Vintage Synth Repair

Yes, I would say it's the PSU. I dont have a diagram for it here but , in a rig with THAT much digital, the +5volts is either a LARGE three terminal regulator ( in a T0-3 case) , or something more discreet. Also, weak solder joints are a problem when a circuit asks for current. So, replace all electrolytic caps in the +5 supply, resolder any connections that appear "grayish" or weak and find the PSU diagram