crate v100h problem

[y-ned]

My v100h is not working on the pre amp stage on both channels. However it work using the return path. Could it be a issue on the preamp valves?
Also i´m trying understand this amp scheatics, but is a no sense on the channels part.

[Jazz P Bass]

Try running a patch cord from the pre send jack to the pre return jack.
You may have a bad normally closed contact on the return jack.