Ms. Pac-Man PCB blowing both 1A fuses upon startup

mike86325

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I just purchased a Ms. Pac-Man upright as my first cabinet, but I have had zero luck getting it to work. I have no signs of life - no sound, video, door lights. Marquee lights up and the monitor does sound like it powers on as well.

I'm completely new to this, but here is what I've done so far:

1) First, I replaced all of the fuses with brand new ones and turned it on. Coin door lights flicked on for a second before going off. Other than that, no other progress. I turned it off and noticed both of the new 1A fuses blew.

2) Disconnected everything, replaced the 1A fuses, and reconnected things one by one until the fuses blew again. Doing this singled out that the fuses were blowing when the board was connected. Also of note, the edge connector appears to be a bit burnt (see pic). I'm not sure if this would lead to the game being completely "dead" or not, but it's the only thing I can easily pick out with my limited knowledge.

3) Checked voltages of the transformer:

120V input: 124.7V

7VAC: 7.59V
7VAC: 7.58V
Combined: 15.21V

12VAC: 12.89V
12VAC: 12.89V
Combined: 25.80V

4) I then measured the voltage at pin 18, C2 and C3 caps, and all of the 7V pins:

7V pins all looked good: ~7.5 on each
pin 18: 2.216 VDC
C2: 2.195 VDC
C3: 2.208 VDC

I know the board has worked in the last few years since I have played it before. This cabinet was owned by my in-laws friend who recently passed. He had it in his house and under his possession since 1993 so it shouldn't be a total hack job situation.

Hoping I can diagnose (and possibly fix!) the issue on my own. If it seems outside my skill, I'll probably hand it over to a pro, but I'm treating this as an opportunity to learn as much as I can about the cabinet. Thanks to this site, I've been able to do a lot of reading this weekend!
 

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(MS) Pac-man has a double stage power design.
A brick transforms the 120VAC from your wall into the needed 12/7 volt AC

The game board itself has a section in the corner (the large heat sink & resistor) that in turn transforms these voltages to 12V DC and 5V DC.

What you want to do at this stage (and be carefull, the ~120 AC input can and will kill).

1. Watch this video: https://www.youtube.com/watch?v=UklLpDWLqqs
2. Disconnect the board again.

- Measure the 12VAC output
- Measure the 7VAC output

If both the 12VAC and 7VAC from the brick are ok, then you have issues with the power section on the board, which would explain the burned edge, but it's also kind of normal.
You could measure the ohms of the 12VAC and 7VAC inputs/phases and see if any shorts exists. I suspect regulator has shorted.

The good news is, you can add a switching power supply in there and bypass the brick on the bottom, and the power section on the MS Pac-man pcb completely, by converting it to DC power. There's a bunch of (non-invasive) ways to do this, just google them.

You might also want to confirm the monitor at least does something when powered on. That's the second most important thing you need to play a game :)
 
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(MS) Pac-man has a double stage power design.
A brick transforms the 120VAC from your wall into the needed 12/7 volt AC

The game board itself has a section in the corner (the large heat sink & resistor) that in turn transforms these voltages to 12V DC and 5V DC.

What you want to do at this stage (and be carefull, the ~120 AC input can and will kill).

1. Watch this video: https://www.youtube.com/watch?v=UklLpDWLqqs
2. Disconnect the board again.

- Measure the 12VAC output
- Measure the 7VAC output

If both the 12VAC and 7VAC from the brick are ok, then you have issues with the power section on the board, which would explain the burned edge, but it's also kind of normal.
You could measure the ohms of the 12VAC and 7VAC inputs/phases and see if any shorts exists. I suspect regulator has shorted.

The good news is, you can add a switching power supply in there and bypass the brick on the bottom, and the power section on the MS Pac-man pcb completely, by converting it to DC power. There's a bunch of (non-invasive) ways to do this, just google them.

You might also want to confirm the monitor at least does something when powered on. That's the second most important thing you need to play a game :)

Thank you for all of the info!!

Here are my voltages:

120V input: 124.7V

7VAC: 7.59V
7VAC: 7.58V
Combined: 15.21V

12VAC: 12.89V
12VAC: 12.89V
Combined: 25.80V

Are these a bit too high? They seem consistent at least. I'll go ahead and check Ohms next. I haven't found out how to test voltage at the board, but it sounds like the solution is to get a switching power supply no matter what?

The monitor seems alright, I can hear it turn on, but without a working game it's hard to know what shape it's in.

Thank you again! :)
 
Voltages from the PSU seem fine.. so

I am assuming you are blowing the 7VAC / 12VAC fuses, not the mains.
If something else than the 7VAC / 12VAC is blowing, pls. let us know.

