Space Invaders - Midway - garbled screen

N24005

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Midway L board Space Invaders..

Power verified at board.. reset signal operating properly. Clock section seems fine. Watchdog fires when H ROM is pulled but does not fire when test ROM is running (as expected). Also following the 8080 test procedure without luck.

cap kit and RAM socketed. Same symptoms before and after. Daughter connector replaced, edge connectors cleaned up and verified. Swapped 8080...

First picture is the display of the v1.3 test ROM. 2nd picture is display of @philmurr test ROM. 3rd is with ROM removed. Also the saucer sound plays continuously.

RAM replaced as indicated by Phil's test. However, no matter how many are replaced, it seems to keep reporting another 1 bad, often 1 I have already replaced.

Chasing my tail. Either the RAM is getting smoked or there is something else wrong.

I have checked the 7404s and there seems to be reliable activity (they have been socketed and swapped).

Grounding pin 7 on various RAM does change the display but does not fix it.

The 8216s are original and appear to have good activity.

This board has seen better days... it was dragged across something that tore up some traces, they were repaired previously. I have verified that there is continuity across all repairs and they match a working board.

I don't know what else I can do to save this board. I'm assuming it is an addressing issue but I don't know where to start
 

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Are you using the version of the test ROM that beeps to indicate the faulty RAM? I included notes as to which RAM it suspects is faulty.

As both the test ROM and the screen display indicate errors you first need to rule out the RAM itself as being the fault. Have you checked for continuity between RAM chips (for all pins except Din & Dout)?
 
looks like you are having ram decodeing problem. need to check outputs of the 9316 or they might be 161 chips feeding the 157 or maybe labled 9322
 
Are you using the version of the test ROM that beeps to indicate the faulty RAM? I included notes as to which RAM it suspects is faulty.

As both the test ROM and the screen display indicate errors you first need to rule out the RAM itself as being the fault. Have you checked for continuity between RAM chips (for all pins except Din & Dout)?
Yes, using the latest version but the saucer sound was drowning out the beeps. Had to turn the volume way up...

10th beep.. replaced 'B'.. then only got the 9th beep, so I replaced A. Then 2nd, replaced 2.. then 10th beep again.

Yes, continuity on all pins and no shorts to adjacent traces or pins.
 
looks like you are having ram decodeing problem. need to check outputs of the 9316 or they might be 161 chips feeding the 157 or maybe labled 9322
Logic probe is showing activity on the outputs of the 9316.. how do I know it's good?
 
probably good then move to next chip in line to ram address. the only signal that is not easily seen with probe is the bank enable going into pin 17 on the rams but it usually is not bad
 
A fellow member figured it out. It was the 157 at F7 itself. All of the outputs were bad. They looked like this on his scope. Thanks Mike!
 

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I was able to get it to boot up by replacing F7, the daughter board needs work to get the game to fully run. I'll returning the board set so he can continue with the rest of the diagnosis. Daughter board needs some love to make it boot and run reliably.
 
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