Repairing bad power supply from Power Mac G4 MDD

[cyclones]

Hi Everyone,
I have a bad PS from a Power Mac G4 MDD or mirror door machine. I took it apart and found a blown fuse. It blows a new fuse immediately upon being plugged in. The computer works fine with another PS.

So there must be a dead short in this thing somewhere. I have a little electronics knowledge but would like to learn more and get some experience repairing electronics.

I don't notice anything burnt in the PS, but I did notice some diodes of this type:
http://www.cartft.com/image_db/1n4001.jpg
that looked like balloons. Like they had pressure inside them and expanded. They are labelled with a DXXX on the board as in D100, D101, etc. (those aren't the exact numbers, but you get the idea). I've found 3 so far, all in the same area. I've never seen a diode that looks like this. Can anyone comment on that observation?

I also noticed this PS board has some CapXon capacitors and as I recall, I have seen a lot of these go bad on both Dell and Apple computers.

I would appreciate any help to tell me where to start troubleshooting this thing. I have a volt-ohm meter and that's it. Do I need any additional tools?

Thanks!!!!

[Toasty]

Re: Repairing bad power supply from Power Mac G4 MDD

Apple PN? Manufacturer?

Common fails - surge protector, main switchers, and primary cap.

[seanc]

Re: Repairing bad power supply from Power Mac G4 MDD

I have two of these at work if anyone wants something for reference. The one I've opened up is branded Samsung, don't have the Apple PN to hand right now.

[cyclones]

Re: Repairing bad power supply from Power Mac G4 MDD

This is a Samsung 360W supply. It is exactly the same as the one in this thread:
https://www.badcaps.net/forum/showthread.php?t=7165

I've attached a picture of the weird-looking diode. Sorry it's kind of poor. I'll try to get a better picture on Monday.

[Toasty]

Re: Repairing bad power supply from Power Mac G4 MDD

The white round with black band is fine and correct.

See post #2 for your problem above.

[cyclones]

Re: Repairing bad power supply from Power Mac G4 MDD

In continuity mode across the big filter cap I get infinite when connecting negative to the striped side and positive to the other side. .380 the other way. The striped side has what appears to be a - on it.

I also found what I believe to be the rectifier bridge. It looks the same as the pictures i have found of these; for example, this one at digikey:
http://parts.digikey.com/1/parts/432...02m-e4-51.html
It is marked BD1 on the board and attached to a heat sink.

Do all rectifier bridges have the same pinouts? I'm assuming they do. I used the data from digikey to attempt to test the thing. I set the multimeter in continuity/diode mode and resistance between either of the two middle leads and the + or - is about .520 in either direction. There is continuity (meter beeps) between the + and - (outside leads) in either direction.

Between the two middle leads (marked ~ on the digrams) I get 1.0.

[Toasty]

Re: Repairing bad power supply from Power Mac G4 MDD

To be clear, it's Apple P/N: 614-0224, Samsung PSCF401601 correct?

The cap sounds okay, but if it's the CapXon HP series, do yourself a favor and replace it. Multiple failures open and shorted.

You need to use the diode check function for the bridge. But, from what your readings are, I'd say the bridge is bad or a component on the output is shorted. There should be no reading either way on the AC input (~) leads.

Check switchers Q1 & Q2.

Toast

[cyclones]

Re: Repairing bad power supply from Power Mac G4 MDD

Yes, those are the exact part numbers I am working with, Toasty.

[cyclones]

Re: Repairing bad power supply from Power Mac G4 MDD

There is continuity between all leads on Q1. Continuity between the middle and one of the outside leads on Q2 (the lead that is towards the edge of the board), .2 for the other combinations.

[cyclones]

Re: Repairing bad power supply from Power Mac G4 MDD

What are Q1 and Q2? I can't read the labels on them.

[Toasty]

Re: Repairing bad power supply from Power Mac G4 MDD

You'll have to pull that whole heatsink out anyway to replace them. Always do them both. Mind the insulator pads on top and around the heatsink. Take multiple pics before you disassemble.

Q1,2: ST W20NM50

[cyclones]

Re: Repairing bad power supply from Power Mac G4 MDD

To be clear, it's Apple P/N: 614-0224, Samsung PSCF401601 correct?

The cap sounds okay, but if it's the CapXon HP series, do yourself a favor and replace it. Multiple failures open and shorted.
Toast

It says "HP VENT" on it. I assume that's one that should be replaced. Any recommendations on replacements?

What about the other capxon caps in here? They're usually OK?

[Toasty]

Re: Repairing bad power supply from Power Mac G4 MDD

You need the datasheet for the CapXon series HP and then find/choose a comparable replacement using the methods presented in many places throughout the forums here.

Without checking the caps with an ESR meter, I cannot say as to the other CapXons well being.

Get the primary working and move on from there.

CapXon: http://www.capxongroup.com/en/prod_all_cat_full.aspx

I'm sure a suitable replacement can be found at either DigiKey or Mouser. FETs are definitely available at both places too.

Toast

[severach]

Re: Repairing bad power supply from Power Mac G4 MDD

I got one of these. A main power transistor W20NM50 was shorted. The part was too expensive on eBay and I have no way of testing it to see if it is good so I never got around to fixing it to see if anything else was bad.

[Toasty]

Re: Repairing bad power supply from Power Mac G4 MDD

I just looked at eBay quickly and they're going for 1/2 of what Mouser or DigiKey want for them. (4 for $15)

I cannot speak to quality or reliability though. I don't trust eBay for these types of items. Since they are coming out of Hong Kong, I am even more suspicious that they would be seconds.

Toast

[cyclones]

Re: Repairing bad power supply from Power Mac G4 MDD

I finally got a chance to do the swap with new transistors. I threw in a new 8A fuse and plugged in. Pop! Fuse blew, slight burning smell. It looks like Q1 or Q2 is again shorted. So there must be something else wrong.

Anyone have suggestions on what else I should check? I want to do a more thorough investigation this time -- replacing Q1 and Q2 isn't cheap!

Thanks!

[Toasty]

Re: Repairing bad power supply from Power Mac G4 MDD

Did you replace mains cap?

[cyclones]

Re: Repairing bad power supply from Power Mac G4 MDD

UPDATE, since I'm not allowed to edit:
Maybe I'm wrong about the shorted Q1 and Q2. It seems I'm getting intermittent continuity. I suppose that's the switchers at work? Either way, it DID blow a fuse.

[sam67]

Re: Repairing bad power supply from Power Mac G4 MDD

Instead of blowing fuses all the time why not use the light bulb method across fuse to see if there is still a short ? saves you blowing them just to test..Just a thought.

[JOHNBOY]

Re: Repairing bad power supply from Power Mac G4 MDD

Did you have any luck fixing this ? I have one with a totally different problem.