Centipede's 6502 Reset Line stuck low

TheDrewster

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I am troubleshooting a few Centipede boards that I bought a while back. I am using my new logic probe (man I love this thing!) and I am poking around to figure out why I am not getting any boot. The Reset line for the CPU is stuck low, so my first inclination was that I had a bad L2. I probed it and it looks like the inputs are stuck high. For fun, I replaced the L2 and had no effect. Since I dont have any schematics handy at this very moment, can someone tell me where L2 gets its inputs from and/or where I need to probe next?
 
Here's a portion of the reset circuit. L8 and 2 transistors. I'm thinking your problem is going to be w/ the watchdog timer. Probe pin 2 of L8. If that is stuck, you could try to ground the watchdis tab.
 

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My watchdog is reading high at the WDDIS test point, which I believe is normal operation (my known good PCB reads the same). Grounding the tab does nothing.
 
Alright!! Board is (mostly) repaired! Here is what I went through, with the help of a very generous KLOVer who guided me through it.

First off, the inputs at L2 were stuck and not pulsing as they should be. I was instructed to check the bi-polar PROM at P4 for pulsing signals on pins 1-7. They were all stuck low. Reseating the chip fixed this issue and I regained the pulse on the inputs of L2.

So now, the reset line was pulsing. I put the game in test mode and upon powering on, I was greeted to a bright red screen with a loud onslaught of various game noises, eventually turning into a never ending series of tones. In order to figure out why, I had to ground the watchdog to allow the CPU to do its thang. I grounded the WDDIS test point and I get 4 solid tones, indicating bad RAM at K5. I replaced K5 and the game officially boots into test mode, where I am greeted to a ROM 0 error, indicating an error on D1. Reseating the ROM also fixed this issue.

I can now boot into attract mode and the game plays. Well, sorta. Now I come to find my 4584 Hex triggers are DOA and the trackball does not move the archer. This is an easy fix I have done before, but I hope this little repair log will help someone in the future!

Stay tuned for episode 2 ;) I have one more board that needs to be fixed!
 
Alright!! Board is (mostly) repaired! Here is what I went through, with the help of a very generous KLOVer who guided me through it.

First off, the inputs at L2 were stuck and not pulsing as they should be. I was instructed to check the bi-polar PROM at P4 for pulsing signals on pins 1-7. They were all stuck low. Reseating the chip fixed this issue and I regained the pulse on the inputs of L2.

So now, the reset line was pulsing. I put the game in test mode and upon powering on, I was greeted to a bright red screen with a loud onslaught of various game noises, eventually turning into a never ending series of tones. In order to figure out why, I had to ground the watchdog to allow the CPU to do its thang. I grounded the WDDIS test point and I get 4 solid tones, indicating bad RAM at K5. I replaced K5 and the game officially boots into test mode, where I am greeted to a ROM 0 error, indicating an error on D1. Reseating the ROM also fixed this issue.

I can now boot into attract mode and the game plays. Well, sorta. Now I come to find my 4584 Hex triggers are DOA and the trackball does not move the archer. This is an easy fix I have done before, but I hope this little repair log will help someone in the future!

Stay tuned for episode 2 ;) I have one more board that needs to be fixed!

I'm pretty impressed that you troubleshot that without having schematics. That's some nice work.
 
Is there a good tutorial out there on getting started with PCB repair using a logic probe? I would love to get into this more and really try to understand what I am doing.

Great work BTW, I am sure that was an awesome feeling when the game booted. I have two dead boards that won't boot I would love to take a stab at (Off road and Rastan).
 
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