Here's an update for those interested.
The PSU seems to function correctly now for a while, however the +12V and -12V regulator get very warm as well as the diode bridge that is before that.
The 7812 and 7912 where plastic housings that do not require any isolation to the heat sink. I ordered new regulators with metal plats (TO-220) and put those in (isolated only the 7912). The temp difference between the regulator package and the heat sink is very low, way lower than it was before but it still gets pretty warm.
(same image as in first post)
I have the top and bottom connectors disconnected and only connected the middle connector which feeds the channel strips. With only half the channels fitted (10) my multi meter said only about 100mA was drawn on the +12V and the -12V lines. I retested with a 1.5 ohm resistor as a shunt (sorry, lowest I could find in my parts bin) and came out a little higher (250mV over 1.5Ohm =166mA) but well under 200mA on both +/-12V power rail lines. Both with my multimeter as with the resistor in series the heat generated was significantly less.
For 10 channels a current of say 160mA does not sound way off. That would mean we stay below 0.5A for the entire device. I cannot imagine that the 160mA I am drawing would heat up these component so much.
I have looked at the rectified levels with a scope and it has a ripple of about 0.5V (p2p) at 20V (instead of the 16V specified in the schematic) - with a typical saw-tooth like wave form of the slowly discharging caps. The regulated +12V and -12V are very flat and I could not measure any ripple.
I did not replace any of the caps in this part of the circuit and am thinking of doing that. I am not sure how this much heat is generated on a relatively small current draw...
As for the MIDI controller, its processor gets very hot when powered up and it does nothing. A new processor is $30,=. So I have given up on that module and removed it from the unit and restored all the channel strips to let the mute work stand alone again.