I removed all modules of this Formant synthesizer to figure out which module is causing a short at the + 15V supply. But the short was in the internal power supply. Unfortunately the supply was soldered to the bus system. Otherwise I had disconnected the power supply from the supply bus. Next time I maybe just cut the lines to see, whether the supply works then.
Most of the power supply failures I repaired so far were caused by an old Tantal capacitor. They often just create a short at a supply line.