PowerMac Dual 1Ghz Mirror Drive Pop&Burn Power Supply

[ran]

Hi everyone!

I have now, a third in my Mac PS series.

this PS comes from a PowerMac 1Ghz G4 Mirror Drive computer.

Sadly, this one appears in the worst shape, as compared to the others which required simple low voltage recapping to regain functionality.

This PS is a Samsung 360W PS (see pics for exact info)

Symptoms: Computer wouldn't cycle, so I pulled out the PS, and opened it up (much easier as they didn't use those stupid anti-tamper screws with the protrusion in the middle of the Philips socket - good thing I have a Dremel!).

Noticed a burnt fuse, and thought, oh! could it be that simple?????

Bought some 250V 8A fuses, popped one in, and, "Pop! Fizzle! Light! Stinky!"

Hrrm...well, that's not good. I took a look and discovered a nasty burn spot right near the AC in point. I've detailed this in several pics shown below.

So, my questions, being a non-electrician:
1.) What do you think happened?
2.) What are those two components? Is the yellow a rectifier, what's the green one with a burnt "whole" on top?
3.) Is this fixable?
4.) If yes to (3), what are your suggestions? I guess I can get replacements online, as I have with capacitors.... right?


Thanks, guys.

Ran

[kc8adu]

Re: PowerMac Dual 1Ghz Mirror Drive Pop&Burn Power Supply

i have one on the bench now.
but i saw the hard blown fuse and investigated the cause.
your burnt part is temperature dependant resistor that serves as a "soft start"
with a dead short they sometimes blow up before the fuse opens.
mine had an open main filter cap and 2 shorted mosfets in the apfc section.
the replacement mosfets arrived yesterday so i will complete it soon.
i autopsyd the cap and the foil connecting from the plate to the terminal was blown open.the cap itself is dry and shorted.

[ran]

Re: PowerMac Dual 1Ghz Mirror Drive Pop&Burn Power Supply

i have one on the bench now.
but i saw the hard blown fuse and investigated the cause.
your burnt part is temperature dependant resistor that serves as a "soft start"
with a dead short they sometimes blow up before the fuse opens.
mine had an open main filter cap and 2 shorted mosfets in the apfc section.
the replacement mosfets arrived yesterday so i will complete it soon.
i autopsyd the cap and the foil connecting from the plate to the terminal was blown open.the cap itself is dry and shorted.

Ummm.. hi!
I understood everything up to the "...sometimes blow up before the fuse opens.." part. Then, you lost me.

Do you think it's as easy as replacing this temp. dependent resistor, or is it merely a symptom of something worse happening?

Ran

[Toasty]

Re: PowerMac Dual 1Ghz Mirror Drive Pop&Burn Power Supply

So, my questions, being a non-electrician:
1.) What do you think happened?

-mains cap shorted
-bridge rectifier shorted
-power MOSFETs shorted

2.) What are those two components? Is the yellow a rectifier, what's the green one with a burnt "whole" on top?

-Yellow plastic devices are X type interference suppression caps; usually metalized polypropylene or polystyrene in construction. Connected across the mains, their used to limit line interference from or to the unit.
-Green burnt device, as KC said, is a current limiter. It's a thermistor of the NTC (negative temperature coefficient) variety. Initially it has a high resistance and as it warms its resistance goes down. It's there to stop, under normal circumstances, the blowing of the fuse from the high inrush current when you power-on the supply.

3.) Is this fixable?

-Most likely

4.) If yes to (3), what are your suggestions?

-check the items I mentioned in the first answer. KC gave you the clues for the mains cap and MOSFETs.

I guess I can get replacements online, as I have with capacitors.... right?

-Yes


Toast

[Toasty]

Re: PowerMac Dual 1Ghz Mirror Drive Pop&Burn Power Supply

Ummm.. hi!
I understood everything up to the "...sometimes blow up before the fuse opens.." part. Then, you lost me.

Open main filter cap is the big black mutha across the board from where this blew.

Shorted MOSFETs in the APFC section. MOSFETs are the devices mounted to the heat sinks.

APFC is Active Power Factor Correction and improves the PSU's efficiency.
Care to bend your mind a bit? - http://en.wikipedia.org/wiki/Power_factor
Pay attention to the lower half of the page starting with "Non-linear Loads". It's an explanation that requires a bit of an egghead. Mine hurts right now... :p

Do you think it's as easy as replacing this temp. dependent resistor, or is it merely a symptom of something worse happening?

Reflecting on history, it's something worse....

Toast

[ran]

Re: PowerMac Dual 1Ghz Mirror Drive Pop&Burn Power Supply

Open main filter cap is the big black mutha across the board from where this blew.

Shorted MOSFETs in the APFC section. MOSFETs are the devices mounted to the heat sinks.

APFC is Active Power Factor Correction and improves the PSU's efficiency.
Care to bend your mind a bit? - http://en.wikipedia.org/wiki/Power_factor
Pay attention to the lower half of the page starting with "Non-linear Loads". It's an explanation that requires a bit of an egghead. Mine hurts right now... :p


Reflecting on history, it's something worse....

