inside BEHRINGER DDM4000, 32bit Digi Mixer(update)

Check socket first & reset the software configuration.. the opamps used are fairly robust; a last shot after eliminating everything else, if comfortable with soldering try swapping over the opamps to determine if one is at fault ..iirc, there are afew people out there selling them, looked them up ages ago.

If you double checked the inputs (RCA), the hedaphones, etc... .then in most oft the times its the socket itself. Open the mixer an check if signal is present at the soldering points of the headphone socket, maybe it´s a loos or cracked soldeirng point. If present and if re-soldering doesn´t fix the problem, change the socket.
Are you familiar with changing und checking OP-Amps?

OK - i plugged in a jack to rca into the headphone socket. getting <0.2 ohm between the 3 wires soldiered onto the socket PCB (earth L and R) and the respective ends of my rca plugged into it - so it would appear the socket is fine from that i guess?

is the headphone pot ok?

I havent seen the diagram for the mixer but I'm guessing the pot is a stereo one. Most digital mixers of course use voltage control and a mono pot but the behringer...well

further update. based on the assumption the pins on these headphone pots are the same as the pots I've seen on the RS website then I get the following pot resistances (multimeter set to 2k range)

https://dl.dropboxusercontent.com/u/30091626/misc/pot%20values.jpg

from that it certainly looks like the "inner" section is not changing as the knob is moved. at loud volume i was hearing some quiet sound in my left ear (right ear blaring out) but i'm assuming this is just some sort of bleed on the pot/headphone socket/my headphones...

gotta be worth changing it out if i can get a new pot cheap surely?? how would i know i'm buying the right one - would one expect a marking on the underside to indicate the total resistance and taper type etc?

Cheers!