Test a Commodore 64 power supply before it reaches the computer

Difficulty	beginner
Time	About an hour, most of it while the supply warms up
Cost	$0 to $22 (USD)
Skills	Using a multimeter's DC and AC voltage ranges, Basic soldering, for the load tester only

Parts

• Digital multimeter with DC and AC voltage ranges (SparkFun) about $21.5 checked 2026-10-26. SparkFun Digital Multimeter - Basic (TOL-12966), listed in stock on the check date. It has a 20 V DC range; its lowest AC range is 200 V, which shows 9 V AC to a tenth of a volt. Any meter with similar ranges will do.
• 7-pin female DIN socket (DIN 45329) for the load tester. Optional. Carlsen suggests taking the power socket from a scrap C64 board. Not priced here.
• Load for the 5 V output: a 12 V #1003 lamp, or a 5 to 8 ohm resistor rated 20 W. Values from Ray Carlsen's tester notes; he says each lamp draws under 1 A. Not priced here.
• Load for the 9 V AC output: a 12 V #1003 lamp, or a 10 to 15 ohm resistor rated 20 W. From the same notes. Not priced here.
• Insulated hook-up wire. To wire the loads and two test leads to the socket.

Tools

• Soldering iron and solder, for the optional load tester
• A heat-resistant surface that is neither plastic nor metal, for the loads
• A watch or timer

What you are building

Mostly a habit, and optionally a small load tester. The original Commodore 64 power supply can fail by passing about 11 V or more down the line that should carry 5 V, and the computer's RAM is usually the first casualty. A multimeter at the brick's 7-pin plug shows which way a supply is heading before it is ever connected to a machine. C64-Wiki recommends checking the 5 V output once a year.

The procedure below comes from the published notes of Ray Carlsen, a retired repair technician who has documented Commodore hardware for years, and from C64-Wiki's guidance. RetroMake has not run it on a bench for this article. The cost range above covers the multimeter only; the lamps or resistors and the DIN socket for the load tester are not priced here. Why supplies fail, and the replacements on sale, are covered in Why the Commodore 64 power brick fails.

How it works

The supply delivers two things on a 7-pin DIN plug, DIN 45329 in C64-Wiki's description. Pin 5 carries regulated +5 V DC and pin 2 is its ground. Pins 6 and 7 carry 9 V AC straight from a transformer winding. The other pins are either extra grounds or unconnected, depending on the supply.

C64-Wiki fixes the positions by their neighbours: pin 1 sits between pins 4 and 6, and pin 3 between pins 5 and 7. Carlsen's drawing, taken from a C64 schematic, shows the socket as you face the right side of the computer. The plug on the brick's cable is the mirror image when you look at its pins.

   9V AC  7           6  9V AC
  ground  3           1  ground
     +5V  5           4  +5V or n/c
                2  ground

A reading with nothing connected catches a supply that is dead or already far out of range. It does not catch everything. Carlsen warns that a failing supply may still read 5 V when it is not properly loaded, and that some bricks only misbehave once they are warm. His answer is an artificial load, a lamp or a power resistor on each output, and half an hour of running before the reading counts. The 9 V AC side is simpler: in his words it is either there or not.

Keelog sells a ready-made alternative, the C64 PSU Tester, at USD 59.99 or EUR 52.99 (checked 26 October 2026). Keelog says it loads the 5 V output at roughly 400 to 500 mA and the 9 V AC at 50 to 150 mA while an OLED display shows voltages, current and ripple.

Steps

1. Inspect the brick. A cracked case or a damaged mains cable ends the test here.
2. Set the meter to DC volts, on its 20 V range if it has manual ranges. Plug the brick into the wall with nothing attached to its DIN plug.
3. Hold the black probe on pin 2 and the red probe on pin 5, one pin each. Carlsen warns against shorting pins together while testing. Note the reading. If pin 5 shows nothing, try pin 4: Carlsen owns a European supply that delivers its 5 V there.
4. Switch the meter to AC volts and probe pins 6 and 7. With no load, Carlsen expects 9 to 11 V AC.
5. Unplug the brick from the wall. If you are building the load tester, solder the 5 V load between pins 5 and 2 of the female DIN socket and the 9 V AC load between pins 6 and 7. Add two short wires on pins 5 and 2 as test points for the meter. Stand the loads on the heat-resistant surface.
6. Push the brick's plug into the socket, then plug the brick into the wall. If you used lamps, both should light steadily.
7. Leave it running for about half an hour, or until the brick's case is warm, and read the 5 V test points every few minutes. Carlsen points out that a leaky regulator can make the voltage rise slowly as the supply warms, which a single reading would miss.
8. Read the 9 V AC across pins 6 and 7 while the load is still connected.
9. Unplug the brick from the wall before you touch the tester. Write down the readings and the date.

Testing

The 5 V figure that matters is the one taken under load with the brick warm.

C64-Wiki adds that most computers killed by their supplies had seen more than 6 V. At the other end, Carlsen links 4.7 V or less to blank screens and program crashes. In his memory-fault notes he gives a rougher rule for the unloaded check: if the 5 V source is high or low by more than a few tenths of a volt, the supply is failing and should be replaced.

Watch the trend as well as the number. Carlsen expects both outputs to stay steady, and if either lamp flickers, goes out or brightens, he calls it time for a new supply.

For the AC side, C64-Wiki explains that the transformer's output drops under load, giving more than 9 V with the computer off and less with it on, and that a C64 works correctly down to about 8.1 V AC. When breadbox64 measured five original supplies at the plug with a computer attached in 2018, they read between 10.19 and 10.63 V AC.

Troubleshooting

Nothing on pin 5 or pin 4 means the supply has failed open, or its 5 V is missing for another reason. Carlsen counts this among the failures that do not hurt the computer, but the potted brick cannot be repaired.

A good reading with no load that sags under the lamp usually points to dried-out filter capacitors, according to Carlsen.

A 5 V reading that climbs as the brick warms is the dangerous case. Carlsen says a reading past about 5.2 V means the regulator is failing. If the 5 V lamp suddenly turns very bright, the regulator has probably shorted and the output has risen to about 11 V; unplug the brick at the wall and retire it.

No 9 V AC does not threaten the computer, but it changes the symptoms. Carlsen writes that an early C64 shows a blank screen with its red light on, while a C64C keeps running without sound, without the cassette port and without the time-of-day clock source for its CIA chips.

A supply that passes every check today can still drift. Repeat the test once a year, and never try a suspect supply on a second computer to see what happens: Carlsen's advice is to test it first, because a failing brick can damage every machine it is plugged into.