A Barn Find 6502 Is Restored

The phrase “Barn find” is normally associated with the world of older cars, where enthusiasts live in the hope that they may one day stumble upon a dusty supercar lurking unloved for decades on a remote farm. It’s not so often found in the context of electronics, but that’s the phrase that [John Culver] uses for a mid-1970s Atari arcade board that had been through a very hard time indeed and was in part coated with cow dung. It’s interesting because it sports a very early example of a MOS 6502 in a ceramic package, whose date code tells us was manufactured in week 22 of 1976.

Finding a microprocessor, even a slightly rare one, is not that great an event in itself. What makes this one interesting is the state it was in when he got it, and the steps he used to retrieve it from the board without it sustaining damage, and then to clean it up and remove accumulated rust on its pins. We are fast approaching a point at which older microprocessors become artifacts rather than mere components, and it’s likely that more than one of us with an interest in such things may one day have to acquire those skills.

We’re rewarded at the end with a picture of the classic chip passing tests with flying colours, and the interesting quirk that this is a chip with the famous rotate right bug that affected early 6502s. If you are interested in the 6502 then you should definitely read our colleague [Bil Herd]’s tribute to its recently-departed designer, [Chuck Peddle].

26 thoughts on “A Barn Find 6502 Is Restored

  1. What are your guys thoughts on putting dirty circuit boards like this through a dishwasher?

    Years ago I had read about electronic collectors using a dishwashers to clean circuit boards that have been through a flood, or extremely dirty.

    I have successfully used a dishwasher to clean several of my cell phone main boards, as a last ditch effort when they started locking up. At the time, I had a dirty job, with lots dirt and fine conductive dust floating around. I’d still use washing detergent, but way less, like 1/8th the normal amount. Use twist ties to suspend the board vertically, and run it though an entire cycle, including the heated dry. Boards came out squeaky clean, and worked great for many years after.

    I think a dishwasher should be looked at as a legit tool for restoring circuit boards like these. Some say “OMG WATER?! NOOO” In my opinion, water is a lot less destructive to a circuit board than the harsh chemicals used by the author. You could always final rinse your board with RO-DI water to get rid of any lingering contaminants. I’d say the only caveat use to make sure everything is 100% dry when done. Baking the parts in a toaster oven at 150’F for 24 hours after washing to drive all the moisture out.

    1. Wash with horsehair brush (be reasonably gentle scrubbing) + dish soap + warm water in a large sink or tub. Rinse. Pat dry with towel. Submerge or soak very well with 100% isopropyl (this draws out remaining water and dissolves some things water and soap won’t). Dry with heat gun on lower temp or hair dryer. I do it all the time for all sorts of grungy circuit boards at work. If you don’t have expensive 100% isopropyl, the 91% from the drug store works fine.

      1. In the last few years, I’ve learned to be careful with isopropyl. Search “isopropyl crazing” for my reason why. It’s not as an innocent chemical as it seems to be. Isopropyl is tough on plastics, specifically acrylic, but damage may not be readily apparent in opaque plastics.

          1. Don’t mix your drinks too strong in your poolside acrylic glasses… Haha

            From what I’ve read, reduce the percentage of isopropyl to water. Use enough alcohol to get the job done, but not cause problems.

            This chart helps too:
            https://www.plasticsintl.com/chemical-resistance-chart

            Don’t get me wrong, isopropyl is a great chemical, but needs to be used in moderation. In full strength, It’s great for loosening hot glue though!

    2. I’m not afraid to use water to clean electronics. Where you’re dealing with things like soda spills, water is the best solvent. I saved a co-worker’s Mac keyboard this way after a Coke spill.

      I lean towards 8bits2wheels’s approach – gentle hand-scrubbing with water, a bit of dish soap and appropriate brushes. I will use distilled water if it’s available. Compressed air is good for (gently!) blasting off the water, then some extended drying.

      Isopropyl alcohol on a Q-tip has been ok for spot-cleaning like removing rosin. I haven’t tried rinsing with it.

      1. Water itself is bad in a sense too. General tap water contains ionised minerals. And these get deposited on your board, when the water dries. And they are both conductive and corrosive. So, you can wash the board with normal tap water. But you should always rinse afterwards with deionized water. Or rinse with something that can displace water (like alcohol, but be careful with alcohol and plastics).

    3. Go for it. Use a dish washing liquid instead of the stuff made for dish washers, it is too harsh. That is how the boards were cleaned after wave soldering during manufacture. Take photos of anything with paper or printed info/labels first.

      1. Up until the time that it was found to be an environmental problem, a Freon® was often used to clean flux from PC boards. Hold the PCB above a boiling tub of Freon®, it condenses on the PCB and washes the flux into the tub. As the use of CFCs became restricted, the search for water-soluble fluxes intensified.

    4. I’d put newer boards through a dishwasher. I have done this before. I always run the machine twice. The first time empty to ensure any remaining detergent was flushed out and the second time with boards (no detergent!). For something you really care about following up with a good distilled-water rinse can’t hurt.

      I thought I remember some of those old CPUs having ceramic rather than plastic cases. Could that absorb water? Same goes for ceramic caps although those aren’t a big deal to replace so long as there aren’t too many.

    1. Flyball’s ROM data can be found by looking through MAME romsets. Most of the board schematic’s in the link I shared above, although it’s notably omitting the CPU and code ROMs schematic

    1. Yup. That crosses a line for me. Previously I would have trashed such a thing had I come across it. But if there are people THAT dedicated to restoration I guess If I’m ever cleaning out an old barn I could seal it up in an air-tight bag and ship it to some tenacious HaD’er.

      1. Cow (herbivore) poo is not usually as disgusting as omnivore (e.g, dog, pig) poo. Sure, you don’t want to tread in any of them, just pointing out that you can find worse poo in any street, any day, than was found in this barn.

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