Repairing a Sega MK-4122

Recently I was working on a project that I needed a 9V power supply for. In my infinite wisdom stupidity I decided to use a Sega model MK-4122 power supply. This particular power supply is for the extremely rare Sega CDX. Well wouldn't you know it that the device I plugged into had a short in it and it toasted the power supply. After plugging it in the power supply was producing 0 volts.

I could just buy a new power supply, although replacing it with the OEM part would be very expensive. Another option is to just buy a similar spec'ed power supply and use that. But I decided to try and repair it first.

Unlike newer power supplies, this one actually has screws to open it up. Therein lies the first and greatest problem. It's a small hex head screw, but it's recessed down in a hole and none of my sockets fit down in there. I ended up buying a 4mm nut driver to be able to remove the screws.



Once open you can see just how simple the power supply is. A transformer, two diodes, a capacitor, a resistor, and a fuse. The problem was the fuse. The replacement fuse cost .97 cents. After soldering it back in the power supply works like new again.



Oh, for anyone who might need it, here's the exact specs on the MK-4122.
Input: 120V AV, 60hz 23W
Output: 9.5V DC 1.5A
Center positive.

Also, the supply is unregulated so if you use a volt meter to test the output without a load connected the output is close to 12.5V.