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I eventually got fed up and took a gamble replacing the intermittent 
EL-backlit LCD screen with a modern 20x2 from Crystalfontz:

<https://www.crystalfontz.com/product/cfah2002a1tmijt-20x2-character-display-module>

I'd seen reports that it's finicky about aftermarket displays but this 
one fired up just fine, and the problem hasn't recurred. The old display 
panel seemed very finicky about glitches on the +5 rail and I tried a 
lot of things but couldn't get it to work consistently, maybe the new EL 
backlight under it was interfering with it somehow, who knows...

But in the process of mounting the new display I managed to crack part 
of the cheap brittle 40 y/o phenolic board and had to jumper some power 
lines to the meter LEDs, ugh...this thing had a lot of problems to begin 
with and like it was definitely built to a budget, stinky single-sided 
phenolic with jumpers and air wires everywhere, it's from 1987 but built 
like a piece of cheap 1970s home stereo equipment, with wobbly input 
jacks and controls mechanically secured only by the PCB.

Kind of a PITA to work on, but it is almost fully digital with only a 
couple LM13700 and companders on VCA duty so there's not too much in the 
SM to adjust once it's running..

The Sony digital reverb unit I have from a few years later (1991) is 
built like a piece of aerospace hardware by comparison. I've only seen a 
couple of these for sale domestically in the US over the years, seems 
like they didn't make too many and they were all screwed-up to begin 
with like this one was so I wouldn't be surprised if it was one of the 
few still in operating condition. Sounds nice, but I don't really want 
to deal with one again like this haha

It does look cool all lit up. Uses an NEC variant of the Z80 at 12 MHz 
for brains.

<https://imgur.com/a/fljL11u?>