right at 60Hz as high as -80db and even higher sometimes.
And as I add channels and busses it gets higher as well.
As long as I have my gain structure properly adjusted I don't have any issues except that I know it is there and my digital card can pick it up.

I have at least a mile of cabling in my system and it gets to be quiet tedious to track down such issues.
Especially when you have two sets of 32 ins/outs to sort through.

If I disconnect all of the cabling then the noise is completely gone except for a little hum that comes out of one side of the amp as that is the amps fualt even when all of the sliders and gains are full wide open.

Anyway, I had some similar issues as well that caused me to revamp my whole system.
One of them was the power distribution board in the mixer had burned up for some reason.
I am pretty sure they were bypass caps that had shorted out and burnt board very badly to the point that it was black and carbonized all of the way through the PC material.
As well as the caps themselves.

This caused a medium to low resistance short to the audio side ground system causing much noise (hum).
Even the all of the cabling disconnected.

There is two ground systems in my board one for power and one for audio signals.
And they were now connected together through the burnt PC board causing a ground loop.

After I repaired the board by grinding out the bad spot and replaced the missing traces with some wire I was back in business.
As I leave the thing on 24/7 it is working better now than the day I got it back in 1994.
It took about a year or two before it started having problems when it was new.

Also when I got the thing back after a few years in 2001 it was soiled with cigarette smoke very badly and only a few channels actually worked.
That was a huge job to take it completely apart and clean every control but it had to be done.
Now it has worked perfectly since except for a few bad pots after all of these years (18), except for the bad PC board issue two years ago.
If I can find the replacements for them I will do so sometime.

I almost got that very same mixer as it is the clone of what I have and was taken off of the market at the same time I was ready to buy it.
So, That is why I ended up with the Mackie instead.
Plus I liked the full parametric filter as well which was what the big huff was all about, otherwise I would have gotten one.

The best way to check for ripple is with a scope.
But you can use your Muti-meter as well, Just use the AC setting on the meter like 2v AC or lower and measure the dc supply lines and it will tell you what your ripple voltage is.

I hope that helps you for some things to check!!

Good Luck !!

jer :)

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