Pac-man board, no video

wolfwood

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Hi,

I have a stock 1980 Midway Pac-man board, and it was working fine until I recently lost video output. The game still boots, as I can hear game sounds after I hit the start button and move the joystick. I tried reseating some of the socketed chips and adjusting the power supply voltages, but it didn't seem to help.

I'm a novice at board repair, so does anyone have any advice about what to check? I just bought a logic probe, but I'm sure I'll need some direction regarding its proper use. :)

Thanks for any suggestions you may have.
 
Are you absolutely sure it's the game board and not the monitor? Monitor failures are rather common things, but a board failing with this symptom is not.

-Ian
 
Are you absolutely sure it's the game board and not the monitor? Monitor failures are rather common things, but a board failing with this symptom is not.

-Ian

Yes, I am sure. I hooked up a different board to the same setup, and it worked fine.
 
There's a PROM at 7F, if it's completely dead or missing, it could cause this. It's socketed, so you can swap it between boards.

-Ian
 
I'd start at the color PROM at 7F. This generates the Red, Green and Blue signals going to your monitor. Check the inputs at pins 10-13 with your logic probe, see if they're toggling.
 
Yeah, check 4A and 7F for activity with a logic probe. There is also a block of 4 74LS161 at the lower left side of the board you should check out.
Can you hear the board playing blind?
If you need the proms I have them on my website, cheapest your going to find them.
 
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I'd start at the color PROM at 7F. This generates the Red, Green and Blue signals going to your monitor. Check the inputs at pins 10-13 with your logic probe, see if they're toggling.

Thank you all for your suggestions.

If 7F is good, should I expect these pins to be constantly toggling? I suppose I'm asking more of a logic probe usage question. For example, if I boot up the board and place the logic probe on pin 10 for some amount of time, should I see the pin go from low to high without any further intervention from me?
 
If 7F is good, should I expect these pins to be constantly toggling? I suppose I'm asking more of a logic probe usage question. For example, if I boot up the board and place the logic probe on pin 10 for some amount of time, should I see the pin go from low to high without any further intervention from me?

Pins 10, 11, 12 and 13 are the inputs to the PROM at 7F. They should be toggling (show up as pulsing on your probe) in normal operation. If they ARE pulsing, and the output of 7F (pins 1 through 8) is NOT, then 7F is bad. If the input is not pulsing, then something further back in the circuit isn't working, and you'll have to follow it back.

-Ian
 
I'm not exactly sure what you mean by this. I can start a game, and I can hear Pac-man chomping pellets if I move the joystick. Is that what you're asking?

"Playing blind" is a phrase used to refer to a game that has no picture on the screen, but the sounds seem to work normally, as if you were playing the game but were blindfolded. It's usually used to refer to a game with a broken monitor, but it works in any situation where a game makes the normal sounds (coins up, start game, hear game sounds) but produces no video.

-Ian
 
I'm not exactly sure what you mean by this. I can start a game, and I can hear Pac-man chomping pellets if I move the joystick. Is that what you're asking?


Yes, it just verifys that the board is booting and working other than the video.
Dont forget it could be the prom socket as well.
 
Pins 10, 11, 12 and 13 are the inputs to the PROM at 7F. They should be toggling (show up as pulsing on your probe) in normal operation. If they ARE pulsing, and the output of 7F (pins 1 through 8) is NOT, then 7F is bad. If the input is not pulsing, then something further back in the circuit isn't working, and you'll have to follow it back.

-Ian

Thanks for explaining that. It makes perfect sense, and I now see how to at least begin to tackle the problem.

On a related note, I'm looking at the board schematic, and it appears as though the video sync line comes out of what I can only guess is some sort of a bus. How would I go about following that kind of a path back?
 
The easiest way is by reading the schematic. Schematics are like a map of the circuit and are easy to follow once you get the hang of it.

I'd suggest looking at this link.

Page 50 describes the PROM at 7F

Page 60 is the schematic. Look at the lower right corner and you'll see the PROM and its outputs. Note the output just above it labeled /CMPSYNC which stands for "composite sync." In other words it has both the horizontal and vertical sync signals combined into one for the monitor to process/use.

If you follow that line back to the big bar it's connected to that signifies a bus or simply a grouping of signals. Follow that bar around until you see the /CMPSYNC signal come off of it. You'll find that over on the left side of the schematic where the signal is coming from pin 11 of the IC at location 5N on the board.

If the sync checks good with a logic probe and you swap 7F but still have no outputs... check backwards in the circuit. If you have inputs on 7F but no outputs then check signals on pins 1 and 15 of 7F as those are the pins that enable the chip to output data.

RJ
 
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