Original Ms. PacMan cockatil fried (montor?)- help to fix/upgrade(?)..

Measure with the connector on or off? I pushed the connector on a bit more firmly this time and I think both fuses are blowing now, or at least the second one just did- I could have slipped with the meter prongs and shorted across 19/20.. With the connector on now, and no 1A fuses in I am getting seemingly arbitrary voltages in th eneightborhood of 2-3V across 19/21 20/21... that seems impossible..

I'm pretty sure I am touching the stripes carefully with the meter tips..
Actually... both fuses blowing makes me feel better, since I don't see how one could blow and not the other.

What we know right now is that your fuses don't blow when the harness is disconnected... so the problem should be on the board. You tested the diodes, and they both tested good (0.7V one direction, open circuit the other).

Two things to check:
1) Confirm that the voltage out of the transformer is good. The fuses can be out for this test... measure the AC voltage between the two fuse holders (on the transformer side). You should see approximately 24VAC.

2) There's only two paths for the voltage to go once it gets to the board. It can either go through R52 into 1R, or to 11A (audio amp). To blow the fuse, one of these paths is drawing too much current. Try desoldering and lifting one side of R52 out of the circuit. Then replace both fuses, reconnect the harness and power up. If the fuse doesn't blow, then it seems there's a problem with 1R. If the fuse still blows, then the problem is likely the audio amp.

I wish I had another board to swap as a simple test :0
Heh, unfortunately with the crazy wiring hackjob, you wouldn't be able to swap another board. :/

DogP
 

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Oh, and one other possibility is that C29 is shorted. With the game off, measure the resistance across C29.

DogP
 

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2) There's only two paths for the voltage to go once it gets to the board. It can either go through R52 into 1R, or to 11A (audio amp). To blow the fuse, one of these paths is drawing too much current. Try desoldering and lifting one side of R52 out of the circuit. Then replace both fuses, reconnect the harness and power up. If the fuse doesn't blow, then it seems there's a problem with 1R. If the fuse still blows, then the problem is likely the audio amp.
Just thinking about this... you can skip the pull R52 test. Even if the other side was shorted to GND, with 16V across a 50 ohm resistor, you wouldn't blow a 1A fuse.

DogP
 
Oh, and if the resistance across C29 reads low, it could actually be C49 or 11A causing it as well (they're all in parallel).

DogP
 
That's a pretty ugly hack. Funny how a machine that looks so clean inside can have that hackery on the game PCB. Take a really close look at that hack and make sure nothing is shorting to ground. It's likely that moving things around may have jostled the wiring. That's probably what all of that glue is there for. It was a bad job to begin with. I'd pull that PCB and repair the edge connector and go from there.
 
That's a pretty ugly hack. Funny how a machine that looks so clean inside can have that hackery on the game PCB. Take a really close look at that hack and make sure nothing is shorting to ground. It's likely that moving things around may have jostled the wiring. That's probably what all of that glue is there for. It was a bad job to begin with. I'd pull that PCB and repair the edge connector and go from there.
While I agree that it's an ugly hack... that's on the 7VAC lines, and the problems are on the 12VAC lines.

I'd almost recommend going with a switching power supply kit (like: http://v2.globalwebcart.com/arcadeshop/i/932/midway-power-supply-kit-3.htm ), but since the board and cabinet have the hacked wiring it wouldn't be plug-and-play. To run a switching power supply with the burned edge connector and existing hacked wiring, I'd just do this hack: http://www.twobits.com/DCpac.gif .

But, it seems that there's a problem with something on the board drawing too much current... not just a faulty power section. So, I don't see installing a replacement power supply fixing the problem.

DogP
 
A little more imagery, info about that weird custom hard-wiring to the edge connector..

There is a blue/red striped wire coming out of the 5th pin from the left (closest to cabinet wall) on the edge connector- it comes out and is connected through a metal ring clip (grounding type?) that is not screwed to anything- also the orange wire you see coming out from that connection goes back through a connector into another wire that is soldered back to that same edge pin location on the board... Judging by the look of the blue wire connectors in this mess matching the ones used on the power cord extension I'd guess this was all done by the same person (before me).

I do not know what the heck is going on with this modification... It's funny because when you slide the edge connector onto the board it actually cannot seat all the way down onto the edge with all of the hard connections, but it seems those hard connected wires are all being spliced back into the appropriate wires coming out of the connector.. just bonkers!?!

This may all be a red herring relating to the fuse blowing since the machine has always worked fine this way before the flyback death..

