centipede pcb need help

RedlandMan

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I purchased a new pcb for my centipede a while back ago and now that a have a test bench i decited to give repairing it a try. So far ive checked all voltages to eproms and cpu. Ive swapped out working cpu and pockey. Removed,cleaned, and checked eproms. I originally had a screen with mostly zeros after everything i did this pic is what i have now. The reset pin to the processor is constantly going low high. I tried disableing the watchdog to try and put it in test mode but no go. That led me to look at L2 outputs on pins 8,9,11 are going low ,high constantly. replaced L2 but no change. I can leave L2 out and i get the same thing on the screen. I checked the pins on L2 against a working board and they are all the same with a logic probe on my non working board with the exceptions of outputs 8,9,11. Im thinking there must be a problem with the input signal at pin 14 on L2 that i wont be able to detect with just a logic probe. Any input would be great. Thanks,luis
 

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i think i made matter worse by reseating everything. i get the same screen even without the cpu in. Did a continuaty test and found rom sockets not making good contact. I have some on order so ill have to wait till they get in.
 
You can find out for shure if you put your meter on ohms and put one lead on the solder side and the other on the leg of the ic chip near the plastic of the chip, more than .3 ohms of resistance there is a problem. You are only checking the resistance from the chip to the board and it would be better if you had a continuity or a meter with a beeper. The beeper is only so you don't have to look at the meter. I did this with my boards while waiting for parts.
Also and not often one of the feed thrugh holes is wacked/broken. That is where the foil trace goes to a "hole" in the board and continues on the other side. Sometimes this will not pass the signal through the feed through. In this case I would simply just solder a wire or a piece of a resistor lead through the hole and cut short. This is much easier to test for, as you just put one lead on one side and on the other side of the hole to hear/see a low ohm reading.
 
i think i made matter worse by reseating everything. i get the same screen even without the cpu in. Did a continuaty test and found rom sockets not making good contact. I have some on order so ill have to wait till they get in.

Best to wait until that is done to continue troubleshooting - those original ROM sockets are horrible.

Bill
 
replaced 4 rom sockets and i get the black screen with the random letters and if i put it in test mode the screen changes to green. cpu reset still pulsing high low. Sorry about upside down pics.
 

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disabled watchdog and put in test mode but nothing happen but once in test mode if i hit reset i got 2 beeps. replaced 2114 ram and game booted to my first picture ran in test mode again but had to disable watchdog again and got 9 beeps replaced ram at N7.Game booted fine (second pic) but would reset.Put in test mode again and it told me i had a bad rom at J1 . Replaced rom and it works great.
 

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disabled watchdog and put in test mode but nothing happen but once in test mode if i hit reset i got 2 beeps. replaced 2114 ram and game booted to my first picture ran in test mode again but had to disable watchdog again and got 9 beeps replaced ram at N7.Game booted fine (second pic) but would reset.Put in test mode again and it told me i had a bad rom at J1 . Replaced rom and it works great.

Nice work. Thanks for following up with the details of your repair.
 
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