Thursday 9 October 2014

Commodore 128 Repair - Part 1 PSU

This is an old post, preserved for reference.
The products and services mentioned within are no longer available.

Here we go again, it's not long since I finishing the repair / rebuild of one of Commodore's first 8-bit computers, the Commodore PET. Now I'm tackling their last 8-bit computer, the Commodore 128.
This one doesn't look too bad, a bit of yellowing on the keys and some marks on the case, but otherwise intact. The power supply has a few issues, so before I began, I gave it an initial checkout, and a short run with an Amiga 500 power supply. The Amiga supply uses the same connector, and two of the pins are the same, 0V and 5V. The Amiga PSU's 5V is rated at 4.3A, the C128 was only 2.5A, so it should be fine. The other two pins are 9V AC at 1A on the C128, and +/- 12V on the Amiga. It works well enough to get the welcome screen, but not recommended for normal use.
The PSU that came with this C128 was taped together, the plastic pillars having snapped. The mains plug was European, that would need to be changed, but since the mains cable was also damaged at the input, I'll just go for a replacement cable.
Inside there were a few more problems. With that replaced, there was no output on either the 5V DC or the 9V AC side. Checking further, the 1.4A fuse on the AC side was blown, and the large inductor in the centre was loose.
Although held in place with a cable tie, both solder joints had cracked, and it was wobbling around. This is the main output inductor on the 5V supply, hence no output. With the fuse and mains cable replaced and the inductor re-soldered, I fired it up and found the 9V supply was there, but the 5V was about 4.5V. There was an adjustment, so I trimmed that for 5V again.
I was hoping I'd be able to replace half or all of the case with one from an Amiga 500 PSU. There were a number of models, all the same shape and size. The original 'heavy' linear one is on the left, the 'light' switch mode one in the centre and the C128 on the right.
It looks like the screw holes are in different places on each one, so I won't be able to do that. I'm not happy with the case of the C128 PSU, so I may put that to one side and look at making a new PSU by adapting one of the A500 ones. All I need to do is disconnect the +/- 12V and install a small 9V 1A transformer. That way I'll get a solid case and a mains switch. The original C128 supply will do for the moment, now onto the main unit....