Surprised they even bothered to use a molex connector! :O Good call. Probably only because they needed to do this hack outside of the machine of course. Otherwise I'm sure they wouldn't have. They never do. :D
 
1) Confirm that the voltage out of the transformer is good. The fuses can be out for this test... measure the AC voltage between the two fuse holders (on the transformer side). You should see approximately 24VAC.

I get ~24.5VAC between the two empty fuse holders (on the transformer side)


Oh, and one other possibility is that C29 is shorted. With the game off, measure the resistance across C29.

DogP

resistance is zero in this case
 
That means it's shorted.

Does that mean the capacitor itself is bad? (I don't see anything shorting across the leads on the bottom of the board...

FYI- the resistance on the multimeter slowly climbs up to about 3.4 before dropping to zero when I measure across the leads of this capacitor..
 
Wait- no! My mistake- when I set the multimeter to very high Ohms C29 just keep creeping up indefinitely.. So I think it is fine...
 
I can't find C49 or 11A on the board...?

mspacman-schematics.jpg


It's on page 88 (out of 99) on the pdf schematics online at: http://arcarc.xmission.com/PDF_Arcade_Bally_Midway/MsPac-Man_Parts_and_Operating_Manual_(Jan_82).pdf

If you own the real deal, it's on page 7-8. I highlighted the two capacitors you were looking for... 11A means 11 amps.
 
And for the record- regarding the burned up edge connector hack- the wires that are hot-glued/soldered to the board just splice into the corresponding wires coming out of the edge connector- so it would seem I *could* just plug the edge connector into a "good" board and it would work- no?
 
I get ~24.5VAC between the two empty fuse holders (on the transformer side)
Okay, that's good.

Wait- no! My mistake- when I set the multimeter to very high Ohms C29 just keep creeping up indefinitely.. So I think it is fine...
Yep, that sounds normal.

And for the record- regarding the burned up edge connector hack- the wires that are hot-glued/soldered to the board just splice into the corresponding wires coming out of the edge connector- so it would seem I *could* just plug the edge connector into a "good" board and it would work- no?
It's possible, though usually when an edge connector is burned up like that, the original pins on the harness side are pretty trashed as well. They might be good enough for a quick test though.

One thing... you said that the other fuse blew once you pushed the connector on tighter... did you have both fuses in place at that time and both of them blow? I'm just curious whether you've tried replacing both fuses, putting everything back together like normal with the harness on tight, and turning it on.

It doesn't seem like you have any shorts on the board, and like you said... everything was working before, and blowing a flyback shouldn't affect the game PCB.

DogP
 
One thing... you said that the other fuse blew once you pushed the connector on tighter... did you have both fuses in place at that time and both of them blow? I'm just curious whether you've tried replacing both fuses, putting everything back together like normal with the harness on tight, and turning it on.

It doesn't seem like you have any shorts on the board, and like you said... everything was working before, and blowing a flyback shouldn't affect the game PCB.

Put the connector on as firmly as I could this morning and they both still blow on power up.
 
You don't hear a loud thump or anything from the speaker before the fuses blow, right? To me, it seems like the problem has to be with the audio amplifier (LM1877 at location 11A).

The 12VAC lines come in, get rectified to 16VDC, and then goes to some caps, the amplifier, and to the 78GUIC through a 50 ohm resistor. The resistor should prevent anything on that path from causing the fuse to blow, and the caps don't appear to be shorted. That should only leave the audio amp (assuming the rectifying diodes are good).

If you can, I'd try to disconnect pin 14 (power pin) of the LM1877 to see if that fixes it. Unfortunately, it's not really a pin you can just "lift" of out circuit, so you may need to cut the trace with an xacto knife somewhere that will be easy to solder it back together.

Also, there's usually a heatsink on that chip... make sure that it's not touching something it shouldn't be.

DogP
 
Assuming this is the LM1877 (right next to the volume control). The heat sink is clear, but it is soldered on through the board- I'm wondering if you can tell which is pin 1 of LM1877 on the back side so can avoid doing surgery to desolder the heatsink. Also- do I cut the trace on the top or bottom of the board associated with that pin?

Sorry for the delay- I barely have time on weekdays to get to this..
 

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According to the manual, the chip has pin 1 pointing away from the edge of the board. So, it should be like the attached drawing. I see a trace on the bottom of the board going to the amp from what looks like the tantalum cap... you should confirm that they didn't also put the trace on the top of the board.

If that trace is only on the bottom, then simply cut that trace with an xacto knife (use your multimeter to make sure there's no continuity afterwards) and try powering it up again.

DogP
 

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