Three of these have come in recently with missing sounds or crackling sounds. Two components seem to be failing with time, M5218AL’s and 2sc2603 transistors. The 2sc2603 transistors are a common crackling sound cause (use something like a BC547 but watch out for different pin-outs).
Dead sounds tend to be the M5218AL IC’s. These are SIP’s and getting rare. I have had great success with mounting a 4558 DIP on a SIP module as can be seen on the picture here.
via This Old Synth – Vintage Analog Synthesizer Repair Portfolio Page.