Hey, know this is an old thread, but did you ever fix your issue? I have an SQ-80 with very similar issues (no sound, makes the sin tone when powering off, and no display). My display randomly started displaying random characters after replacing my faders (loose connector maybe?). My power supply voltages all match the service manual too.
(If you want to see my post about my SQ-80, you can see it here - Help trying to resurrect an SQ-80 )
Thanks!