Commodore 128 PSU
01.09.2006 17:23
Here are some photos of a Commodore 128 external power supply from Cyberpipe's computer museum.
This is an interesting design (schematics), because it first uses a transformer and RC rectifier to get cca. 20V DC and then a switching regulator to provide 5V DC for the computer.
From my experience most home computers at that time used a cheap linear regulator IC (e. g. 7805) instead of a switching regulator. Why exactly they chose a more expensive design isn't clear. With 2A of rated maximum current at 5V it would be possible to get the same result with a linear regulator and a large heat sink. Perhaps designers didn't want Commodore to heat up as much as their competition.
This power supply had a heavily damaged printed circuit board because the over-voltage protection shunt obviously overheated and caught fire. I don't know how exactly this could have happened, because the power supply has both a fuse and an over-current protection circuit.
A seriously bad capacitor. Probably damaged by the heat of the fire nearby (note the shape of rubber seal at the bottom).
This is the circuit with new capacitor and shunt installed.
Hello , i have a C128 PSU damaged for no reason, it lost the 5 DCV .. 9 AC is perfect... i do know electronics.. but... i read a lot...
can you help me in this battle to recover it?
thanks , see my email ..
greetings
Hi
It's hard to say what went wrong with your power supply - 9V AC is provided directly by the transformer and is pretty independent from the 5V DC that is provided by the switcher. If nothing is obviously burned out, I would start by checking if the fuse, chip and power transistors in that order. Also check if you get any voltage on the secondary winding of the transformer that is powering the switcher.
