c84 v1.3 repair

[brekke]

Hello

Ive used a rack with C84 preamps since 2007, and never had any problems with them, done many great recordings with them. However, recently one of the preamp-modules stopped working. theres no audio until the volume is loud, then it distorts badly. I would like to try to fix it myself..
What should I measure with my volt-meter? The preamp looks fine cosmetically.

The preamp is version 1.3, does anybody have documentation for this version?

I would appreciate any help.
Thanks,
Morten