Vintage Synth Repair

Christian, r. Do you have the p/n for the 3rd thing on the heatsink?  2. Both the regulators 7805 & 7806 have grounded  and MUST be screwed to the hs  or they will thermal shutdown.  3. If the middle wire is not connected to gnd on the pc bd, then the hs is the gnd connection.  Might explain voltage reading. 

Where are u located?  U and your synth need a visit to my basement.  Absent that, how about some pics of this or diagram with  voltages measured with red probe and blk on gnd or hs.  We'll git it going, barring anything unobtanium that might have been smoked.  

Also sounds like more caps might need repl; bad & dragging voltages down.  More pics of boards needed.
Hugh