Repairing bad power supply from Power Mac G4 MDD

[JOHNBOY]

Re: Repairing bad power supply from Power Mac G4 MDD

The other guy should know.. I could measure mine but most likely mine is bad to begin with. If you want I can give it a try.


I'll be getting my first capacitor tester tomorrow. An entry level multimeter for $45. It should do the trick for now and if I'm successful in the repairs I will get one of those EST things.

[cyclones]

Re: Repairing bad power supply from Power Mac G4 MDD

The light bulb trick was pretty neat. I had to replace Q1 and Q2 again because the previous fuse blow didn't save them. I started it up this time and the light bulb didn't come on. At this point I can check the 25V output and see if I'm getting anything there.

[Toasty]

Re: Repairing bad power supply from Power Mac G4 MDD

You should have 5vsb & 25v without jumpering it on. On initial PSU plug in, the light bulb should briefly pulse. Then on startup, it may blink several times as the circuit comes up. With no load, it may remain just barely glowing, and will increase in brightness as it is loaded. As long as it remains dim with no load, you should be safe to go to full power (no bulb) for testing.

[JOHNBOY]

Re: Repairing bad power supply from Power Mac G4 MDD

I changed Q1 and Q2 and still popped a fuse. What should this diode be reading? I pulled one and on the 200k setting of the DMM it reads 7.5 going one way and about 150 going opposite.

I'm about to transfer the entire Q1 and Q2 assembly and the two diodes to another board and see if the fuse blows on that as well.

[Toasty]

Re: Repairing bad power supply from Power Mac G4 MDD

Did you replace the mains cap?

What diodes?

[JOHNBOY]

Re: Repairing bad power supply from Power Mac G4 MDD

Did you replace the mains cap?

What diodes?

It was the D101/102 . The 1N9158 or something close to that.

All of the caps look fine and seem to be within range. They are capxon but the ones I pulled including the big one were right on.

[ronbak]

Re: Repairing bad power supply from Power Mac G4 MDD

My ps just went. I found Q1 AND Q2 shorted and will replace them. I can not find D101/102. I found D104 under the TF.

[Toasty]

Re: Repairing bad power supply from Power Mac G4 MDD

>>I found D104 under the TF.<<

Where? You mean L01?

D101 & 102 are between the power resistor (see post 29) and the center heatsink, next to Q2. You'll see them once you get the heatsink out to change the MOSFETs.

Be -very- careful you don't damage the thermistor attached to the heatsink. Make sure all the holes are clear and the component legs are free before you go pulling on it to remove.

Now, once again for the cheap seats....

>> REPLACE THE MAINS CAP <<

The entire series of Capxon HP are known failures. It is a factory defect.

Sure, it might look and test good today, but when (not if) it goes, you'll be back with new MOSFETS, fuses, and an ICL.

Toast

[Agent24]

Re: Repairing bad power supply from Power Mac G4 MDD

Now, once again for the cheap seats....

>> REPLACE THE MAINS CAP <<

The entire series of Capxon HP are known failures. It is a factory defect.

I thought the entire Capxon product line are failures

[Toasty]

Re: Repairing bad power supply from Power Mac G4 MDD

Surprisingly, the output caps are very solid. I've not found a bad one there yet. I replace them as a matter of course. The small ones up between the main heatsink/switchers and PWM/PFC daughterboard are always bad and a real trouble spot. No start symptom.

Toast