Work all the pots on the back left and right then return them to Orginal spot. See if that clears up your oddity.

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Ok I can try that tonight. It's been a while since I've opened up the old Mac, are the pot locations obvious or is there a picture I can look at? What are those pots for?

BTW here's a bigger version of that picture: http://www.atariprotos.com/temp/Mac.jpg