RM 80 crackle problem

I've got two identical RM80 combos (they're basically an RM50 with 6550 power tubes and 2 x Celestion G12T-75 speakers arranged diagonally in the case).

On one of them, when using high gain modules I get a crackle in the background which becomes audible when a struck power chord decays.

It's almost as if something is loose inside and is causing the crackle. It is not an issue at lower volumes.

Having two means I've been able to test all of the usual suspects, like microphonic tubes, bad modules etc. Swapping everything back and forth has not helped. Running the head outside the combo and probing around inside with an insulated tool has also not revealed any loose connection etc.

Does anyone have any ideas? I've even played it removed from the case in the pitch dark to see if there was any arcing, but could detect nothing of the sort.

Possibly try cleaning the tube sockets and the effects loop jacks with contact cleaner.

Cleaned the tube sockets like you suggested and that's fixed the crackle! Liberal application of CRC contact cleaner to the sockets and tube pins has done the trick.

Never had this issue before, but I suppose this amp has been sitting sealed in plastic in its box for 12 years so fumes from the glues and plasticizers used in its construction may have gummed things up.

Thanks again Spencer4hire for your awesome suggestion.

You are welcome. I had the same issue on my RM100 and I thought on no
something is going out and it is going to be expensive. I read a new guitar mag that had an article on amp issues and fixes and it worked.