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I'm writing and posting this mainly for my own record as I'm trying to teach myself how to diagnose and repair basic board issues, but also maybe this will help someone down the line. Full disclosure, I'm going at this with only a DMM and logic probe. I know I know I know, I need a scope, especially since I own both an Asteroids and Tempest. But I wouldn't know what to do with a scope if I had one. That will be next on my list of things to learn once I get better at simple fixes.


Background: I picked up a non-working Centipede -01 PCB for $50 on ebay. The board is pretty clean overall. There is no scorching on the edge connector. It is missing all 6 ROMs, the EAROM, both 8-pin dip switches, and a LS42 @ H3.

The board had three broken traces on the backside near the edge connector that were jumpered. After cleaning all the socketed chip legs and washing/drying the entire board overnight the first step was to get rid of the jumpers and repair the broken traces. It was a fairly easy fix. I tested for continuity afterwards and confirmed it was good.

I burned new ROMs, socketed/replaced the LS42 @ H3, the missing dip switches, and EAROM. Time to plug it in to my Centipede cab..

+5 voltage was good (4.998), TIP102/6044s all tested good. -15 was good. +15 was dead. Replaced it with a new 7815. The screen was filled with garbage at startup. Sometimes it's a black screen with red garbage, sometimes a yellow screen with red garbage, sometimes it's just a white screen with nothing at all. I was unable to get a test screen or boot to test. Just solid white screens or garbage screens.

Pin 37 on the 6502 is showing a clock. I'm not sure if it's a good clock, but a clock none the less. The clock has been the only constant while probing around the last two days. Pressing the reset button has no effect on the clock while probing. Pin 40 was stuck high (which should be normal operating procedure?) After I took a break and turned it back on, it was stuck low. Reset button has no effect on it while probing

I probed all of the address and data lines on the 6502. Every time I turn the game on and probe them I would get something totally different.

First check of the data lines showed everything stuck low. Based on the schematics, I immediately thought the LS245 @ E2 is bad. However, after turning the game off and then back on, the data lines were almost all pulsing, with only one or two stuck low. Same issue with the address lines; sometimes high, sometimes low, sometimes pulsing. What are the odds that the 244's AND 245 are bad?

I placed a DigiKey order for a bunch various chips that I might need for repairs, but I'm in a holding pattern as my order wont be here until after the New Year. Don't worry, I'm not shotgunning anything, I just want to be prepared.

In the meantime, more reading and watching videos. Happy New Year...

EDIT 12/31/24: After actually thinking about this a little more, it appears that there is probably something wrong within the reset circuit. For the most part, pin 40 on the 6502 is stuck low. Sometimes when turning the game on it was stuck high and would then get stuck low after hitting the reset. Regardless, there was no immediate low to high shift when powering up the board. I do have a functioning Centipede board here to compare with while I wait.
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