Behringer MDX-8000 seems to be stuck in bypass mode. Help!

[analogggman]

I just picked one of these up but it's not working. All the meters work but it just appears to be passing Unity gain only. The meters reflect gain changes and output changes, but seems to be stuck in bypass mode, because the audio doesn't change. Does anyone have any idea what could be going wrong here. Or how to get it out of bypass mode?

 

[Khron]

Internal photos, at least?

 

[analogggman]

Internal photos, at least?

 

[user 130681]

Check the routing pre sets, could be toggled to bypass or monitor only.

 

[analogggman]

Check the routing pre sets, could be toggled to bypass or monitor only.

How do I check those? Would it be some kind of dipswitch internally?

http://240260280.com/music/mdx-8000 combinator/MDX 8000 Combinator Users Manual.pdf

 

[user 130681]

Follow the signal path in the service manual, this assumes you are familiar with this sort of equipment and are suitably qualified to fault find and repair at component level.

 

[CJ]

Try some deoxit on àll the switches , I fixed a Beringer preamp yesterday by doing this. And pay no attention to suppressive people.

 

[analogggman]

Try some deoxit on àll the switches , I fixed a Beringer preamp yesterday by doing this. And pay no attention to suppressive people.

Thanks, I will try tomorrow... was yours stuck in bypass too?

 

[analogggman]

Follow the signal path in the service manual, this assumes you are familiar with this sort of equipment and are suitably qualified to fault find and repair at component level.

According to the manual it has a DC controlled "hard bypass" relay. Could this have failed? i don't know anything about those.

I tried cleaning everything, no results. It's as if the "process" switch doesn't do anything. Like it's stuck in hard bypass. But the meters all work and function as if it is in process mode, whether that button is pressed or not.

 

[Khron]

I'd rather suspect that whatever is driving the relay has failed, rather than the relay itself. Does that relay itself click at all, when you switch the bypass on and off?

 

[analogggman]

Yeah, it doesn't seem to be failing. It clicks and the process light turns from red to green.

 

[user 130681]

Yeah, it doesn't seem to be failing. It clicks and the process light turns from red to green.

Does the signal move through the contacts as it should?

 

[analogggman]

Does the signal move through the contacts as it should?

I think so. The meters wouldn't be lighting up if it wasn't based on the signal flow from the manual.

 

[RuudNL]

Many years ago, when the MDX-8000 just appeared on the market, I had a serious problem with it.
All 'S'-sounds were heavily distorted. I called Behringer and spoke with the designer.
The distortion was caused by the clipper, because the attack time of the compressor was too slow.
It was 'impossible' to change the attack time, I was told, because this would have a a serious impact on the working of the unit. I signed a 6-fold non-disclosure agreement(!) and got the schematics.
I looked at it and said: 'that one, that one and that one'... I changed some resistors and the 'S' distortion was gone!
The last sentence the designer spoke before I hung up was: 'if you find a solution, please let us know'.
I am afraid I 'forgot' to do that...

 

[analogggman]

hahaha.

Many years ago, when the MDX-8000 just appeared on the market, I had a serious problem with it.
All 'S'-sounds were heavily distorted. I called Behringer and spoke with the designer.
The distortion was caused by the clipper, because the attack time of the compressor was too slow.
It was 'impossible' to change the attack time, I was told, because this would have a a serious impact on the working of the unit. I signed a 6-fold non-disclosure agreement(!) and got the schematics.
I looked at it and said: 'that one, that one and that one'... I changed some resistors and the 'S' distortion was gone!
The last sentence the designer spoke before I hung up was: 'if you find a solution, please let us know'.
I am afraid I 'forgot' to do that...

hahaha... do you have any clue what is going on with why it would be stuck in hard bypass?

 

[RuudNL]

I would have to check the schematics (if I can still find them after so many years).
I don't remember if there is a 'hard' bypass with a relay or a switch.
I will try to find the schematics tomorrow. IF I find them, I will come back to you!

 

[RuudNL]

I just looked for the set of schematics. I remember having seen them not too long ago, but they are no longer in the drawer where they were last time...
But: I looked closer at one of your pictures and if I am not mistaken I see a relay at the back of the PCB, close to the XLR connectors. This could be a so called 'safety relay', so that in case there is an interruption in the mains voltage, the input is connected to the output, so audio is still passed. (Of course without processing, but better to have unprocessed sound than no sound at all!) With the enclosure open, you should be able to hear soft clicking, maybe at the moment you apply power to the MDX-8000, or if you use the bypass switch.
If the relay gets no voltage on the coil, or the relay is defect, this could cause the problem you are experiencing.
But...I will try to dig deeper in my pile of documentation!

 

[RuudNL]

A miracle has happened! After some 'excavations' I found the schematics of the MDX-8000...
It is indeed what I thought: the input of the MDX-8000 goes straight into an OpAmp (differential amplifier) AND to the bypass relay.
This relay is driven by a BC557 (depending on the version number T1 or T2) The base of this transistor gets a voltage through a 20 or 22 K.ohm resistor (R5 or R10 depending on the version).
The button 'process' switches the voltage to the base of the BC557.
So I would try to locate the combination of a 20 (22) K.ohm resistor, a 100 K.ohm resistor (between base and emitter) and a BC557. these components should drive the relay. And of course check the relay (coil) itself!

 

[analogggman]

A miracle has happened! After some 'excavations' I found the schematics of the MDX-8000...
It is indeed what I thought: the input of the MDX-8000 goes straight into an OpAmp (differential amplifier) AND to the bypass relay.
This relay is driven by a BC557 (depending on the version number T1 or T2) The base of this transistor gets a voltage through a 20 or 22 K.ohm resistor (R5 or R10 depending on the version).
The button 'process' switches the voltage to the base of the BC557.
So I would try to locate the combination of a 20 (22) K.ohm resistor, a 100 K.ohm resistor (between base and emitter) and a BC557. these components should drive the relay. And of course check the relay (coil) itself!

Big thanks!!!! I will get down to investigating now.

 

[Script]

Just what I had been looking for. Thank you for posting.