I have a faulty cruise amp from a 1987 300d. what typically goes bad in the amp? I have pulled it apart and see no obvious problems or cold solders.

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1987 300D 180k
1985 300SD 330k Helga
1992 300D Lola 269k- Sold
1986 325 coupe Bessie >300k Original engine(ODO Broke) sold
2006 2500HD Druamax Linda- Tows boat and looks pretty in garage thats about it