Thanks for the ideas. Indeed, it might be a PSU problem. For the full latest details, please see my thread on Vintage Synth Explorer: http://www.vintagesynth.com/forum/viewtopic.php?f=5&t=78014
But in brief, the cable at the back of the FDD does not supply +12 V - only 0.05 V, even if the synth is powered off! And the +12 V supply seems to come through the fuse F4, rectifier S1WBA10, a bunch of transistors and other things, and finally an electrolytic capacitor. While I could unscrew the PSU and replace everything, it would be nice to have a better idea of what to test - and which things are worth replacing. :-)
Once someone can lend a pair of hands to operate the power on/off switch, I can use a voltmeter to test suggested points on the PSU.
Maybe there's still hope to resurrect this dusty old FZ...
Cheers,
Dan