Upgrading a Commodore 64 power supply

One of the first things I bought when I came back to retro computing was a Commodore 64. I’d never had one of these before, having been firmly in the Spectrum camp when I was growing up. For some reason I’d never really had the curiosity to explore this before but I decided the time was right and bought a decent bundle off eBay, which came with a handsome bundle of games which were fun to explore. It was also interesting experiencing tape loading complete with music which didn’t happen on a Spectrum.

One thing I quickly discovered on reading around was that the original power supply was something of an accident waiting to happen, with some quite sensational articles with dire warnings of impending doom. Unlike a lot of early computers it is a reasonably complex power supply with a 9V AC and a 5V DC rail which feeds a DIN plug, and so it’s not absolutely straightforward to replace.

The original design of these is extremely odd (it seems to me) in that the they are fairly simple devices with a transformer and voltage regulator, but then the whole lot is encased in epoxy resin making it a heavy and completely solid block. I really can’t imagine why they did this, other than perhaps to make absolutely sure that no-one actually tried to service it. I’ve not seen any other computer manufacturer of the day do anything like this. The problem referred to above is due to a breakdown of the DC voltage regulator, which is buried in the epoxy resulting in a significant over-voltage and generally a destruction of the C64 itself. The epoxy is also poor at radiating heat which I’m sure doesn’t help the situation

There are various opinions out there on what to do about this – most people recommend avoiding the stock PSU, although I tested the voltages on mine and it was fine. There are quite a number of third party units which are very nice, but rather expensive for what they are:

After a lot of debate, I took the view that the only issue with the stock PSU was the regulator, and after finding a few other examples of people who had modified it I thought I would do the same. There are various different types of these, but mine is the so-called ‘wedge’. Others are just big rectangular bricks, but all of them have the same epoxy construction.

The first thing to do is get into it – easier said than done. The plastic base is glued on, and part of it is buried in the epoxy itself. So it does take a lot of pushing and prying (and some cutting with a sharp knife) but eventually it comes off. All you can see is what is below – most of the wiring is buried in the epoxy but there are taps straight of the transformer for the AC supply, and then via a regulator (circled in red) for the 5V DC.

So there are two jobs to do – the first is disconnect the existing regulator, and the second is to provide some kind of modern alternative. The first is a fairly simple matter of desoldering the pads and cutting off and removing the regulator legs. You could probably manage without this but I really wanted to make sure there was no danger of issues down the line.

There are various modules out there for regulating / rectifying voltages (Buck converters etc) but the guide I linked to above suggested using a ‘UBEC’ (Universal Battery Eliminator Circuit). This is something used in remote control vehicles for getting high voltage battery pack outputs converted down into the lower voltages needed. So it’s ideal for my needs, which is to convert the 9V AC from the transformed into the 5V that I need.

These are cheap and easy to find on eBay and all the other usual sources:

These are not particularly sophisticated devices, and there is apparently a fair bit of electrical noise on the output but these older computers are not sensitive enough to be troubled by it. You do have to configure the UBEC with jumpers to get the 5V you need. It’s also small enough to fit neatly inside. You need to connect the input to the UBEC from the main smoothing capacitor, and the output to the output pin from the voltage regulator (the right hand one on the the photo below). The negative output can simply be left unconnected.

If you’ve done it right, then you should be able to measure about 5V coming out and off to the C64. It’s then just a matter of fitting everything back together again.

Overall it’s a fairly easy job and I am now confident that I can use the power supply without issue. A nice spinoff is that it runs much cooler and still looks authentic. It’s been running happily for long periods with no problems at all.