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Do the date codes in a retro computer matter?

Youtuber Dave Plummer made an interesting argument in his PET 2001 repair video. He said he wants the date codes on his chips to match as closely as possible, arguing that it matters to vintage car enthusiasts, so it’s going to matter in the future on vintage computers too.

He has a point.

The arguments against matching date codes

date codes on vintage computers

The date codes on this Commodore 64 motherboard raise questions. Most of the chips range from the 9th to the 28th week of 1982. The 6510 CPU dates to the 7th week of 1983. That suggests this computer was repaired at some point.

There will always be some segment of the population that doesn’t care about the date codes matching. As long as the part fits and it works, they’re happy.

Some will draw the line at different places. They may prefer original parts but not care about when the parts were made. Or they may want modern parts, with the theory being that modern replacements will be more reliable. Most probably draw the line somewhere in between, as a matter of practicality. I put 1980s 65C22 chips in my Commodore 1541s so they run cooler, for example.

And a modern GAL-based PLA is probably the most practical and cost effective way to replace the least reliable chip in a C-64.

The other thing to keep in mind is that a machine doesn’t necessarily have to remain in that state forever. If the machine doesn’t work and you want to actually use the machine, it’s probably worth more to you in a functioning state, even if that means a mishmash of parts, than it is sitting on your bench in pieces for years or even decades. You can always leave notes to yourself stating what non original parts are in there, with an eye toward replacing them as parts of matching vintage become available to you.

The argument for matching date codes

The most valuable collector cars have parts of matching vintage all around. The ideal is a car that sat parked in a garage or a barn for decades, was barely driven at all, and basically sat untouched for most of its life.

Those make for great stories, but I wouldn’t say it’s something that happens frequently. What happens more frequently is someone buys a car and then restores it. If they have a lot of patience and a lot of money to spend, they may be able to turn half a car they found in a field into something that looks like that barn find. Someone who likes cars but has a modest budget might be happy just to get the thing running.

And there’s nothing wrong with that approach. It’s supposed to be fun. That vintage car that is a mishmash of parts that either happened to work together or that a tinkerer was able to make work together will never be as valuable as an example where everything matches.

And I think that is Dave Plummer’s argument.

What constitutes a close enough match?

When you open up a computer and look at the date codes on the parts, they are probably not going to match exactly. The newer the computer is, the closer the matches will probably be, because of tighter supply chains and just in time manufacturing. But when you are talking systems built before about 1990, it really seems like manufacturers but chips, sorted them into bins, and the new inventory ended up piled on top of whatever old inventory was in the bin. There was no reason to rotate it, because computer chips don’t spoil like groceries. Or if they do, it takes decades, and these devices were intended to last 3 years.

A board assembled at the beginning of the day probably has parts with reasonably consistent dates on it. A board assembled toward the end of the day, with parts from the bottom of the bin, are more likely to have more variety.

You can find maximum variety with popular machines that had supply chain issues built at the height of those supply chain issues. For example, in 1983 and 1984, when Commodore was selling Commodore 64’s faster than they could make them. So they would assemble boards as completely as they could. If they were missing parts, they just left a socket in its place. Then they stored the boards up until parts arrived. When parts arrived, they would pull the boards off the stack. They’d populate the missing parts, and then finish assembling a complete computer from it.

Generally speaking, it’s hard to go wrong with parts that are within six months of the rest of the parts on the machine. But the more you know about the specific machine, the better judgment you can make about what would be normal and period correct.

I’d pay more attention to it on an early Commodore 64 than I would on a common revision.

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