Hi there,
The voltage seems high, even before your 20V measurement, I guess for most uses a 12V supply should be max.
What kind of PSU it is? Regulated or Unregulated?
Could you provide us with more information, which kind of MIDIbox SID (SammichSid, Modular, MB6582) it is? Which and how many SIDs are you using?
Depending on the type of your MIDIbox SID, there are instructions in the Wiki how to measure correct voltages on various locations on the PCB.
If you are confident, that your PSU is suitable for your MIDIbox (I am not sure right now), first remove your SIDs to protect them, then measure voltages and report back.
Best regards
Edit: just saw your previous posts... 15V is ok for a 6581 which requires 12V (and you use a 12v voltage regulator)... can you remove the SID and measure voltages then?