The problem is caused by a capacitor in the +/- 15 V supply.
The supply is no longer available.

The faulty part is C2. It is the 100uF/ 60V, orange, electrolytic cap located between the two , large 3500uF filter caps.

The PCB feed thru's for this cap are VERY small and you will probably need to drill them out to get modern leads through. I leave a gap beneath the base to gain access to the top side of the PCB. This allows you to assure a good solder connection, since drilling will remove the thru-hole plating...