JCM900 - urgent advice needed

[Kermit]

Hi all, Ive got a 4500 model (head) I'm getting intermittent volume drops and can't figure out what's happening...

This model has a separate master volume for each channel but its happening on both which says to me it could be the power amp stage...

had a look at the valves (marshall branded EL34's for those who care) they're all glowing happily, Ive run it at 25 watts and tried 1 valve at a time,
tried with different leads, tried cleaning the pots and checked the entire thing for dodgy soldering/connections, tried wiggling leads at i/p n o/p, different cabs etc

I'm stumped and have an v important gig in about 21 hours or so and my only backups are a pod xt live through the PA or my mate's valvestate - nuff said eh?

On the plus side though its not half as bad once she's warmed up a bit but I can't gig it in that state - any advice/diagnosis???

[Garydean]

Do you have an effects loop? Try jumping that with a cable and see if that clears it up. If so, those jacks could need some cleaning.

[Gtr_tech]

Do you have an effects loop? Try jumping that with a cable and see if that clears it up. If so, those jacks could need some cleaning.

What he said......

[Kermit]

Genius! cheers Theres a load of gunk on the send socket, bridging seems to help so I'll pick a new 1 up tomorrow

1 more prob though... theres a fx level dial which peaks halfway in between high and low then drop either side... don't suppose ya know if thats normal or whats causing it?