I got a dead Univox Mini-Korg MK2 for repair. After initial testing it turns out that besides 2 broken keys the power supply is not generating any voltages. The transformer and fuses are ok, so I focused on the voltage regulator, which is an EHD RD3243R 20V voltage regulator with integrated rectifier.
After measurement I found that the regulator was blown and needed replacement, as did the all electrolytic capacitors on the pcb, showing signs of bulging at the tips. Â A little research revealed no sources for ordering a replacement voltage regulator nor any cross references.
I guess that it will become more common with parts having been manufactured in the 70’ties, they will get harder and harder to come by in future.
But anyway, since a voltage regulating circuit is quite a common application, I decided to replace the obsolete hybrid part by a new circuit built on prototyping board out of commonly available parts.
I based my circuit on a full wave rectifier, a standard LM317 voltage regulator with adjustment potentiometer for voltage calibration and a protection diode. I wired it in a way so that I could reuse the existing capacitors on the original circuit board.
The -10V power supply is dependent on the +20V supply, so once that one had been fixed the negative power supply would work again as well.
After all functions had been tested and found working, I glued the broken key with some acetone and put everything back together. Good to go!
Obsoletetechnology 