hifi engine

Marantz 4240 balance control issues.

I bought a Marantz 4240 back in 1975 or so when I was 18. I sold it to a friend around 1977 when the Quad phase pooped out. Last week I visited my old buddy and he insisted I take my old Marantz back for old times sake and I gladly accepted! I still have my Century 100 JBL's, turn table, and all my old LP's.
Sorry for the rambling....the front R&L balance control causes speaker cut out. When I hook up to Rear Speaker output, this clears up. I'm hoping a cleaning of the slide controls will correct this. I don't know which product to use and I'm also looking for any tips on the cleaning process of the whole unit in general. There is a thin coating of goo on the electronics, most likely THC.