Pac-man power issues 3.1v on the 5vt

80sarcade

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I have a Pac-man board that is dead. I am going over the power voltages and determined that Q6 looked to be dead. I am getting only 3.1 volts out of the Base leg. The voltages on the power brick look fine and I checked the fuses, all check ok. (yes I removed the fuses to check and tested the voltages at the brick) :)
Here are some of the other voltages I am getting.
R52 - 16v should be 12v
Q6 B= 2.2v C=8.3v E=3.1v
R53 3.1v
C3 3.1v
C8 and C9 8.3v

Looking at this, it appears that Q6 is dead. I picked up a TIP31 and replace, no change to the board, dead. (did pick this up at RS and this may be my new problem, it is listed as a TIP31a)

So any more suggestions? Does this look like a bad Q6 and did I replace it with a bad one from Radio Shack, of is it something else?

Paul
 
I ran a cross reference on NTE just for the hell of it.

the TIP31 and TIP31a are returning different NTE part #s, so I'm guessing they're not the same. :(
 
I did a little digging and the only thing I found out is the tip31 a b and c differences are the max voltages they can handle, starting at 40v up to 100v. So I am guessing the tip31 I put in is good and have another issue.
 
has the power supply board ever been rebuilt? I'm sure rebuild kits exist for those on bobroberts or wherever, could just be its time. (failing caps, transistors, etc.)

I've never mucked with one of these, so I can't tell you what to do otherwise, lol

I say rebuild it, if it fixes it, fantastic, if it doesn't, then start asking the classic gaming geniuses here what to try next.

now would probably be a good time to change out the fuse holders if you haven't done so already. I hear those like disintegrate on these!
 
The power supplies, besides the power brick, are in the Pac-man board. Looking at the schematic, not much is involved with the power part. Looks like it could be the caps or the one other four pin component.

Any of the techs have any suggestions? My next step is to replace the caps.

Thanks in advance
Paul
 
Have you tried just connecting a switcher to the pac board to see if it is working properly? Pac's have AC input that is converted to DC by some diodes, and it's normally higher at the fuses without the board connected than what it would be with it connected.

If your board will work fine with the switcher attached, and your AC is coming to the connector properly, and you have that AC at one side of the diode, but don't have +5vdc approximately on the other side of the diode, then replace the diodes. Otherwise, check to make sure you're getting proper power transfer through the connector to the board pads.

If it won't work with the switcher, then you definitely need to fix the board...
 
The TIP31A will work. I use C on them.
Have you checked the fuses in the cab? If you loose one leg of the AC coming in you can get readings around 3v. You might have a bad 78GU1C regulator but those dont go bad very often.
 
Also, if your edge connector on the board has burnt through the contact to the fiberglass circuit board below then it won't make good connection and you'll end up with the same symptoms as a bad fuse holder.

Check the edge connector carefully.
 
Thanks guys some great suggestions. The edge connector looks very clean, no burn marks or real wear and tear. The fuse block checks out but I will double check the voltages at the board at both sides and each rail.
I think I either have a bad cap or a bad regulator.

modessitt, great info and link. I would like to keep original but for testing purposes your suggestion on using a switcher is a great one, thanks.
So why did they run 7 volt AC to the board when it needs 5v DC? Would it have been easier to run 5v and just convert it from AC to DC and not add the extra step down??

Anyway, thanks again and I am making progress.

Paul
 
So why did they run 7 volt AC to the board when it needs 5v DC? Would it have been easier to run 5v and just convert it from AC to DC and not add the extra step down??

Cost less to put the power supply on the main board than to install a standalone power supply. The pacman power supply is not a bad design, just the fuse holders are not very good. The brass contacts only flex so many times before they start cracking.
 
...The brass contacts only flex so many times before they start cracking.

And they only sit in the hot nasty environment of the arcade cab so long before they start corroding and making bad contact.
 
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