If yes, it is likely your pcb is broken, and something is shorting out, causing the fuses to blow... unless:

First, do you have the correct fuses in the right place? You mention 1A, but the 7AC needs 5A fuse for example.

All Cabinets

1) 2 Amp Slo-Blow Fuse for the main AC input. Short in electric wall plug may blow this fuse.
1) 2 Amp Slo Blo Fuse for monitor feed (this is not the fuses on the monitor itself)

Upright & Mini
2) 5 Amp Fuses for 7VAC
1) 1 Amp Fuses for 12 VAC
1) 2 Amp Slow blow for lights & 12 VAC or older cabinets may just be 1 Amp!

More detailled information is here:
http://lawnmowerman.rotheblog.com/

What you can do, and this is easy, just measure for shorts when the board is connected but not powered on. You should have decent resistance on the 12VAC and 7VAC / 0 leads and 0 to your pcb.

Or alternatively, remove the pcb, check for shorts on pins A/B to C/D and 1/2 to 3/4 in all combinations. Post some pics of the power section and big diodes near/behind the edge connector (d3/d4/d6/d8 and so on.)
 
You're correct, the two 12VAC fuses are the ones blowing. They are both supposed to be 1A for my cabinet according to the paper beside the fusebox on the board. All fuses are definitely to spec, I was really careful of that when replacing them as to not do damage.

So it's most likely the board? :(

Ok, good to hear there are ways to check the board without it powered on. I saw a video of a guy measuring voltage across the capacitors with it powered on so I gave that a shot. I was getting some weird readings until I realized that I was dumb to try to read the voltage when the fuses are always blown.

So with that in mind, I should replace the fuses before checking for shorts using your first method I would assume. For the first method with everything in the cab - you're saying connect the black pin of the multimeter to the transformer (either 0, 7VAC, or 12VAC) then the red on the corresponding (0, 7, or 12VAC) part of the edge connector on the PCB? (Somewhat guessing, I'm very new to this).

I could also try the second method. If you know of a link to a guide or youtube video for dummies that explains either method, that would be awesome. I'll take pictures and work on this later today.
 
You're correct, the two 12VAC fuses are the ones blowing. They are both supposed to be 1A for my cabinet according to the paper beside the fusebox on the board. All fuses are definitely to spec, I was really careful of that when replacing them as to not do damage.

So you mean, the +7VAC never blows? That's great!

So the board shorts 12V to ground i'd say. LM1877N is a likely culprit (audio amp).
Let's get back to it later. First we need to get the game up & running.

If you remove the fuses from the 12VAC, that should leave only the +7AC output and allow it to transform into +5VDC on the game board. The 12VAC is run by diodes to the amp at 12VDC, so not really important. It is only used for sound amplification so the game can run without it, it will just have no sounds.

This test without the 12V would at least prove the game board/logic runs.
The 7VAC becomes 5VAC by regulator/big heat sink on the edge of the board, you can check the regulator output voltage, there should be 5V DC out there.

If you don't want to measure at the regulator itself, you can try pin 18 on the edge connector, it should have +5V DC output.

Assuming +5V is there, you should have something on the screen at least.
From your earlier posts it doesn't sound like you saw anything on screen :(

Let's see if you have +5V, and go from there.
 
You're correct, the two 12VAC fuses are the ones blowing. They are both supposed to be 1A for my cabinet according to the paper beside the fusebox on the board. All fuses are definitely to spec, I was really careful of that when replacing them as to not do damage.

So it's most likely the board? :(

Yes, but not a big deal. The 12VAC is only for sound. This can be repaired.

Ok, good to hear there are ways to check the board without it powered on. I saw a video of a guy measuring voltage across the capacitors with it powered on so I gave that a shot. I was getting some weird readings until I realized that I was dumb to try to read the voltage when the fuses are always blown.

So with that in mind, I should replace the fuses before checking for shorts using your first method I would assume. For the first method with everything in the cab - you're saying connect the black pin of the multimeter to the transformer (either 0, 7VAC, or 12VAC) then the red on the corresponding (0, 7, or 12VAC) part of the edge connector on the PCB? (Somewhat guessing, I'm very new to this).

I could also try the second method. If you know of a link to a guide or youtube video for dummies that explains either method, that would be awesome. I'll take pictures and work on this later today.

