Atari 2600 - Part 3
I don’t appear to have written about it, but quite a few years ago now I bought an Atari 2600 lot. I think I paid about a hundred bucks for it, “unknown condition” (I’ve since learned that it’s convenient to “lose” the power supply and go “yeah I can’t test it” when it absolutely does not work… not saying that’s definitely what happened here but it’s a possibility!). I bought a power supply for it, fired it up, and yep it did not work.
I sold off a couple of the games, I think I got back about $40AUD, so it seemed a reasonable price to me. I’d later learn that Star Raiders, with the touchpad controller, is worth a bit so I still did okay even if the console itself is a shelf ornament and nothing more.
Fast forward more than a year, and I got an oscilloscope. I fashioned together what I think is a functioning NOP generator, unfortunately I didn’t write down the URL I got the idea from and now I can’t find it. With this, I confirmed with reasonable certainty that the 6507 CPU was cactus, and put a pause on the whole thing because a 6507 is $40 on eBay by itself, I wasn’t completely certain it was the 6507 that was dead, and I wasn’t sure nothing else was dead too!
Fast forward once more to this weekend, where someone was advertising a 4-switch 2600 for $40, “doesn’t work, couldn’t get it going”. At least they’re honest. It was about an hour away in a very rural town, but that’d make the cost of it just a bit more and I didn’t want to give up any of my weekend, so I asked if they were coming in to Horsham any time soon, and could they bring it in… I didn’t mind if someone else turned up with cash in the mean time, don’t hold it for me or anything, but yeah. I figured that there was a fair-to-good chance it was anything but the CPU that had died (from what I understand, the CPU, the RIOT, and the TIA, all fail at about the same rate, the chance that two chips died notwithstanding that puts me at about a 66% chance?). Anyway, they were coming in to Horsham on Wednesday, so if they still had it they’d bring it then, and sure enough they dropped it off this morning.
After work, cracked it open and had a look. With a cart in it, I apply some power, and probe the data+address lines of the CPU, and I’ve got signals so I’m pretty sure the CPU works, but it too has a black screen. But since I’ve got signs of life from the CPU, let’s throw it in my light-sixer and see what happens?
Sure enough, after a couple of cycles inserting the cart (figured out later because I wast testing with Pitfall, and it’s the one cart I have that’s a bit dicky on insert) the game came up! Only two issues remain: the sounds are terrible (basically snow), and the fire button on the left player doesn’t work.
I traced that around a bit, reflowed some possible dry joints, and figured out that A203, a “hex non-inverting buffer” used as a driver for a couple input and output lines to the TIA looks like it’s cactus. That might be why the sound’s dead too, not sure if the sound goes via it (the schematics are a bit hard to read in the PDF I have) but on the right player fire button it’s held at 5VDC by a 10kohm resistor and on the same pin on the left player fire button it’s floating at around 1.5VDC.
Thinking maybe I could rob A203 from the other board, I had a look and there isn’t one! Check the service manual and yep, they’ve done away with that for cost-saving reasons. But I wonder if I can throw all three chips into the four-switcher? The part numbers on the TIA were different, but there’s nothing in the service manual that says I can’t. Did that, and I have a fully functioning 2600A, so that’s good enough for me.
So I buttoned everything up, finished up a really long thread on Mastodon documenting it, marked the 6507 as definitely bad with a paint pen, and that’ll do for now.