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1980 pacman arcade fix


Prince Vegeta
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Hello everyone I need some guidance. I bought an original pacman aracde machine at a garage sale for $150. I knew it needed work. When I turn it on the marquee lights up and I hear this extremely loud buzzing noise from the speaker and also the only thing I get on the screen is a very faint horizontal white line in the middle that flickers. it's not as prominent as the videos I've seen of vertical collapse. I guess my original thought was to troubleshoot the crt but I'm conflicted due to the speaker issue. Could a bad crt also affect the speaker or could it just be a bad pcb causing both. I watched how to discharge the crt before hand and was gonna look for any cold solder joints or anything obvious just didnt know where or what part to begin with.

 

Thanks a bunch

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Hi, the problem with a garage sale (or any pickup from an unknown source) is you don't know who has messed with it already and your first priority must be to ensure it is electrically safe.

 

Start with it unplugged and inspect it inside for anything loose or any tampering or alteration, especially in the mains and power supply areas to make sure no safety measures have been bypassed including providing power to the monitor. Early machines usually required an isolation transformer between the mains input and the monitor chassis. If in doubt try to find a local arcade machine expert or an electrician to confirm the wiring is safe.

 

Having said that, an original Pacman would have a vertically oriented (portrait style) monitor which is really just a standard unit installed 'sideways' so if it had vertical collapse a line would run from top to bottom as you look at the machine, not left to right and doesn't explain the loud buzzing noise from the speaker as you say.

 

Once you're certain the wiring is safe and with the machine unplugged I would suggest locating the edge connector connected to the game PCB and unplug it, slide it off and leave it hanging safely without touching any other components. Then turn the machine on (if there is an interlock switch on the back door either put the door back or set it to service mode, usually by pulling the plastic switch plunger outwards slightly) and observe the monitor.

 

Any buzzing from the speaker should have stopped as it is driven by the game PCB. Is the monitor running quietly, has the horizontal line disappeared? If you look carefully can you see a background image or raster on the monitor? you might need to locate the monitor brightness and turn it up slightly to see the backgrond raster and it should be a full screen, not just a line.

 

If that worked and you were able to see a background raster on the screen I'd asume for the moment that your monitor is working and the fault is in the power supply area on the game PCB.

 

US Midway Pacman PCBs have the low voltage supply rectification, filtering and regulation on the game PCB itself and a fault in this area could cause the game not to run as well as the buzzing problem if a rectifier or filter capacitor has failed leaving large amounts of AC hum on what should be the regulated DC voltages. It's still a job for an expert familiar with this type of repair and important to avoid running the board for too long in that state to avoid further damage. There is plenty of service information available online for this machine which will help.

 

Hope that helps to identify the problem area at least. Regards, John

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John thanks for the reply. Last night I took out the monitor chassis it appears to be a wg 4600. I couldn't find any cold solder joints or anything else bad on the boards visually. I did find and broken pin on the main board where one of the smaller boards plugs into it. I thought this might be the issue I soldered a wire from the one board to the other bypassing that broken pin and now have continuity between those two points. Threw everything back in still what I believe to be a horizontal collapse. Vertical mounted monitor single line from left to right. After I fixed that pin the line is now yellow and blue instead of white. My only question is the machine in my opinion looked pretty untouched inside but the actual tube itself has an RCA sticker on its rear. Is it possible it was swapped out and the original wg4600 chassis stayed? Also after looking at pictures of the chassis online I did notice my horizontal width coil that is mounted on the metal plate that holds the board has absolutely no wires going to it. And it's weird because I didnt even see any wires just dangling there like they broke off. Is this something absolutely required? Could its absence cause a horizontal collapse. All the pictures I see of that board look exactly like mine except this coil is connected. Didn't know if maybe someone swapped the tube out and disconnected this.

 

Thanks for your time and help.

Jack

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Hi Jack,

 

I'm still doubtful about that monitor issue, 'Horizontal collapse' doesn.t usually occur in the same way as vertical because the EHT generation is part of the horizontal output stage so usually if the horizontal is not working you have no picture at all. Also, if the image is collapsed ( as does happen with the vertical deflection sometimes) the line you see is incredibly intense, so much so that it will quickly burn the phosphor and important not to run the monitor for too long in that state. Effectively every line of the image is superimposed into that one line on the screen, multiplyintg the brightness hundreds of times over.

