Arcade Switching Power Supply Voltage Skew...

Hypersport

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Does anyone have an answer to what is going on internally in a standard switching power supply when the voltages start to skew? For example, I've got a PS that used to be OK (when adjusting +5V output, +12 was about 12.30V with game board attached). Over time, that gap has increased - 6 months ago if I set +5 correctly, +12 would be at 13.89 Volts, and now, if I set my +5 to +5, my 12V is at 14.99. Different game boards produce similar results, so not a game board issue.

I opened up the supply and found 5 bad caps with my ESR meter. A few were bulging as well and I replaced all 5 caps. I was hoping the repair would fix it, but nothing has changed what so ever. If I adjust +5 to 5.00, +12 is still at 14.99. So if not bad caps, then what else is failing here to cause this? The caps replaced were the biggest ones - 2 x 3300MFD 16V, 2 x 1000MFD, 25V, and 1 470MFD, 25V.

Also, the high +12 voltage is causing distortion on my monitor -- if I turn it back down to 12.0 volts, that distortion goes away (although at 12.0V, my +5 is at 3.89 Volts). I did manage to get one board to run with this setting - a Mega Zone.

I actually have the same problem with several other supplies. In fact, I have about 10 -- now I know I can purchase a new one real cheap, but that doesn't help me today. And, if I can get by with making some working ones out of my 10 bad ones, then that's money saved, and trash parts put to good use.

So, if has anyone been down this road and understands the failure and can explain why the new caps didn't fix the problem, I'd love to hear from you.
 
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Adjust the PS to get good 5VDC, with the game PCB attached and running. 12VDC is commonly only used for audio amplification.

Is there a little 12VDC fan hooked up inside of the PS? Some have such a fan which serves two purposes; provide cooling, and provide a minimum load to the 12V suppy.

Do you know the model of your power supply? Looking at a schematic would make it much easier to determine why your 12V line is too high.

PS--Randy Fromm described a similar issue on a -5VDC line in this article: http://www.cityofberwyn.com/info/psrepair.txt The issue was a bad coil. You might look for one in your 12VDC output circuit.
 
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I was using this same article when repairing the supply. Unfortunately, the article mostly deals with dead power supplies. Mine is fully working, but not accurately. Randy suggests that the CAPs I replaced are the ususal cause of problems. Other than that, he mentions the transistors on the +5 and +12 lines. He suggests to look for a short - well, mine aren't shorted. And, he suggests that these transistors get shorted due to bad CAPs, so with good caps now, things should be fine.

Also, I don't suspect a diode problem, because Randy suggests that a diode failure will cause the PS to shut down.

I guess that really only leaves the regulator IC. But, I don't really know how to test one of these ICs, and simply swapping parts for swapping parts sake isn't really what I'm after.

I guess I'd like to understand how the switching transistors and the IC interact a little better. From what Randy says, the IC looks at the voltage levels coming from the transistors. If too high, the transistor on time is reduced. If too low, then the on time is increased. So, that implies that the IC adjusts until it thinks the supply voltages are good -- but that's not what I'm seeing on output. So, what would cause the IC to think the voltages are good when they aren't?

I don't know the model of the PS unfortunately - just a black box with no manufactuer labels or markings. It's probably a mid 80s model (not a peter chou), just a small +5V 7A supply. I would certainly be looking for schematics if I knew the model.

I can continue to poke around and test parts, but again I'd like to understand the root cause of the failure. If I can make that leap, then I can probably repair the pile of bad ones I have cheaper than I can replace them. That's the goal anyway.

Thanks for the help - I found Randy's article at a different link than the one you supplied. Looks like he'll be busy for the rest of his life trying to get all these sites displaying his copyrighted information to take it down.
 
I was referring to the section headed "Minus 5 Volt Output Too High". It could just as well apply to the 12V output:

"If you find that the +5VDC and +12 VDC outputs are normal but the -5 VDC output is too high (more than -6 VDC), try replacing the -5 output filter choke.

It's easy to locate the -5 volt filter choke, even without a schematic
diagram. Just follow the trace on the printed circuit board back from the
-5 VDC output of the power supply. You will eventually come to a
component that may look something like a capacitor but will be clearly
labeled "L" on the board and will generally be accompanied by the
schematic symbol for a coil as well. The coil is wound on a ferrite coil
and is covered with a plastic sleeve that has been heat-shrinked over it.
Examine the coil. If the heat-shrinked cover has been melted or is
missing entirely, the coil may be bad.

This is not a component that you can boogie down to Radio Shack for a
replacement. There are a couple of options for obtaining a replacement
coil. The preferred method is to take the coil off a junk power supply.
Alternately, you can pull the burned wire off the ferrite core and rewind
the choke yourself using the appropriate gauge wire. There aren't that
many turns of wire on it that you can't rewind a new coil in five minutes."

Try reverse-engineering the 12V output. Tracing backwards from the line out back to the transformer secondary. You should find, at a minimum, a high-freq diode, a choke and a cap. Perhaps a coule of chokes and a couple of caps, perhaps more than one diode... perhaps a regulator, no telling until you look closely at it. In any case, the feedback for the control IC is likely done on the 5V line, and the 12V is left to do what it will at that operating point. Luckily there typically aren't but a handful of components in an aux voltage output.
 
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Sometimes the integrated circuit control chip can go whacky and not provide correct voltages. The chip is cheap to replace.

Usually the chip is a type TL494CN (same as KA7500B). If your power supply has two integrated circuits (a TL494CN and a LM339) then replace both of them.

My repair guides are here:

 
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did manage to get one board to run with this setting - a Mega Zone.

if I can get by with making some working ones out of my 10 bad ones, then that's money saved, and trash parts put to good use.

how much is a mega zone going for these days
i use one of these old 39 in 1 multiboards for my test rig and who cares if it gets fried

over time, as the capacitors slowly dry out and change the mathematical equation of a circuit, the voltage change can put stress on other components which can lead to more unwanted failure. one doesnt want that. no how, no way am i going to put an old power supply to run my Escape from the planet of the Robot Monsters printed circuit board. these go for 350 bucks if you can even find one which brings me to where i love to save money and thats on buying a brand new power supply at any drop of a hat and be done with it good luck
 
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