Toast

Hah - funny you should provide that wiki link. I've realized that, looking at these boards, that I should know a lot more about them if I'm going to be tinkering with them, so I actually was reading that the other night. Unfortunately, my brain is so full of science (my field is immunology), that I don't seem to have much room for this stuff

But, slowly I'm sure osmosis will have a net positive effect..... at least I hope Especially if I hang around here long enough

Now, I've looked at that big ol' cap in the back there, and nothing seems obviously wrong with it, I am unaware of a way to test it with my multimeter without actually removing it from the circuit board - is there a way?

Regarding the other components, Iguess I can use my multimeter to test each of those and see if they're dead without removal as well?

Fun Fun... Wish I had more time for this...

[Toasty]

Re: PowerMac Dual 1Ghz Mirror Drive Pop&Burn Power Supply

There is a guide on here somewhere for testing MOSFETs. What you're looking for is most likely shorted components. Make sure you discharge that cap before you try to read it. Use volts scale first to be sure it's discharged.

KC should have some answers as he said he's got one now. He can point you better. Me, I need pics to tell you what to look for and test.

You might want to pop that out of the case and make sure nothings burnt underneath.

Toast

[ran]

Re: PowerMac Dual 1Ghz Mirror Drive Pop&Burn Power Supply

There is a guide on here somewhere for testing MOSFETs. What you're looking for is most likely shorted components. Make sure you discharge that cap before you try to read it. Use volts scale first to be sure it's discharged.

KC should have some answers as he said he's got one now. He can point you better. Me, I need pics to tell you what to look for and test.

You might want to pop that out of the case and make sure nothings burnt underneath.

Toast

Thanks, Toasty - I was going to take the board out this weekend and do just that. I'll look around on the site, I did look through the sticky FAQ's, but didn't see much, but I know there are about a bazillion "guides" on how to test whoseewhatsits, gigamaroles, and transmogrifiers, so I should be good on that end, I think.

Will report back on my findings next week.

[Toasty]

Re: PowerMac Dual 1Ghz Mirror Drive Pop&Burn Power Supply

Yeah. Just stay away from that thread on transmogrifiers. The last guy they got in there...

Well, lets just say they're still finding the tasty bits...

Toast

[ran]

Re: PowerMac Dual 1Ghz Mirror Drive Pop&Burn Power Supply

I completely removed the board from the casing, and see no obvious damage on top or bottom, besides in the pics I showed you. Nothing appears bulging or anything

So, I've been playing around with my multimeter.

I haven't removed anything from the board, so I don't know if any of this is accurate.

I tested the capacitance (uF-2000) of several caps on the board, and the resulting values were all within about 30uF of the rated value. When I tried this with the main cap, by touching the cap leads on the underside of the board, I get nothing. Does this mean anything, as it is still soldered to the board?

I've been trying to test some of the MOSFET's, according to instructions here I believe I correctly identified the Source/Drain/Gate pins, but I can't seem to see what they describe. After setting my multimeter to the diode position, If I touch the two outer pins, I see no value. Touching what I think are the source (black) and drain (red) pins, I simply see an accumulating voltage starting at 0.000v, and slowly climbing, until I remove the probe tips around 0.8xx v (though it would go higher, I think). But I can't seem to discharge by putting my finger over the leads while maintaining contact with the probes. Again, this is all done while they are still on the board - does that make a difference?

[Toasty]

Re: PowerMac Dual 1Ghz Mirror Drive Pop&Burn Power Supply

Reading the "When MOSFETS fail..." paragraph is what you're looking for. A failed one usually goes shorted -zero ohms- drain to gate. Or, is obvious by a hole in it or a chunk blown out or a fracture of the casing.

On the board testing is not reliable as the other components interfere. I think they are talking in that how-to about a loose in your hand unit or at least one that's desoldered from the circuit.

The mains cap should read something. Try an ohmmeter across it and see if you get any reading connecting first one way (red + / black -) then the other with your leads. You should see a lower reading that rises in one of those hook ups. If not, it's time to desolder at least one leg and try that for a uF reading. You may have the same situation that KC found, that being the cap is open circuit - no connection.

Good Hunting!
Toast

[ran]

Re: PowerMac Dual 1Ghz Mirror Drive Pop&Burn Power Supply

Hrrm. So, following Toasty's suggestion, I set my multimeter to Ohms, and tested the resistance across the soldered leads of the big black cap

I get a reading of 0.435 M Ohms (black to neg, red to pos, OL when reversed). Umm..does that mean anything regarding whether the cap is good or not? Am I still going to have to desolder it?

[ran]

Re: PowerMac Dual 1Ghz Mirror Drive Pop&Burn Power Supply

... sorry this wasn't in the previous post, but in addition to measuring resistance across the Big cap, I also looked at several MOSFET's


Resistance values here varied tremendously. On most of the ones on the side opposite the burnt section near the AC in, I was getting resistances of 0.23 - 0.33 k Ohms. On the ones near the burnt section, values of 4.5 M Ohms were being read. (D - black, G - Red)

Again, do these values tell me they're ok or are they relatively meaningless?