There is AC voltage involved and the 120AC input is nearby, so be carefull.
- In order to check for short circuits just pull out the board.
- Set the meter to OHMS (10K or so).
- Put the lead in the right socket. Some meters have different holes for volts and ohms.
- Put the black lead on position X
- Put the red lead on position Y
The display will tell you if there is a short, it displays a 0 or very very close to it. Some meters beep. I won't go into full details here.

Anyway, check the board pinout. You will see that pins A/B and 1/2 are all 0 ohms, they are connected together. Pins A/B/1/2 should not be shorted to C/D/3/4 so the meter should not drop to 0 when you measure with black on A/B/1/2 and red on C/D/3/4.
Same for black on Y/Z/21/22 and red on W/X/19/20. There should be resistance there.

For extra tests think if this: Bend the middle pins out of a fork. You know not to stick into wall outlet. Take the meter & put black on the left pin, and red on the right pin of the fork. Or touch the pins of the meter together to observe 0 ohms on the meter. That's a short circuit. The fuses detect how fast power is sucked into the board. A short circuit sucks at very high speed, causing the fuse to melt itself, which is what happens to your 12VAC fuse.

Be carefull around AC power, always.
 
I removed the 1A fuses as you advised and then tested all other fuses for continuity (just wanted to be sure I'm not missing something else being blown). Powered on and nothing happened.

I then measured the voltage at pin 18, C2 and C3 caps, and all of the 7V pins:

7V pins all looked good: ~7.5 on each
pin 18: 2.216 VDC
C2: 2.195 VDC
C3: 2.208 VDC

You were right!

I checked for shorts on both ends of the board using the method you described (excellent instructions by the way!) and found resistance with every combination (except the ones you mentioned are connected obviously). I kept the board in the cabinet for this and just removed the edge connector before testing. I also took a few pictures of the board and the inside of the cabinet just to give everyone a better idea of what I'm working with. It seems I can only upload 2 per post due to size, but I'll work on finding another way to share them all.

Unfortunately, I'm getting a little worried about this monitor too. I hear it power up like I mentioned earlier but I'm not seeing any significant light from the screen. I also can see the "orange neck glow" after turning off the lights in the garage. I'm not sure how to determine if it is working without a known working game though. If it is not working, it will make diagnosing this board even more difficult.
 

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7V pins all looked good: ~7.5 on each
pin 18: 2.216 VDC
C2: 2.195 VDC
C3: 2.208 VDC

If you are sure the meter is set to DC when measuring pin 18, there should be +5.x VDC output there. So if this measurement is correct, your on-board power is shot. Which would explain there not being anything visible on screen.

Unfortunately, I'm getting a little worried about this monitor too. I hear it power up like I mentioned earlier but I'm not seeing any significant light from the screen. I also can see the "orange neck glow" after turning off the lights in the garage. I'm not sure how to determine if it is working without a known working game though. If it is not working, it will make diagnosing this board even more difficult.

I'm not an expert on monitors, but assuming that Pac Man has a monitor with 'screen' and 'focus' pots, you should be able to turn on the cabinet, and turn up the 'screen' volts a bit, so you end up with a white-ish screen with retrace lines. (turn screen down again after that.) Be carefull around the high voltage: 10.000V there can kill you.

I like to cheat & make lucky guesses, i think everything is fine except the pcb, and throwing a new pcb inside will make this cab work.

It would be better to try & get the pcb running with DC volts on a test bench, bypassing the power stage on the PCB. And side effect would be that you convert the cab to DC at some point as well. You'll need a DC power supply at some point anyway.

You _could_ get something like this:
http://www.arcadeshop.com/i/932/midway-power-supply-kit-3.htm

It's pretty non-invasive, you just tap the input AC 120 power from the monitor.
If you install that in its current state it will blow the fuse on the 12V also by the way.
So you'll need to sort that out first. (also the manual does not tell you to unplug the old 7AC/12AC transformer but you should do that, or remove the fuses to ensure no shorts.)

Or this complete kit:
https://www.mikesarcade.com/cgi-bin/store.pl?sku=60in1Kit-Pac-Upright

And use the 60-in-1 for testing (and later playing) ;) Set aside the original Pac board. etc.

If the pcb was with me, i would hook up a cheap PC power supply for the +5v and GND, then a monitor to RGB/S/G on the test bench and see what happens. But i'm not sure you have a test bench with working 15Khz monitor.

But there is probably plenty of people who repair these pcb's in the USA.

