What to look for in a Commodore 64

If you’re into retro computing, the Commodore 64 is hard to escape. It was the most popular computer model of all time, so it was everywhere in the 1980s. But the machine does have its quirks, which can be either endearing or maddening. Here’s what to look for in a Commodore 64.

Certain models and eras of Commodore 64 are more dependable than others. Early models will appeal to people who want something slightly challenging to repair, while a newer model is definitely better for someone who wants a retro machine that works without putting up a fuss.

The challenge of the C-64

what to look for in a Commodore 64
Finding a Commodore 64 isn’t too hard. But finding a working C-64 can be if you don’t know what to look for in one.

Commodore diehards were every bit as loyal and dedicated as Apple diehards. The difference is there weren’t as many of them. Much like Apple machines, Commodore machines weren’t perfect, and that’s a turnoff for some.

It helps to understand what Commodore was. Commodore was a vertically integrated company that made its own chips. That included CPUs, I/O, video and sound chips, and even the TTL glue logic chips that held it all together. Commodore’s chipmaking subsidiary, MOS Technology (not to be confused with MOSTEK), tried to make it all, including memory chips. MOS realized early on it wasn’t good at making memory chips, so you won’t generally find MOS memory chips in a C-64. Every other chip on the board was fair game. MOS couldn’t keep up with demand so Commodore did turn to outside suppliers for some chips, but keeping things in-house helped Commodore meet those aggressive price points it maintained from 1980 to 1984.

The various reasons why MOS chips fail in C-64s

Many MOS-brand chips are failure prone, and that’s led some people to conclude MOS didn’t know what it was doing. But different MOS chips fail in different ways. The I/O chips are failure prone, but one of the main reasons they fail is from hot-plugging peripherals while the machine is powered on. Commodore machines weren’t designed for that. If you don’t hot-plug them, MOS I/O chips are no worse than anyone else’s from the era.

But some MOS chips really were junk. There’s a chip in the C-64 called the PLA that accounted for 40% of C-64 failures once Commodore started producing that chip in-house. Commodore messed up the chemistry in that chip and eventually the MOS PLAs self-destruct, even if they’re just sitting on the shelf. The MOS TTL logic chips also weren’t great. Eventually MOS realized they weren’t good at making those either, and stopped.

The power supply

The other problem with Commodore machines was its power supply. Commodore cheaped out on the C-64 power supply, and its 5-volt rail has a tendency to deliver much more than 5 volts when it fails. Usually it’s a memory chip that fails first, and the result is a Commodore 64 that shows an incorrect memory size when you power it up, a scrambled screen, or even a blank black screen. The power supply you use is important.

Heat

MOS chips run hot. Unlike other chip makers who set out to make their chips smaller and more efficient every couple of years, Commodore revised its chip manufacturing processes as little as possible. That means the chips run hot. And for several years Commodore used metalized cardboard to keep C-64s from interfering with television broadcasts. This trapped the heat inside the machines. Eventually Commodore switched to metal shielding and used the shield as a giant heat sink. The shield wasn’t a great heat sink, but it dissipated heat rather than trapping it.

What to look for in a Commodore 64

If you want a Commodore 64 that’s going to work without a lot of fuss, you’re much better off looking for a Commodore 64C, the streamlined cream-colored model from 1986-1993. Commodore revised this design a lot over its 7-year lifespan, but it had several things going for it from the beginning. By 1986, Commodore was using metal shielding. Also, by then Commodore had a good idea what chips it could make reliably, so you’re much less likely to end up with obscure TTL faults in a minor chip. And by 1988, Commodore integrated the dreaded PLA chip with some other chips and farmed out production to other companies. Those companies got the chemistry right, so their equivalent chips are much more reliable.

Later 64Cs also had fewer memory chips. The smaller number of memory chips meant less failures, and when you do have a memory issue, it makes it much easier to fix. It’s only two chips, so replace the one that feels hotter than the other, or just replace both of them on the theory that if one chip died, the other isn’t far behind it.

And while the 64C shipped with the same dreaded power supply, its tighter integration made it more power efficient so the power supply was a little less likely to fail catastrophically.

Similarly, if you find a newer-model 1541-II disk drive, it’s more likely to be in working order than the infamous brown bomber 1541s.

What to look for in a breadbin-style Commodore 64

But what if you like the darker pre-1986 Commodore 64? While a brown breadbin-style C-64 is less likely to be reliable, certain models did fine. By 1984, Commodore was using metal shielding, so if you can find a 1984-1986 breadbin model, there’s a much better chance it’ll still be in working order.

Unfortunately there’s nothing on the outside that tells you definitively what year the machine is from. The best you can do is look at the serial number, if you can. Then look at the C-64 serial registry and scroll down. Find some serial numbers in close proximity to the machine you’re examining. If they have a 250425 motherboard revision in them, it’s a 1984 or newer model.

If you can open the machine before purchasing, look at the shielding. If it’s metal with tabs and holes, it’s a new-enough model with a reasonable chance of working. If it has a cardboard shield, it’s an earlier machine. My success rate with 1983 machines is very low. I’ve owned three of them over the years. One was broken when I got it. A second failed on me within days. A third one worked, and I packed it away in working condition. But when I pulled it out of storage in 2019 it failed on me within an hour.

Frequently you can piece together a working machine using parts from several. But in my case, my dead chips weren’t in sockets, so I couldn’t swap them with chips from another one. That’s another nice thing about the 250425 boards–most of the critical chips in those boards are in sockets, so you can swap them.

Fixing Commodore 64s

If you have desoldering equipment and an oscilloscope and like using them, fixing 1983-era Commodore 64s can be fun, especially if you have a working machine you can swap chips from. It can be profitable too, since there are more people who want a C-64 than there are people who have the knowledge and equipment to fix them.

If that’s you, fantastic. If it’s not you, that’s fine too. I was never great at that kind of work and no longer have a steady enough hand for it myself.

Even if you don’t have that kind of equipment, if the chips are in sockets, you can swap in chips. The single most failure-prone chip in the C-64 is the PLA, and there are modern, reliable replacement PLAs available. If you have a dead C-64, especially one that shows a black screen or scrambled screen, there’s about a 50% chance it’s memory and a 50% chance it’s the PLA. The PLA in older breadbin-style 64s is the 28-pin chip near the center of the board not marked 6581. Commodore’s part numbers for it were 906114-01, 251064-01, 7700, or 8700.

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