Vintage Synth Repair

>>The schematics don't say anything in this regard (or am I missing something?)<<

the schematic I found by googling dates from 1981, & shows that the synth uses +/- 15V & +/- 12V as well as a couple of other lower voltages, all regulated on the power supply board. elsewhere in the synth there is some local 5V regulation.

http://people.su.se/~viklund/service_manuals/flat/

so..

you need to give it something that will do 15-0-15. a centre tapped iron core trafo that can do 15-0-15, or a dual secondary toroidal with two lots of 15V would do. somewhere on the back of the synth should be a power rating, though I would estimate from the schemos that somewhere around 80VA would be adequate.

you might want to change a few components, speculatively, in the power supply first. if the transformer really has been burned up, then there are other problems, most likely electrolytics or failed regulators.

hth-
duncan.