AJ,

Thanks a lot for your answer and hint.
I'm not a full time musician but, yes, when playing live I frequently do it in "smokey" environments.

How do you recommend I should clean the contacts? Do you refer to the external female output jacks or also to internal plugs? I suppose my attention should be directed to the circuits in the neighborhood of the audio outputs... Is that so?

I wish the solution is that simple! Thanks very much!

-- José.