Commodore 64 power suppliers should be tested before being sold off. In this article, I discuss test procedures, acceptance criteria and some of the concerns that call for awareness when using old power supplies connected directly to the power grid.

Why you should test a power supply

It is widely known that the use of old electronics and in particular power supplies comes with a risk, and for that reason, my warranty doesn’t cover damaged cause by the use of original power supply. Yet, I am fully aware that the majority of buyers will be using the original power supply that I provided with the computer. For that reason, I have instated a small but effective test regime for the power supplies.

There are three main reasons to test original power supplies:

• First and foremost, to check that the power supply is working. A fair share don’t work any longer. The by far most common problem today.
• Second, to check that the PSU can run without catching fire. The small PCB that converts AC into DC contains parts that may burn. For the same reason, you should never leave a C64 PSU powered on without supervision.
• Third, to prevent electronic damage. The 5V voltage regulator inside the PSU can die in a way which leads to an output level will rising significantly above 5V, which in turn will damage the computer.

It goes without saying that tests can’t predict the future, although the results may give away some hints when failure is imminent.

Test equipment

My go-to test device for C64 power suppliers is a Power supply tester from Keelog. You can use a multimeter instead, but the Keelog-device offers some clear advantages. Firstly, it provides a more valid measurement of voltage levels because it measures under a load of 400-500 mA. Secondly, it measures ripple, which I will revert to shortly.

An attention point with the Keelog tester is that earlier generations didn’t take into account that Commodore changed the pinout of the power supplies, moving 5V DC from pin 4 to pin 5, meaning that the Keelog tester didn’t wouldn’t work with original breadbin power supplies. That issue is fixed in later versions.

The Keelog tester is powered by the 5V DC line, meaning that if it doesn’t show any output, the 5V line has failed.

Expected outputs

All C64 power supplies are specified to deliver 5V DC and 9V AC, but they never do. I have tested hundreds of C64 power supplies, and I am yet to see a power supply that delivers significantly less than 10V AC. This is however no concern, and the test I perform in relation to the AC output is mainly about ensuring its presence.

Testing the 5V DC output is serious business. Here we have concerns about both overvoltage and undervoltage. I consider anything between 4.9 and 5.3V within acceptable tolerances when testing under load. Anything lower indicates that something in the power supply about to fail. Anything higher indicates that the power supply is at risk of frying your computer.

And then there is ripple, i.e. the residual periodic variation. Ripple is undesirable for many reasons, but I primarily measure it in order to get an indication of the health condition of the components in the PSU. Ripple should not exceed 0.01V. Anything above that level could indicate that the large smoothing capacitor inside the power supply is failing. Ripple should be measured over a few minutes to see if conditions change as the power supply heats up.

Power supply variants

Commodore produced many different power supplies, which behave slightly differently. I have gathered some notes around then to help you interpret the outputs you are getting. A main point here is that you should apply slightly different criteria to various power supply designs.

	Original “wedge” PSU Sold with the breadbins and came in several editions with 0, 1 or 2 fuses and with and without build-in circuit breaker. These units vary more in voltage output, but shouldn’t go above 5.3V. (Part No. 902503-06 and 902503-11)
	Later wedge-shaped PSU but the angled surface is shorter. This power supply has screws in the base and only a small amount of resin inside. It was sold with C64C and perhaps later breadbins. Produces around 5.3V DC when working normally. (Part No. 251053-11)
	Brick-shaped power supply sold with breadbins I have only seen this unit once, and based on memory, it was similar to the later, wedge-shaped unit with the exception that the based was fastened with plastic rivets instead of screws. (Part No. 251053-11)
	Regular C64C power supply Beige, brick-shaped unit sold with C64C, Aldi and C64G (Part No. 251053-11). Tends to deliver a very stable around 5.1V DC. (Part No. 251053-11)
	Power supply for the C64 REU (I presume) This is a very rare power supply, which works for the C64 but is a switch-mode power supply, meaning that it doesn’t suffer from the overvoltage issues that can be observed for other models. To my knowledge, it was sold with the C64 REU. Should output a steady 5.1V DC. (Part No. 251053-11)

Apart from the power supplies listed above, Commodore produced other variants:

• A black rendition of the C64C power supply. It has Part No. 310200-04 and was sold with C64GS and C64C.
• A metal cased black C64 power supply sold in Australia and perhaps elsewhere.

I have no data on any of those devices, as they are rare in Europa.