The "brilliance" was provided by my headband mounted flashlight, that for some reason, I received two identical gifts of recently. Glad to have taken seriously the overwhelming consensus among bricksters the 89's computer was gonna die someday. I've never been completely satisfied with the "replace the prom rebuild" touted as THE fix. I suppose there could have been a batch flaw with the outfit doing the mask programming, but you would think someone by now would have dumped a still good prom and offered a bunch of eprom copies for sale with or without code improvements. I think Matt L has the more realistic answer; the defects are various.

Thanks, Steve, I'll certainly give that resistor a look-see, which by the way is R315 not R15 as I posted.
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Art Benstein near Baltimore