Warning, you'll probably want multi games soon, so the 60-in-1 might not be a bad idea at all, it has a variety of classics :)
Vid of someone installing it here: https://www.youtube.com/watch?v=8K8-wbDP9fM

Short version: Try replacing Q6 (TIP31A) to get back +5V and hope the pcb runs. The 12V power stage it is likely (one of) the diodes causing the fuse to blow check if they are shorting. You say all those pins do not short out (no GND to 12VAC shorts) but there must be a short somewhere, or the fuse won't blow.
Converting the pcb to DC, best i can find you is this: http://www.ukvac.com/forum/tut-how-to-dc-mod-a-pacman-pcb_topic343979.html
 
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If you are sure the meter is set to DC when measuring pin 18, there should be +5.x VDC output there. So if this measurement is correct, your on-board power is shot. Which would explain there not being anything visible on screen.



I'm not an expert on monitors, but assuming that Pac Man has a monitor with 'screen' and 'focus' pots, you should be able to turn on the cabinet, and turn up the 'screen' volts a bit, so you end up with a white-ish screen with retrace lines. (turn screen down again after that.) Be carefull around the high voltage: 10.000V there can kill you.

I like to cheat & make lucky guesses, i think everything is fine except the pcb, and throwing a new pcb inside will make this cab work.

It would be better to try & get the pcb running with DC volts on a test bench, bypassing the power stage on the PCB. And side effect would be that you convert the cab to DC at some point as well. You'll need a DC power supply at some point anyway.

You _could_ get something like this:
http://www.arcadeshop.com/i/932/midway-power-supply-kit-3.htm

It's pretty non-invasive, you just tap the input AC 120 power from the monitor.
If you install that in its current state it will blow the fuse on the 12V also by the way.
So you'll need to sort that out first. (also the manual does not tell you to unplug the old 7AC/12AC transformer but you should do that, or remove the fuses to ensure no shorts.)

Or this complete kit:
https://www.mikesarcade.com/cgi-bin/store.pl?sku=60in1Kit-Pac-Upright

And use the 60-in-1 for testing (and later playing) ;) Set aside the original Pac board. etc.

If the pcb was with me, i would hook up a cheap PC power supply for the +5v and GND, then a monitor to RGB/S/G on the test bench and see what happens. But i'm not sure you have a test bench with working 15Khz monitor.

But there is probably plenty of people who repair these pcb's in the USA.

Warning, you'll probably want multi games soon, so the 60-in-1 might not be a bad idea at all, it has a variety of classics :)
Vid of someone installing it here: https://www.youtube.com/watch?v=8K8-wbDP9fM

Short version: Try replacing Q6 (TIP31A) to get back +5V and hope the pcb runs. The 12V power stage it is likely (one of) the diodes causing the fuse to blow check if they are shorting. You say all those pins do not short out (no GND to 12VAC shorts) but there must be a short somewhere, or the fuse won't blow.
Converting the pcb to DC, best i can find you is this: http://www.ukvac.com/forum/tut-how-to-dc-mod-a-pacman-pcb_topic343979.html

Unfortunately I think you are right about the on-board power being shot. I think I will test all the different diodes to see if one of them is to blame. When I have the time, I'd like to get a soldering iron and (after practicing a bit) attempt to fix this one. It's a skill I've always wanted to learn and it will pay off in the long run as other issues come up with the game in the future.

In any case, repairing it now is outside my ability so I took your advice and ordered something to get to gaming. I went ahead and ordered a working and refurbished pac-man board from ebay (seller happens to be a fellow KLOV member as well). I can use my daughter board with that to get up and running or maybe purchase the multipac kit that highscoresaves.com sells. Either way, I should finally be able to (hopefully) start enjoying my first arcade cabinet fairly soon.

Thanks for the monitor advice! Adjusting the "screen" setting on the monitor did allow me so I could finally see that it does work!

Regarding the idea of switching to a DC power supply like the one you linked. Is it necessary to mod the board for DC? I thought it was OK to run a switching power supply with these boards since it will just bypass the power section on the board. I've seen so many tutorials and videos where people switch to one of these, but never alter the board itself. In theory, switching to one would allow the game to work 100% besides the sound, which is powered by the 12VAC, right? I might be missing something.
Also, you said that the transformer should be disconnected if switching to one of these, but you also warned that my 12VAC fuses would continue to blow with a switching power supply. If I disconnected the transformer, that would eliminate those 12 and 7VAC fuses from the system though right since they are connected to the transformer?


Probably a shorted diode in the 12 volt power section of the game board.

I think you are definitely right. Any specific ones to test first?
 
