Hi, I have an old PowerMac 8100 power supply from a company MagneTek.
I need some advice repairing it. I am not sure if I got the right SMPS controller IC.
When I got it, the primary side was completely blown, both MOSFETs blown, control IC blown its cap off, etc.
I slowly got it to work, first I disconnected all the visually blown and shorted components, until all there was left was a standby power supply. There was a major darkening in the standby area, but it was caused by an overheating resistor (which still tests good - 68 ohms). This was caused by dried up capacitors, at first I decided not to recap it, but when I tried it first, the power supply drew 20W thorugh the lamp limiter. Recapping made it much happier (but still a bit high) at 9W. The resistor gets only warm now.
Next, I tackled the primary switcher. I ordered two new MOSFETs, new diodes, few resistors, two new optocouplers, etc. But there was a problem. The main SMPS control chip was completely blown, so I have not been able to identify it. I thought it would be most likely an UC3843 (Vcc, GND, Output pins were all in right places), so I put this one in.
Also I was not able to get the same MOSFETs, the original ones were STP5NA80FI which aren't made anymore, so I put in STP8N80K5 ones, main difference is that the Rds is lower (should only make it more efficient).
Now, the power supply kinda works, but something is still very wrong.
1. It starts automatically when you apply power (it has soft power, so it should wait for PS_ON signal)
2. The voltages on the secondary are too high, I get 6.4V on 5V rail, and 12.5V on 12V rail.
I checked the schematics of some of the UC3843 power supplies and none of them uses two optocouplers as this one does. So I was thinking, perhaps the main issue is that I just happened to get a wrong SMPS IC that just coincidentally kinda works.
I attached some photos of the circuit.
Thanks
I need some advice repairing it. I am not sure if I got the right SMPS controller IC.
When I got it, the primary side was completely blown, both MOSFETs blown, control IC blown its cap off, etc.
I slowly got it to work, first I disconnected all the visually blown and shorted components, until all there was left was a standby power supply. There was a major darkening in the standby area, but it was caused by an overheating resistor (which still tests good - 68 ohms). This was caused by dried up capacitors, at first I decided not to recap it, but when I tried it first, the power supply drew 20W thorugh the lamp limiter. Recapping made it much happier (but still a bit high) at 9W. The resistor gets only warm now.
Next, I tackled the primary switcher. I ordered two new MOSFETs, new diodes, few resistors, two new optocouplers, etc. But there was a problem. The main SMPS control chip was completely blown, so I have not been able to identify it. I thought it would be most likely an UC3843 (Vcc, GND, Output pins were all in right places), so I put this one in.
Also I was not able to get the same MOSFETs, the original ones were STP5NA80FI which aren't made anymore, so I put in STP8N80K5 ones, main difference is that the Rds is lower (should only make it more efficient).
Now, the power supply kinda works, but something is still very wrong.
1. It starts automatically when you apply power (it has soft power, so it should wait for PS_ON signal)
2. The voltages on the secondary are too high, I get 6.4V on 5V rail, and 12.5V on 12V rail.
I checked the schematics of some of the UC3843 power supplies and none of them uses two optocouplers as this one does. So I was thinking, perhaps the main issue is that I just happened to get a wrong SMPS IC that just coincidentally kinda works.
I attached some photos of the circuit.
Thanks
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