 

WG didn't make their own CRTs so it's normal to see tubes from another manufacturer in their monitors. That was the case with most arcade monitors and most TVs, for that matter. I'm not sure if the tube originally fitted was an RCA, there are probably a few types which would work and it definitely won't be the original CRT, when these machines were new and being operated almost non stop a CRT would have worn out in a few years. It's probably been replaced several times over. Pacman also has one of the worst 'screen burn' issues as the maze part of the image never moves and is displayed almost constantly, even in attract mode.

 

I'm not familiar with that specific monitor and without seeing any pictures I'm not sure what you mean about the horizontal width coil. It would be required in any case but the actual coil may be concealed within the wider part of the body and would normally be soldered to the PCB. A plastic tube usually extends from it with a ferrite grub screw inside. The image width is adjusted by winding the grub screw in or out from the centre of the coil, using a plastic adjusting tool. It's also important not to use a metallic tool as this will interfere with the adjustment, also the ferrite grub screws are easily broken.

 

With a machine like this where nothing seems to be working it usually helps to look at each section separately if possible. If you can try the monitor with a known source and have the game board checked on a test bench or in another machine that would be a great start. If there's a repairer or someone in your area with a collection of machines they may be able to help with that. Apart from this site the KLOV forums may have members near you with the same machine - at least with Pacman there were quite a few made...

 

Regards, John

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Hi john, the first two pictures are my machine. The first is my screen the second is the board which is at the moment installed. The third picture is one I grabbed online of the same board but that coil is wired up to the board but on mine it is not. I didnt know if the lack of that coil could cause my issue or if that's something more for a fine adjustment. conflicted on whether I should wire it up or not. If I'm gonna throw a bunch of money at this machine I'm just gonna upgrade the screen to LCD I'm hoping to fix it myself but dont really wanna use money on expensive parts that may or may not work or pay for repairs.

 

Thanks

Jack

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Hi Jack, photos help a lot, I see what you mean about the coil, slightly different to ones I've seen which mount to the PCB.. I'll have a look at a circuit diagram & get back to you soon, regards John. Edited by jbtech
fixing typo
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Hi again, Yes, if the width coil is meant to be connected but not, that would effectively leave one end of the Horizontal Deflection coil floating which could explain the issue in your photo...

 

But take care, there are many variations of the WG 4600 chassis, some with provision for width adjustment and apparently some without so some of the components may be deliberarely omitted from the PCB. It could even be that PCB has been swapped at some stage with another which was not an exact match for the original.

 

If you can find an exact model number on the monitor somewhere (e.g. 19K4601 etc.) and compare the circuit around the horizontal deflection with the diagram / parts listing for that model it may provide the answer, what is missing (and why?)

 

As for an LCD display, if you were planning to go down the multigame path an LCD monitor would make sense as most multigame boards support VGA monitors, but an original Pacman PCB won't, as least not without adding a signal converter. And by the time you've replaced the monitor, game PCB and added a suitable power supply the only original thing remaining would be the box...

 

Regards, John

Edited by jbtech
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Hi john, I really appreciate you taking the time to help me. I replaced a capacitor that was broken off I plan I replacing the rest eventually. I tried it still the same problem. I was unable to locate the specific model number of the monitor board which is some variation of the wg4600 so I went ahead and connected that coil. I think it did need to be there because I believe I had and open circuit between the two solder points now I don't. So now my screen does light up when I have the game board disconnected it's a white screen that flashes in kind of a rolling pattern from left to right with the game board plugged in it flashes still but has a mixture of blue and yellow colors. Any idea if this is a step in the right direction? Or what this could be a symptom of? Could it just be a cap issue. I have the reat of them I was trying to isolate the problem before I wasted effort changing them.

Thanks

Jack

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Hi Jack, it looks like a step in the right direction - can you get the image to stop rolling with the horizontal hold / frequency and then vertical hold? Regards John

 

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Hi John, hope your doing well. Since we've last talked I have completely replaced all capacitors and reflowed all the solder on the wg4600 chasis. When I turn the monitor on without the pacman pcb video plugged into the monitor. I get a completely steady white screen which I am able to fully adjust brightness and all the other pots. When I plug the pacman video signal into it everything is moving super fast Its all scrabbled. I see the occasional stuff I recognize like fruit or parts of the maze appear extremely fast and disappear. My question is as far as the monitor goes. Would I be able to tell if it's fixed without any video signal plugged in. Or is it possible my image is all crazy do to the monitor being bad when video is received? I'm just trying to decide if I can move on with troubleshooting the pcb.

 

Thanks again

Jack

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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

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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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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.

 

 

 

 

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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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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

 

 

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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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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...

Edited by jbtech
fixing typos
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