Open the backbox while the speakers are crackling.
On the sound board, see if you can grip the big heat sink for more than a second or two. If not, then you amp is saving itself by cutting of.
There are several caps that need changing to solve this issue.
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Chris Hibler - CARGPB #31
Http://chrishiblerpinball.com/contact
http://www.PinWiki.com/ - The new place for pinball repair info