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.

Posted by Tomaž | Categories: Analog


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 ..


Posted by Fer


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.

Posted by Tomaž


My C128 PSU has the same issue. This Thyristor has heated up and burnt. I want to replace it but can't find the proper replacement part. The thyristor has the following written on: X0403BE and TAG8542. What component can I use to replace?

Posted by Hege

Hege, I don't remember what part I used or I how I chose it. Either I found the datasheet for the original part used and chose a similar thyristor or I estimated the values needed from the rest of the circuit - thyristor in this case acts as an overvoltage protection, so it should trigger at the correct voltage and sustain the short-circuit current long enough for a fuse to blow.

Posted by Tomaž

Hi there, I see this is a fair old topic on your blog, but hope you help me to win a faulty C128 Psu battle:)

I have exactly the PSU showed on the pictures, a 220/230 Volt version.

First problem apparently was a common one and not difficult to fix:

1. No +5 DC on output. After checking for blown fuses I found the problem was created by a snapped leg on the secondary part of the transformer. I took apart the transformer from the board, soldered all legs back to the board with wired and I got my +5 Volt back.


After re attaching the transformer back to the board ( the snapped leg was fixed with a piece of wire) and measured it again, I got +1.3 DC only??? Any idea where that is coming from.?

After this, I checked the sec. part which gave the +/- 9 V ac and 22 V ac, which should be Ok, i assume right?

I re re and again re soldered the transformer, as it must be a bad solder job from my side, but this wasn't the case for sure. so then the "shooting" began....

Following things I have tried so far:

1. Took out all Diodes from the rectifier bridge circuit to measure them, all were fine.
2. Re-soldered all components
3. checked 2 big polarized Caps, all were fine (replaced the 2200u one, as I had a new one spare anyway)
4. took out smaller (green and yellow) caps to check, No problem too.
5. Did notice that that X0403BE component (which turned out to be a SCR as part of the protective " crowbar" circuit) was getting really hot very quick. I took it out with the idea that it should not really have a purpose with respect to the DC part (please correct me if i am wrong if you know more about that, as I am not a fully trained electronics engineer as you might noticed:) and i got some high readings on the DC output part, but not sure what conclusion I can make of that.

I did not check those other 2 TO202 looking things yet and the 16-pin chip too (any idea how to check those maybe??)

None of the fuses have ever blown.

hope one can help me revive this old strange, but valuable me at least:)

Posted by Youri

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