Going from memory so I might be off a bit… Check the wire connections between board 5 and 6 for cracked solder joints. Adjust the volume control trim pots on board 6 per the service manual. Clean or replace any trim pots that don’t adjust properly. Make sure that the plastic film display slider window moves freely. I may have missed something, but what this should be a decent start.
Replacing the belt with a new one from Danish Sound Parts would be good preventative maintenance. The belt is the weak link in the design and a new one will save you headaches.
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