Repairing Switching Power Supplies

OK - yes I know that switching supplies are cheap, but hey... sometimes you just have to fix one. Maybe it's for your PS2 (which is why I wrote this over on the neo-geo.com forums) or maybe it's because you can't get one in time or you are just impatient and want to play the game right away! Anway - I'm sure that CactusJack or some of the other smart folks here can add to this.

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Switching power supplies aren't typically too hard to repair... They tend to fall into some categories:

1. Dead & silent with fuse blown
2. Dead & silent with fuse good
3. Dead & chirping/clicking with fuse good
4. Voltage output is OK but game acts goofy on this supply

#2 is the hardest to fix.

Switching power supplies run like this:

High voltage side: Brute force rectification of the mains voltage through a set of diodes - either separate ones or a 4 leaded bridge rectifier. This is filtered through a capacitor & goes to the switching circuit (after being stepped down through other components) and to the main switching transistor. Problems in here are related to #1 and are fairly easy to fix.

Regulation: this circuitry kicks off the supply & makes sure the output is correct. It runs the oscillation of the main switching transistor and monitors the output of the high frequency stepdown transformer through a feedback mechanism. Problems in here are related to #2 - the hard one to fix.

Low voltage side: Here are the rectifier diodes, filter choke coils, and capacitors that turn that high frequency AC output from the transformer into the DC output that is needed by the game. There's a small part of the circuitry here that gives the feedback to the regulation circutry to keep things running stable. Problems here are related to #3 and #4.

DISCLAIMER: *ALL* listed methods of troubleshooting are done with the power OFF. Keep in mind that problems listed as #2, #3, and #4 are related to things where the fuse is GOOD and the high voltage section of the board may have a charge on the large filter capacitors. Some power supplies have bleeder resistors across them. Others do NOT. Use a 150k 1/2w resistor to bleed those caps & test the voltage with your meter to keep yourself from getting a nasty shock. DC makes your muscles contract and if you pick up a power supply you might find yourself unable to let go. Yes - I have had this happen to me once. Take the proper precautions. That's how I learned that not all power supplies have bleeder resistors for those main filter caps on the high voltage side. Damn Apple II power supplies...

OK... back to business...

Fixing high voltage side:

Use your ohmmeter and check the resistance across all combinations of the 4 legs of the bridge rectifier. They should NOT read zero ohms. If they do, reverse the leads and check again... if they do... replace the component.

Do the same test across the legs of the main switching transistor and any other semiconductor (diode/transistor) in the high voltage section. Replace any shorted components.

Be aware that some switching supplies use low value resistors around the switching transistor. If you read around 2 ohms then you may be reading those. A shorted component is usually 1/2 ohm or less.

If you find shorted components anywhere in the high voltage section you should check the resistors for any open ones and replace as necessary. Replace the fuse, fix any cracked solder joints, reassemble, and test...

Fixing the low voltage side: Chirping supplies generally mean problems with the output. It could be a problem with the regulation portion too but I've never seen that as the case myself. Every case of chirping supplies I've worked on ended up having a shorted rectifier diode in the low voltage section.

Some of the diodes are dual-diodes that look like transistors. Look at the circuit board as most of them are labeled as "D#" or "CR#". Test these components with the ohmmeter and look for one that reads shorted both ways. It is common for the high speed dual rectifiers to read very low resistance one way - almost shorted looking - but they will read high the other way unless they are shorted.

Replace any shorted rectifiers, fix any cracked solder joints, reassemble, and test.

Power Supply works, but the game is flakey on it: Check the filter capacitors on the output section of the power supply. Look for ones that have split tops or ones that have tilted over or lifted up because the rubber plug has popped out of the bottom. If they all look OK then either shotgun them or check the outputs with an oscilloscope and look for trash high frequency AC ripple on them. Replace the caps as necessary to clean up those outputs, fix any cracked solder joint, reassemble, and test.

Problem in the regulation section: Well, these can be a BI*** to figure out. The only times I've been successful at fixing these without a schematic (which isn't very often since you cannot usually get schematics for these) is when shotgunning the caps in the regulation section or finding a cracked solder joint.

What if I have a problem related to #1 or #3 and can't find a shorted component? Well, this gets trickier. Sometimes a semiconductor doesn't short out. Sometimes it gets "leaky" meaning the forward resistance is low like normal but the reverse path resistance is lower than it should be. When you run into these situations then check the components carefully. If you find one that is low one way and around 500 to 1000 or so ohms (maybe a little more, maybe a little less) then desolder one leg of the part, lift that leg out of the board and test the part out of circuit. If it reads low one way and not high the other (should be tens if not hundreds of thousands of ohms or higher the other way) then replace it as it may be leaky.

Where do I get components? Well, not your local Radio Shack... Try Mouser, DigiKey, or Jameco on the web... Check your local Yellow Pages for Electronic Component stores but be aware that many sell only large quantities to businesses - you need one with a retail location. Also check with your local Amateur Radio Operators. Most of them know where to find parts.

I've fixed hundreds of switching supplies over the years - Apple II and older Mac II, SE, SE/30, and lots of PC clone ones. I've also repaired them for various pieces of network gear. Keep the safety precautions in mind and make sure the caps are discharged and you should be safe.

RJ