Here's a quick one. I wasn't going to do any repairs tonite, but I was pretty bored so I figured I'd start testing a pile of arcade boards. On the bench is a galaga. After cleaning the chip legs of the customs (they are always corroded), I fired up the game. To my suprise it booted up just fine!
The problem was that some sprite colors were off, specifically any of them that move such as the ship and the bugs.
Thinking about the issue, it dawned upon me that the sprites are controlled by the set of eight 2147 SRAMs. One thing I noticed while probing around on the RAMs, is that the data bits on 6D and 6K were stuck high. Tracing it back, it goes to two 74ls365 buffer chips. Probing those showed that those were functioning as they should. Which means that it was either the PROM at 1C or whatever was controlling it.
Looking at it with the logic probe, data bit 1 was stuck high, which is not correct for the chip. Swapping it with a known good one brought back all the colors. This is kind of a bummer, as the PROM is a 82S129, which are getting harder to come across. On top of that, I don't even have a way to program them currently.
One more thing, I attempted to do a youtube video on this! Give me some feedback!