E-Piano was a bit more complicated repair compared to the Vermona string synth. PSU had dried up filter electrolytics and one of the earth wires fell off.
Following a pot cleanup all was working well except tremolo.
See the unusual but very reliable slider pot
I noticed some of the electrolytic caps were getting warm due to ESR. After the replacement piano was quieter with less strain on the PSU however the tremolo generator still wasn’t oscillating.
Both of the transistors were OK, all the surrounding resistors as well, electrolytes were replaced so the only culprit left were the foil capacitors and indeed once replaced by small electrolytic caps (the only 470nF I had in my spares drawer) tremolo sprung to life. I managed to later replace electrolytics with foil polypropylene condensers and tremolo was much nicer and more stable sounding afterwards.