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1980 pacman arcade fix

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Anything to do with repairs.

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  • #16
    Hi Jack, that could still be the monitor or could be the game board but it's an issue with sync either way.

    I assume you've re-tried horizontal and vertical hold on the monitor, can you change the rate of skewing / rolling at all?

    I'm leaning towards it being the monitor not syncing but the other possibility is no sync from the game PCB.

    If you don't have another arcade monitor or game PCB to use as a signal source or other test equipment to view the signals then one rough & ready test would be to take the sync and ground from the game pcb and connect it to the video input of a spare standard tv, using an RCA cable and set it to AV input.

    If the TV complains of no sync your game PCB has an issue, if it just shows a black screen your monitor is bad.

    Hope that helps, regards John
    more from John's Retro Workshop - www.jbtech.linkpc.net

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    • #17
      John thanks for the fast reply. I tried the rca test method. When I hooked the pcb ground wire to the wire that is bare in the rca cable and goes to the outer ring and then hook the sync cable to the insulated wire that goes to the male post on the rca I get a crazy picture like my sync is off. But if I flip the wires the other way it's a calm black screen. So not sure which one was right. I know that pcb has a negative sync so any idea which wire goes to what on the rca cable?

      Thanks
      Jack

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      • #18
        Also I do not have any control over the rate of rolling or skewing as far as the vertical and horizontal hold pots on the machine itself.

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        • #19
          Hi Jack,

          Yes, I'm thinking the sync and the video from the game PCB is not stable -

          Most game PCBs / arcade monitors use 'negative sync' as does the TV.

          To check the sync with the TV the sync signal should go to the tip of the RCA connector and 0V or ground on the game board goes to the shield of the RCA so your first example was correct.

          Ideally the video and sync going to the arcade monitor should first be unplugged so the load on the sync line is correct and it's not going to 2 destinations at once, though that may not make any difference in practice.

          Most modern TVs will give a blue screen or 'no signal' warning if there is no sync at all but will attempt to lock if there is a signal of some sort.

          The other clue is your horizontal and vertical hold on the arcade monitor, even if the monitor refuses to sync or there is no sync present, if the video is stable it should be possible to adjust the horizontal and vertical holds so the 'free running' frequency of the monitor is very close to the correct video rate and the picture should almost appear stationary for a moment before drifting off.

          So the fact you can't get the picture any better makes me think the video is scrambled anyway. It's just a guess and not conclusive unfortunately and doesn't prove the monitor is perfect either.

          Regards, John.



          more from John's Retro Workshop - www.jbtech.linkpc.net

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          • #20
            Hi John, just wanna give another update. I was able to finagle a good test signal to use using a spliced rgb cable and hdmi to rgb converter and program a raspberry pi to output a 15khz signal. The monitor is all good now. How hard is it to trouble shoot a pcb besides any obvious signs like cold solder joints and what not. I'd love to keep this as close as possible to original. Dont really wanna drop $150 on a working board though when there are other ways to get more bang for my buck.Thanks again for all your help

            Jack

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            • #21

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              • #22
                Hi Jack, that's great to see the monitor working. I'm assuming the original board still shows the skewed / scrambled image (?)

                The Pacman PCB could definitely be repaired and I think the issue could be in the sync generation area which would also affect the video timing but it might take some proper test equipment to trace the problem. It's good that there is some output which resembles the game image at least.

                I'd be starting at the sync output with an oscilloscope and if that waveform is not correct I'd be tracing back through the logic to find the point where the timing goes wrong. If that is resolved the game may work but I can't be absolutely certain just from the pictures.

                I'd think the cost to repair would be better than the replacement cost, that would certainly be the case here where complete working PCBs are not so common.

                There's a lot of troubleshooting information online for the Midway Pacman PCB, maybe if you spend a bit of time searching the symptoms you will find some suggestions to narrow the search down to a smaller area or even a specific IC, with some luck.

                Regards, John

                more from John's Retro Workshop - www.jbtech.linkpc.net

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                • #23
                  There are 4 74LS161 or LS163 chips at 2R/2S, 3R/3S on the PCB... check them for failures. They are common failure points and are clock dividers necessary for proper operation of the board.

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                  • #24
                    Just wanted to give another update I was looking closer at the original power supply transformer and noticed that one of the 12v leads was ripped clean off to the fuse block. wire was missing completely the whole tap was missing from the transformer. Purchased a new switching power supply and bam. She fired right up. So the only other question I have is I'm totally happy with the picture I could live with it. But is there any other way to focus the picture. I've tried the focus adjustment on the flyback which does absolutely nothing. Also I really didnt give her a.chance to warm up. maybe a minute or two. But it is totally livable I'm not really sure how sharp these old crt ever where. i realize this isnt an lcd. Other than that i just wanted to thank everyone for the help especially John. Its crazy, i realize I got this from a garage sale non working but man I would of never realized I'd have entire wires missing from the width coil and power supply. With no trace they where even there to begin with not even hanging there. Guess you can never assume what someone has or hasn't done to something u pick up like that. I'm super excited and feel like I found a new hobby.

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                    • #25
                      Hi Jack, well done spotting that and getting it running!

                      Yes, that picture is very fuzzy and the focus pot should fix that or at least make some improvement. If it makes no difference at all something is not right.

                      You could double check the focus wire, which should emerge from the flyback transformer but is much smaller than the EHT wire, should be properly connected to the focus pin on the CRT neck via the CRT socket / neck board - Do that with the power removed - make sure it´s not loose or disconnected entirely.

                      Otherwise it would probably need a replacement flyback transformer, which would be readily available for that chassis and not too expensive especially in the U.S.

                      Regards, John.

                      P.S. It´s a great hobby, very rewarding and very addictive...
                      Last edited by jbtech; 28 July 2021, 03:49 PM. Reason: fixing typos
                      more from John's Retro Workshop - www.jbtech.linkpc.net

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