Repairing bad power supply from Power Mac G4 MDD

[cyclones]

Re: Repairing bad power supply from Power Mac G4 MDD

I have not had a chance to work on it lately, but will be starting on it again soon. I'm going to replace the main filter cap, which is a cRapXon. Should I replace anything else while I'm at it?

Also, could someone explain the "light bulb method" to me?

[Toasty]

Re: Repairing bad power supply from Power Mac G4 MDD

>>Also, could someone explain the "light bulb method" to me?<<

See following thread, Post 23. Good read & same machine PSU.

https://www.badcaps.net/forum/showthread.php?t=7165

[cyclones]

Re: Repairing bad power supply from Power Mac G4 MDD

Any suggestions on rectifier bridge replacement for this PS? Brands, specs? If I'm going to desolder the heatsink assembly again, I might as well replace the bridge.

[Toasty]

Re: Repairing bad power supply from Power Mac G4 MDD

Why? Is it defective? They don't "wear out".

[cyclones]

Re: Repairing bad power supply from Power Mac G4 MDD

I just pulled out the heatsink assembly for this one. I'm going to take some high-quality pictures later and post them. Even thought I was being really careful, I almost busted the thermistor. I think it is still OK. I was trying to figure out why the PS still has a dead short in it, and I think I may have stumbled upon something. With the heatsink assembly out, I have continuity between two of the holes for Q2. I can't tell at this point if it's sloppy soldering on my part or something else, but I don't think it's supposed to be that way. The continuity is between the middle pin hole for Q2 and the pin hole towards the edge of the board.

[cyclones]

Re: Repairing bad power supply from Power Mac G4 MDD

Why? Is it defective? They don't "wear out".

I'm going to pull it out and check it, but I think so. After your post, I started doubting myself, but I really think it is. I think this is why I'm getting the weird continuity on the Q1 and Q2 holes.

We'll see here in a few minutes...

[cyclones]

Re: Repairing bad power supply from Power Mac G4 MDD

Bridge rectifier checks out fine. The + and - holes on the board have continuity. So there must be something else shorting out. Don't know what it is yet.

[Toasty]

Re: Repairing bad power supply from Power Mac G4 MDD

Try the Q2 holes with Diode Check function. Should be low one way and high the other. If not, did you pull the Mains cap out?

[cyclones]

Re: Repairing bad power supply from Power Mac G4 MDD

I started poking around checking resistance between things and got an itching to take this thing out (see picture -- it's bad; sorry I don't have a good camera with me today). I have no idea what it is, but it looks awfully complex and big for a jumper. It's marked R107 on the board. After removing this thing I removed still has 100% continuity in both directions on its two leads. I then tested the holes for the rectifier bridge and they no longer have continuity. So whatever it is, it is my hypothesis that it's bad and causing the short in the PSU. the only markings are
HMR
5W 0.06[omega]J
I'm not 100% sure on the omega, but it looks closer to that than anything else.

[Toasty]

Re: Repairing bad power supply from Power Mac G4 MDD

5 watt, .06Ω (ohm) resistor

It's fine.

[cyclones]

Re: Repairing bad power supply from Power Mac G4 MDD

wow way different from the other resistors I've encountered.
Any thoughts on where I should look next for the short? Something caused that fuse to blow.

[cyclones]

Re: Repairing bad power supply from Power Mac G4 MDD

Try the Q2 holes with Diode Check function. Should be low one way and high the other. If not, did you pull the Mains cap out?

Yep pulled the big cap out log ago. Will try your check on the q2 holes when I get a chance. My recollection is there is no difference whichever direction.

[cyclones]

Re: Repairing bad power supply from Power Mac G4 MDD

5 watt, .06Ω (ohm) resistor

It's fine.

My meter measured .04. Assume that's close enough.

[cyclones]

Re: Repairing bad power supply from Power Mac G4 MDD

This morning I found diode D102 has continuity in BOTH directions. I desoldered it from the board to verify. No resistance either direction.

[cyclones]

Re: Repairing bad power supply from Power Mac G4 MDD

Only info on this diode is
IN5819
0340G

[Toasty]

Re: Repairing bad power supply from Power Mac G4 MDD

ONE - N - 5819 ... 1N5819

Schottky diode, 1A, 40v

D101 is the same type. Double-check your measurements. On diode check, both should read about 0.2Ω when forward biased (black probe on banded end).

[cyclones]

Re: Repairing bad power supply from Power Mac G4 MDD

Double checked them and compared them to the other diode which I had already noticed was the exact same diode type. WAY different measurements. Ordered replacement from Digikey. Came in today. Soldered new diode in and that resistor back in. Checked at rectifier bridge holes for continuity. NONE. Yay! Now I just have to solder the BR back in, the big heat sink, and hook up the light bulb test and check it out.

[JOHNBOY]

Re: Repairing bad power supply from Power Mac G4 MDD

Let us know. I have one of the samsungs too and may give it a shot. How long did it take to get the heatsink out.

[cyclones]

Re: Repairing bad power supply from Power Mac G4 MDD

Let us know. I have one of the samsungs too and may give it a shot. How long did it take to get the heatsink out.

Approx. 10-15 minutes if you do it carefully. Take it slow. Use my photos and make sure the thermistor is thoroughly desoldered as Toasty said.

[cyclones]

Re: Repairing bad power supply from Power Mac G4 MDD

What kind of resistance should I be getting between the outer two through-holes for Q1 and Q2? I am currently getting about 160 Ohms.