Regarding the idea of switching to a DC power supply like the one you linked. Is it necessary to mod the board for DC? I thought it was OK to run a switching power supply with these boards since it will just bypass the power section on the board. I've seen so many tutorials and videos where people switch to one of these, but never alter the board itself. In theory, switching to one would allow the game to work 100% besides the sound, which is powered by the 12VAC, right? I might be missing something.
Also, you said that the transformer should be disconnected if switching to one of these, but you also warned that my 12VAC fuses would continue to blow with a switching power supply. If I disconnected the transformer, that would eliminate those 12 and 7VAC fuses from the system though right since they are connected to the transformer?
Good to hear the monitor has life in it and shows retrace lines! If you know nothing about electronics, the monitor is a big no-go area, so good for you this works :)

If you add a DC power supply you should indeed disconnect the transformer so no shorts can occur there.

If you use DC with the broken board and fuse block the 12V will blow the fuse, that's what i meant.

You will be ok not modding the board depending how you feed it +5VDC and bypass the regulator. Most mods put the +5VDC into the output pin 18 which works, as that is connected to the regulator's +5 output and then power flows from there into the rest of the board.

In theory, yes, if the board is OK and you give it +5VDC+GND it should run and output something on the screen. You could try it, if you're impatient like me. Even a 5V 2A DC adapter for some device in your home(like a modem, wifi router etc.) could do this. GND to A/B/1/2 and +5V to the Coin Multiplier pin 18 should show some result.
Use a multimeter to find the positive lead if you cut the plug of such an adapter. Some soldering required :)

Edit: Don't run the board forever with the +5V on Pin 18, it's just for testing & bypassing the 7VAC diodes' voltage drop.
 
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You _could_ get something like this:
http://www.arcadeshop.com/i/932/midway-power-supply-kit-3.htm

It's pretty non-invasive, you just tap the input AC 120 power from the monitor.
If you install that in its current state it will blow the fuse on the 12V also by the way.
So you'll need to sort that out first. (also the manual does not tell you to unplug the old 7AC/12AC transformer but you should do that, or remove the fuses to ensure no shorts.)

This is really good information to have, as the instructions I have, and have read, do not mention this at all. I read them three times to make sure I wasn't going blind. I was scratching my head, until trying to discover information for another problem I'm having, led me to this thread and this post.

Thank you very much
 
Sorry to rain on the parade, but the 12V section is needed in order to generate the regulated 5V from the rectified 7VAC.

The way this works is the 12VAC is rectified and filtered buy C29 and C5. The result is about 16VDC. That 16VDC is used to feed the audio section, but it is also used to feed the 78GUIC regulator. The regulator takes this voltage as input and outputs a voltage to drive the pass transistor (the TO-220 on the heatsink). The pass transistor take the 7.5VDC generated from rectifying and filtering the 7VAC input and regulates it down to 5VDC. The regulator adjusts its output voltage such that the pass transistor outputs 5VDC as read from the regulator adjustment input.

Without the 12VAC input, the pass transistor isn't doing anything so the current feeding the board is purely being pulled through R53. At about 1.3A, it will drop ~5V across the resistor, leaving you with 2ish volts across the two capacitors.

There are only a couple of things that could short out the 12VAC fuses. D3 and D4 should be checked as a shorted diode would do it. C29 or C49 being shorted or, as others have pointed out, the audio amp.

*edit* The above applies to using the original power supply. If feeding the +5V directly to the board from another power supply, then the 12VAC is not needed in order other than for audio.
 
Sorry to rain on the parade, but the 12V section is needed in order to generate the regulated 5V from the rectified 7VAC.

The way this works is the 12VAC is rectified and filtered buy C29 and C5. The result is about 16VDC. That 16VDC is used to feed the audio section, but it is also used to feed the 78GUIC regulator. The regulator takes this voltage as input and outputs a voltage to drive the pass transistor (the TO-220 on the heatsink). The pass transistor take the 7.5VDC generated from rectifying and filtering the 7VAC input and regulates it down to 5VDC. The regulator adjusts its output voltage such that the pass transistor outputs 5VDC as read from the regulator adjustment input.

Without the 12VAC input, the pass transistor isn't doing anything so the current feeding the board is purely being pulled through R53. At about 1.3A, it will drop ~5V across the resistor, leaving you with 2ish volts across the two capacitors.

There are only a couple of things that could short out the 12VAC fuses. D3 and D4 should be checked as a shorted diode would do it. C29 or C49 being shorted or, as others have pointed out, the audio amp.

*edit* The above applies to using the original power supply. If feeding the +5V directly to the board from another power supply, then the 12VAC is not needed in order other than for audio.
I need you to come on a scuba dive trip with me. Every time I read one of your answers I feel like I have at least two new answers to jeopardy questions now. Thx again for more help.
 
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