The stock Commodore 64 power supply was notorious. I can’t overstate how big of a piece of junk it was. It was terrible in Commodore’s heyday and it’s no better now. If you have a Commodore 64 and want to keep it working, you need to consider which Commodore power supplies are safe to use, and make sure you have a good one. Otherwise, at the very least, you need to consider repair and protection for your vintage supply to prevent damage to your 64.
If your Commodore-branded power supply doesn’t have the Commodore logo in the corner of the unit, like part number 390205-01 for the 64 or part number 310416-06 for the 128, you have to assume it’s unsafe to use. Other Commodore power supplies for the C-64 fail in such a way that they deliver unsafe voltages that cause serious damage to a C-64 motherboard.
Aftermarket Commodore power supplies
There was a booming market for alternative supplies in the 80s. It dried up over time, and today there aren’t a lot of people making modern direct replacements. Why? Because the 64, unlike most electronics, needs two distinct voltages and it’s very picky about the two it gets. So it’s a more complex replacement than a power supply for a game system from the same time period, which usually only needs one voltage.
Commodore4ever’s Atom power supplies are reliable and repairable. They are pricey, but if they save you a repair, they more than pay for themselves. Spending $80 to protect $100 worth of vintage chips, plus the labor of replacing them, seems like a reasonable deal to me. If you don’t already have the tools for fix a 64, including a 20W soldering iron and supplies and a continuity tester and a logic probe, the price starts to look like an even bigger bargain.
One of Commodore’s good power supplies from the past will probably be cheaper than an Atom unit, but not as readily available. And a modern Atom unit is less likely to need repair in the near future than a power supply that’s now on the wrong side of 30, age-wise.
If you find a vintage third-party power supply that is repairable, such as a Recoton C-1004 or one of the power supplies Tenex Computer Express used to sell, be sure to open it up and replace the 5VDC voltage regulator in it to ensure it’s reliable today.
And when all else fails, it’s not terribly difficult or expensive to just build your own. I had fun building mine.
The problems with the stock Commodore 64 power supply
The stock power supply had at least three problems. If your power supply isn’t light tan, brick-shaped with the Commodore logo in the corner of the unit, and a replaceable fuse on the underside, you have a stock unit that’s problematic.
It doesn’t fail safe
First, when the 5-volt line fails, it can overvolt the computer. When this happens, it frequently damages RAM chips. It can also take out the PLA, or worse yet, the SID. SID chips are worth 50 bucks when you can find one, so you want to save that chip. And Commodore didn’t socket its RAM chips from the factory, so replacing those means desoldering chips and soldering in sockets to replace them. If you’ve never soldered before, I don’t recommend this as a first project.
You can test the power supply ahead of time, but just because it delivers a steady 5 volts now doesn’t mean it will do so forever. The only way to continue to use a vintage power brick of this type is to use a Commodore overvoltage protection circuit that cuts power when the power supply fails and starts delivering too much voltage.
Poor component quality
Second, Commodore didn’t exactly use the highest quality components in the power supply. This makes them more likely to fail in the first place. If you’re lucky, they just die. But if you’re unlucky, they overvolt components and you end up with a C-64 that powers on but only gives you a black screen and doesn’t function. Hopefully you notice the problem before it starts frying chips you can’t easily get anymore.
Commodore 64 power supply repair
Finally, Commodore 64 power supply repair is difficult. Commodore encased the whole assembly in a brick of epoxy. Ex-Commodore engineer Bil Herd says this was a safety feature, to keep power supplies from creating house fires, because it’s nearly impossible to set a blob of epoxy on fire. This may have been the original motivation, but the side effect is it makes it nearly impossible to repair them. That provided Commodore with a reliable revenue stream. A third side effect is that it made it run hot, since epoxy is a lousy conductor of heat. This increased the failure rate. So when these things break due to a five-cent part popping, you can’t open them up and replace them with a 10-cent part.
In other countries, Commodore sometimes skipped the step of turning the power supply into an epoxy brick, but the design is still unreliable. If you have one of those power supplies, open it up and replace the 5V regulator, likely a 7805, before you use it.
If Commodore had a reputation for making junk, the lousy power supplies they made were a big part of it.
Commodore 64 power supply protection
The only way to continue to use a vintage power brick of this type is to use a Commodore overvoltage protection circuit that cuts power when the power supply fails and starts delivering too much voltage.
Safe Commodore power supplies to use
Commodore knew how to make good power bricks. They just didn’t usually choose to do so. But there are two power supplies that fail safe, so you can use them without concern and without an overvoltage protection circuit.
Commodore 64 power supply #390205-01
Commodore bundled a heavy-duty power supply with its 1764 RAM Expansion Units. The part number was 390205-01. These units were overbuilt because they were designed to power expanded 64s. Just as importantly, they aren’t epoxy bricks, so the components can breathe. And when this particular power supply fails, it fails in a safe manner that doesn’t overvolt the computer.
This power supply is distinctive because the Commodore logo is in the corner, rather than in the center of the unit.
Commodore 128 power supply #310416-06
Because the 64’s power supply had such a horrid reputation, many people wonder about the 128 power supply. The 128 power supply, like part# 390205-01, is safe to use. Bil Herd designed the power supply he wanted to design. Like the good 64 power supply, this one has the Commodore logo in the corner, rather than the center.
The thing about the 128 power supply is it doesn’t work with the 64 unmodified. The 128 used a different power connector, which keeps people from plugging it into the wrong port and blowing up a CIA chip. Some people buy 128 power supplies and swap in a cord from a 64 supply. As a 128 fan, I wish people wouldn’t do that.
But if you find a 128 and you’re worried about damaging it by just setting it up and powering it on, don’t. It may or may not work, but you won’t damage it any further than it already might be.
DIY Commodore 64 power supply replacement
For a DIY Commodore 64 power supply replacement, get a 9VAC alarm panel transformer and a 5VDC AC adapter of at least 2 amps, but 4 amps is better if you intend to use RAM expansion. The 5V adapter is dirt common. You’ll also need the cord and DIN connector off an original 64 power supply. If your power supply is missing, buy any original untested 64 brick off Ebay.
The 9VAC lines from the alarm panel transformer connect to pins 6 and 7 on the C-64 power connector. Polarity on these pins don’t matter. The +5V line from your 5VDC adapter connect to pin 5 of the C-64 power connector. The GND line from your 5VDC adapter connects to pin 2. The polarity on pins 2 and 5 is absolutely critical.
Double-check all wires with the pins for continuity and label them as you go, as you can’t assume the color of the wires is consistent.
Wiring up a DIY Commodore 64 power supply replacement isn’t a bad first project. Just make sure you check and double check all connections because there are a lot of chips on the 64’s motherboard that haven’t been made since 1992.