Vintage Synth Repair

>>Yeah,  3-terminal regulators do fail.  Occasionally.  *Very*
occasionally.  
I'd suspect a lot of other tihngs before blindly replacing those,
unless I had reason to suspect that the design was poor and that the
device had become thermally stressed.<<

well.... I've been in broadcast engineering for 25 years, &
electronics for slightly longer, & the three-terminal regulator (78xx,
79xx) is definitely in my top-5 of things that go wrong. 

electrolytic caps, bridge rectifiers, those racehorse transistors in
SMPSUs, tantalum caps, lithium batteries, the odd transistor, op-amps...

I have seen 78xx/79xx go open & even go short, input-to-output. this
latter happened in a moog prodigy (which survived the 17V) & a pro-1
(which didn't, needed a new VCF).

I think, with kit you don't know the full service history of, the best
approach is to suspect everything. see if the psu works on it's own,
if possible. a good sense of smell can be useful, if something's
getting too warm in a device that seems ok otherwise- you can
sometimes find that a badly-replaced reg runs hotter than it wants to
(& will fail again) because the repair-guy before you didn't have any
heatsink compound. in a symmetrical power supply, I check that the
positive & negative components are running at around the same
temperature & that the on-load volt-drop is similar.

duncan.