[Toasty]

Re: PowerMac Dual 1Ghz Mirror Drive Pop&Burn Power Supply

>>OL when reversed<<
What does that mean?

The readings sound high to me on the cap. Plus, you should see a rise in the reading at least in one direction. I'd just desolder a leg or both and pull it to test. The board isn't double sided, like the iMac was, so it should be fairly easy.

The readings on the MOSFETs would indicate they are not shorted. You are likely reading the other components in the circuit.

Is there a bridge rectifier in that PSU or individual diodes? I would check it/them as I indicated in my first response. It/they may be shorted.

Toast

[ran]

Re: PowerMac Dual 1Ghz Mirror Drive Pop&Burn Power Supply

>>OL when reversed<<
What does that mean?

sorry - that's out of limit/range, no reading.

The readings sound high to me on the cap. Plus, you should see a rise in the reading at least in one direction. I'd just desolder a leg or both and pull it to test. The board isn't double sided, like the iMac was, so it should be fairly easy.

Yep, figured I'd have to do that. Will try to over the next couple of evenings...

Is there a bridge rectifier in that PSU or individual diodes? I would check it/them as I indicated in my first response. It/they may be shorted.

I believe there is attached to one of those heatsinks. I will look check it tonight. Umm... regarding testing it, I just need to do a continuity/resistance test, correct? I should see different readings in each direction (high vs. low) and if I see the same, then that would mean the rectifier is shot, yes?

[Toasty]

Re: PowerMac Dual 1Ghz Mirror Drive Pop&Burn Power Supply

>>that's out of limit/range, no reading.<<
I think then that the mains cap might be open.

>>regarding testing it, I just need to do a continuity/resistance test, correct? I should see different readings in each direction (high vs. low)...<<
Use the diode-check function and you should see high one way and low the other. The bridge rectifier packs are usually marked. Each of the 4 pins is 2 diodes connected as follows-

Set 1: Anode to Anode - Out
Set 2: Cathode to Cathode + Out
Sets 3 & 4: Anode to Cathode ~(A/C) In

See: http://en.wikipedia.org/wiki/Diode_bridge

>>...and if I see the same, then that would mean the rectifier is shot, yes?<<
Yes, usually. Again, out of circuit may be necessary to confirm as whatever is across it may be influencing the readings.

Toast

[ran]

Re: PowerMac Dual 1Ghz Mirror Drive Pop&Burn Power Supply

Well, I pulled the main cap off this weekend (390uF, 420V) and get no reading across the leads when set to capacitance.

As an aside, whenever I test good caps - still in bags (e.g. 2200uF, 330uF, etc) I never seem to see the full reading. Is this normal? For the 2200uF, I only see about 1800uF, and for the 330uF, I'll see maybe 300...

Anyway, do you think I could just replace that one big cap and the temp. dependent resistor that blew, or are there more likely other bad components?

I suppose to be thorough, I should go through and try to test all transformers/rectifiers... eh?

[ran]

Re: PowerMac Dual 1Ghz Mirror Drive Pop&Burn Power Supply

Well, as indicated above, I do get low readings on all the rectifiers and such. Maybe they're ok?

I was also going to replace the X-capacitor near the fried temp. dependent resistor that got scorched. However, the only way I can think to get that out is to remove the closest toroid, which, after trying to work back and forth to get all four of the leads desoldered seemed really hard.

So, I just checked with my multimeter and I get a reading of about 6.6-7.0uF across (still in board). I compared with the other X capacitor a little ways away, which gave me the same reading. So, do you think it's probably ok, even though the outer casing got a little scorched when the resistor blew?

If so, then I may just order resistor (how do I identify what it is so that I can replace???) and the main cap and see if the PS works....

Other thoughts?

[Toasty]

Re: PowerMac Dual 1Ghz Mirror Drive Pop&Burn Power Supply

KC should have the info on the thermistor. He should be able to point out which MOSFET's were blown too. The fact that the thermistor and the fuse blew indicates something else is probably shorted. Unless of course, it was the cap that now shows open.

If the rectifier pack shows high one way and low the other, it's probably okay. Double check.

If you have a variable isolation transformer, use that to power it up once you get the parts replaced. If not, you can use a 110v, 100 watt bulb in series with the mains. If the lamp comes on strong with no load connected to the supply, it means you still have a short.

Toast

[ran]

Re: PowerMac Dual 1Ghz Mirror Drive Pop&Burn Power Supply

Hiya Toasty!

Thanks for responding so quickly!

KC should have the info on the thermistor. He should be able to point out which MOSFET's were blown too. The fact that the thermistor and the fuse blew indicates something else is probably shorted. Unless of course, it was the cap that now shows open.

I figured it was just the main cap, but is that unlikely to be the primary component gone bad, but more a secondary effect?

If the rectifier pack shows high one way and low the other, it's probably okay. Double check.

Ok.

If you have a variable isolation transformer... <snip>

The only thing I have is a "Speed Controller". See pics for what it is. Not sure if that would really work or not...

...If not, you can use a 110v, 100 watt bulb in series with the mains. If the lamp comes on strong with no load connected to the supply, it means you still have a short.

Huh, I don't think I'd have an idea of how to set that up...