[sdiy] Glowing diode on Casio CT-510 analog board?

Benjamin Tremblay btremblay at me.com
Tue May 23 02:41:42 CEST 2023

Sorry if this is way way way off topic. I’m trying to fix this Casiotone keyboard and there are no schematics. The power supply seems to be putting out around 4 volts. The logic board runs but the 4558, 4066, and power amplifiers seem inert. 
I’m trying to trace the power supply. Unlike most Casios from the 80s, the 5 volt regulator is just a zener diode, not a 7805. There’s one more non-zener diode on the board, and it’s faintly glowing red (!)
I know all diodes emit radiation when they are operating, but this one has such and led-red color.
Were such diodes ever used to indicate an overload on the board? I was more interested in fluorescent hair gel than power supplies  when this keyboard was made. 
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(Lower left corner, a normal looking glass diode)
Benjamin Tremblay

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