Behringer gmx212 problems with volume knob

[Engusjazz]

good morning all, and thanks for that wonderful forum.
in fact I'm a behringer gmx212 owner. I'm having trouble with my amp.
When connecting guitar to the front input, I'm having very low signal (sound). On both channels. The volume knob is acting very strange (on the reverse way) when I bring it to max, I have low volume, to min it's gives high volume (but not as higher that it was in the past). And the clean sound is saturated.
But when I connect anything on the back on slave input (usually my guitar pedal gr55), I have a good sound, the pedal piloting the amplifier, the volume being controlled by the pedal volume knob.
I would to use my front guitar input like in the past.
Thanks enough in advance for your help.

[soundguruman]

A tech will remove the control, and clean it or replace it.
I don't think you can clean it without taking it off the circuit board.

[Engusjazz]

A tech will remove the control, and clean it or replace it.
I don't think you can clean it without taking it off the circuit board.

Thanks soundguruman,
But the Amplifier has been 2 times to 2 different workshops without any success.
I think that I should add the event that leads to the problem I'm having.
One day a friend of mine wrongly connected the speakers output of one amplifier in the AUX input of my behringer gmx212. I saw a little blue spark at the AUX input. From that day I'm having those problems:

- AUX input producing very low sound with the input knob at the back seeming to not work.
-Front Input very low sound, with the volume knob acting on the reverse way.
- Clean sound saturated a little.
- when playing with the mid knob of the equalizer the volume come back powerful with the knob acting in reverse way, and after a while goes back down, and need to play again with the mid knob.

--- connecting the guitar pedals on slave in, no problems the sound is perfect as the pedals is controlling the amplifier.

Thanks enough for your kind assistance

[Fragger]

Read through this thread for more ideas:

http://music-electronics-forum.com/t21673/

[Jazz P Bass]

Well now, we do not know what the two repair tech's did, do we?

In all likelyhood, the blue spark damaged at least the Aux in opamp.
That opamp may be pulling down the supply voltages for all of them.
Or the supply itself may be damaged.

Going in Slave in L or R bypasses all of the opamps.

[Jazz P Bass]

The schematic can be found here: BEHRINGER GMX212 SCH Service Manual free download, schematics, eeprom, repair info for electronics

[Engusjazz]

Well now, we do not know what the two repair tech's did, do we?

In all likelyhood, the blue spark damaged at least the Aux in opamp.
That opamp may be pulling down the supply voltages for all of them.
Or the supply itself may be damaged.

Going in Slave in L or R bypasses all of the opamps.

Thanks a lot for your suggestions.
The two techs did absolutely nothing, they just opened the amplifier and they gave it back to
Me saying that as it was an electronic amplifier i would need to change all the mother board.
I think those techs were weak.
I will load the schematic and bring it with.the.amp to another guy.