Behringer GMX110

[goldtop5]

I have a Behringer GMX110 guitar amp that's not producing full signal. I'm suspecting the fault may be in the input jack. Is there a way to bypass the input jack circuit board? Does anyone have a schematic? Thanks.

[Jazz P Bass]

I find it odd that you think it's the input jack.
Usually the signal will cutout, not drop in amplitude.
Resoldering the jack board should fix it up, unless the jack itself is mangled internally.

Being that it's a 'modeling' amp, try this:
Does the Clean Channel, with it's volume control at max, match the Modeling Amp volume.

You could try to insert your guitar into the Slave In jack, which will bypass the preamp.
Or you could go in the loop Return jack.

This is a very complicated rig, so you need to narrow down where the problem is.
Preamp or power amp.

[goldtop5]

The distortion channel was very weak and the clean channel was almost non-existent until you turn it all the way up. I used a guitar and guitar cable as a probe to attach to the ground pin on the jack and the hot to one of the pins on the 4-pin connector of the preamp board. The full signal came back and it was as loud as can be.

[Jazz P Bass]

Well, if I recall, there is an opamp on the input jack board.
That is used as a buffer for the digital part of the preamp. (which is after the 4 pin header)
I would not recommend bypassing it.

[Jazz P Bass]

Behringer GMX212 Schematic

Oops.
I could not have been any more incorrect on that stab in the dark.
Here is a schematic shot of the input jack.

There is a Jfet (T2/ J112) in the signal path on the jack board.
That Jfet is being used as a buffer to header pin 2.
From there is goes to opamps, so it is not digital as yet.

That Jfet may be the fault.
The circuit voltages are easy enough to check to locate the fault.


BEHRINGER+GMX212 input jack.pdf
Attached is the complete schematic.

[52 Bill]

By sending the signal around the input board, you haven't proved that the input jack isn't at fault. I'd suggest inspecting the board for bad solder or broken pc traces first, and then check the fet circuit as Jazz has suggested.

[goldtop5]

Problem fixed. It was a bad j112.