diagnosing midway pac-man arcade with logic probe or scope

JimD53

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I have a pac-man that I need to diagnose to figure out why I have a white screen and no sound. Looking at the board I can't figure out a way to use a logic probe or oscilloscope the parts under the two daughter cards. I need to look at the z80 pins to see if it is running. Short of flipping the board upside down... Ideas will be greatly appreciated.

Are there any informal repair log documents for a pac-man arcade?
 
Will the game run, albeit in a degraded mode with the 284 & 285 daughter cards removed? That would simplify probing the z80 & 2114 ram.
 
Parts of the board might run, but you're not going to get much out of it with them missing.

First test would be to check the power at the PCB by checking it at a TTL chip.

Testing +5 on TTL.jpg

You can flip the PCB over and probe it from the bottom, or you can use the small Test Edge Connector:

1730516092544.png

The pin out of that connector matches the schematics. The number in the circle is the pin number on the edge connector.

1730516238098.png


After testing voltage, I would check the Clock (Pin 6 on the CPU, Pin 18 of the Test Connector) to make sure that is working.
If you have a clock, then check to see if the CPU is being reset, or if it's held high or low all the time.
 
I'm pretty new as well working on a board. I wasn't getting a boot until i got a known working daughter board. There was no way to know if 285 was good or not.

You can try https://lawnmowerman.rotheblog.com with your boot diagnosis

You can also try fixyourpacman and mikes arcade repair logs

Without 285 you will get something like the attached
 

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Parts of the board might run, but you're not going to get much out of it with them missing.

First test would be to check the power at the PCB by checking it at a TTL chip.

View attachment 778667

You can flip the PCB over and probe it from the bottom, or you can use the small Test Edge Connector:

View attachment 778665

The pin out of that connector matches the schematics. The number in the circle is the pin number on the edge connector.

View attachment 778666


After testing voltage, I would check the Clock (Pin 6 on the CPU, Pin 18 of the Test Connector) to make sure that is working.
If you have a clock, then check to see if the CPU is being reset, or if it's held high or low all the time.
what is the correct clock at pin 6 of the z80? I want to make sure the clock is working properly
 
I found https://lawnmowerman.rotheblog.com/#signsoflife . I also found the pacman/ms pacman troubleshooting guide V2. The clock is 3mhz and looks fine going into the Z80. I also checked the reset pin which stays high most of the time but I see the signal going low very frequently. Now to figure out why it keeps resetting.
 
Last edited:
I found https://lawnmowerman.rotheblog.com/#signsoflife . I also found the pacman/ms pacman troubleshooting guide V2. The clock is 3mhz and looks fine going into the Z80. I also checked the reset pin which stays high most of the time but I see the signal going low very frequently. Now to figure out why it keeps resetting.
Sorry, you are correct, that should have said 3 but I had 1 on the brain :)
 
This thread served its purpose so I'll open a new one on the correct forum to address the reset problem I observed.
 
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