Help me repair an Amiga power supply?

[Jeff]

I recently picked up an A1060 sidecar with a dead power supply board. I have a fair electronics background with good component replacement skills, but I have never worked on troubleshooting a dead SMPS beyond replacing old caps.

I would like to keep the power supply board original if possible. So far I have replaced the rectifiers and electrolytic caps, and either tested or replaced most of the rest of the discreet compontents including the switching transistor.

The "shotgun" approach has always worked for me in the past, but I just can't get this one going. I don't have a schematic for it either. I can provide good photos of both sides of the board, or send it to somebody and offer to pay you for your time if you can take a look at it for me.

I can also sub in a board from a modern power supply and be done with it, but I was trying not to.

Thanks and Happy Holidays,
Jeff

[Jeff]

Here is a link to the pictures.
Power Supply Front Side
Power Supply Back Side

The line voltage goes in at the spades by the fuse. The output should be +12vdc, +5vdc, and -12vdc.

I do have 300vdc out of the main rectifier and across the filter caps with no AC leakage. I can follow it to the main switching transistor, but it seems to stop there. I don't understand the switching circuitry beyond that.

One of the low voltage rectifiers was open (they have both been replaced). I have also replaced both of the small IC's along with all of the electrolytics. One is an LM393N and the other is an LM339N.

I can email higher res versions of these pictures if needed. I  am pretty much stuck and I hate to give up on it at this point. There is low resistance across the windings on both sides of the main transformer, but I'm not sure what it should be.

There are also a couple of components that I can't identify and don't know how to test (Like the blue Schaffer box at the bottom).

Anyway here are the pictures, and thank you in advance for any ideas. BTW I do have an older Tektronics scope too if that will help.

Thank You,
Jeff

[Jeff]

Yes, I have done all of my tests with a proper load on the output. I forgot to mention that.

I will begin the search for an open resistor. That's how I was able to test the small NPN and PNP transistors, by lifting a leg.

Thanks again,
Jeff

[Jeff]

It ended up being a bad winding in the transformer. I won't be repairing that:-(

On the lighter side, I have fitted the board from a old 250 watt AT power supply in it's place and it works great. I guess I should have done that to start with. My A1060 is finally finished and working!

Thanks for all the advice.
Jeff