All replacement capacitors in iMac G5 PSU burned up

[i4004]

Re: All replacement capacitors in iMac G5 PSU burned up

no, i actually gave few theories(see above..paragraph below the pic) and am questioning your theories too...
feel free to question my theories.

but that is most interesting aspect of this malfunction, the burning coil issue...

[nwd]

Re: All replacement capacitors in iMac G5 PSU burned up

So the so-called electronics shop does not carry inductors. At Mouser on the net, the inductor search filter asks for a whole pile of specs I don't know - can't specify by number of windings or gauge of wire of course. I looked up a few 10 and 15uH capacitors and they only show generic gif files of the same inductor for every toroid. I am therefore stuck until I can somehow figure out how to get to old electronics boards hunting for a coil with same number of windings, gauge, core type and size in the hope that it matches.

I will happily pay shipping to and from for this thread-expanding PSU to anyone who wants to give it a go. Otherwise, it will probably be awhile before I can post results. I am in no way giving up - not after all the thought and energy you all have put into this.

Once again, your help is very much appreciated.

-Nate

[Toasty]

Re: All replacement capacitors in iMac G5 PSU burned up

Well, this seems to be a well documented, known problem to Apple. It particularly applies to those units produced in late 2004:

Mac Owners Support Group

From reading this, it seems that the defective PSU will/can/does/possibly/likely/more often than not, take out what they call the "midplane". (Motherboard to us common folk)

Sad

[i4004]

Re: All replacement capacitors in iMac G5 PSU burned up

>defective PSU will/can/does/possibly/likely/more often than not

i like this too!
hehe...good sense of humor...

i don't think mac users are of much help when it comes to coils, though...hehe

>I will happily pay shipping to and from for this thread-expanding PSU to anyone who wants to give it a go.

well, wizard dissapointed me...he says he wants bad psu that was not tempered with
https://www.badcaps.net/forum/showthread.php?t=6580
otoh, i would be glad to have a look at that psu, to tinker with different loads, to see and test coil, to measure other cap's esr and alike. and for free.
but i'm not near you...

dunno if toasty or pcbonez have such interest, though...
would you pay them(what they ask/if they ask) if they were able to fix it?

about "midplane"(funny indeed!); this machine works, so i don't think it can be in such a poor condition....also, if we know for sure psu is ok(ie fixed) then if problems appear again(monitoring the temp of psu) he could send mobo to topcat, he can change caps there...

btw. i didn't know macs have such problems...i thought price tag was justified...
oh...wait...actually i didn't....i always thought apple sells normal hardware with excessive price tag...

[nwd]

Re: All replacement capacitors in iMac G5 PSU burned up

More info per Toasty's request. The black square component mounted to the heatsink has the following on it:

ON (in a circle)
MBR2045CT
CO(or zero)413
aka

The inductor on that side of the heat sink has a couple black spots on two of the windings - I don't think it is damage but could be. I'm not sure if the windings on the badly burned inductor were black originally (some kind of coating) but they are completely black now.

So far, info on the burned inductor - vertical alignment, 9 turns, powdered iron core, 5mm wide, 8mm inner diameter, 14mm outer diameter core. I still need to find the wire gauge. Any chance that it will be a non-AWG gauge?

Once I have that, I will try to get it from West Florida Components - I found a similar looking inductor offered by them on ebay.

[nwd]

Re: All replacement capacitors in iMac G5 PSU burned up

Also, when Toasty says "check voltages" with the PSU disconnected and then connected from the load, what do I need to do?

-Nate

[i4004]

Re: All replacement capacitors in iMac G5 PSU burned up

you need digital (or analog) multimeter so you can read the voltages...
you can find guides on multimeter usage on youtube

[nwd]

Re: All replacement capacitors in iMac G5 PSU burned up

Also, also - the wire on the burned inductor appears to have been black by design - not from damage according to the photos on Jim Warholic's page. Does that mean the replacement inductor also needs to have black-coated wire?

Is it possible I shorted the inductor by scraping away the black coating on the inductor wire with the exacto knife revealing the bare wire?

[i4004]

Re: All replacement capacitors in iMac G5 PSU burned up

coating color or type doesn't matter.

revelaing bare wire?
well, if you scraped away the isolation and then made sure that those (non-isolated) portions of the wire are touching the non-isolated portions of the core-metal, then yes, you made a short circuit there...
but i think that's almost science-fiction, isn't it?
because you would actually need to unwind the wire from the core to do that lil mischief properly(as wire can touch the core only from the inner side, ie side facing the core)...hehe...

other than that, you could of make "s.c." between windings, but stil, you know how i feel about calling something with 8 turns short circuited anyway...heh...

so overall, no, i don't think you did it...i think something is loading the coil too much and it's failing because of that, not because of you..

[nwd]

Re: All replacement capacitors in iMac G5 PSU burned up

Thanks for the thoughts i4004. What I meant to ask is does the replacement inductor need to be coated, or is a non-coated (i.e., bare wire) inductor acceptable.

-Nate

[i4004]

Re: All replacement capacitors in iMac G5 PSU burned up

if it looks "bare" it probably has lacquer layer on it....
but you don't have to worry about that at all.
what matters is the inductivity....


hm..this got me wondering if esr meter could be used to check coils...hm..i should try that...

[PCBONEZ]

Re: All replacement capacitors in iMac G5 PSU burned up

That's correct.
Even though it looks like it is the wire is not bare.
It's either insulated or it's coated "transformer wire".

[Toasty]

Re: All replacement capacitors in iMac G5 PSU burned up

I hate pics on the web!

Here I thought we were looking at a major coil and now come to find out it's no more than the size of 4 Dimes stacked together! ( I think that's 4 - 1 Lipa for you, i4004 )

LOL

Unfortunately - Not more than 5 weeks ago, I cleaned out the shop and tossed a whole pile of ancient PSU's and other assorted electronic crap that had collected over the years. In there was a PSU from a HP scanner, and I know it had some of these in it.

But, ya gotta clean house once in a while, lest you end up on Dr. Phil having an intervention...

...this thread-expanding PSU...

Toast

[Toasty]

Re: All replacement capacitors in iMac G5 PSU burned up


Schematic of the problem area...

[Wizard]

Re: All replacement capacitors in iMac G5 PSU burned up

This forms a buck regulator for 3.3V using pulses from the transformer via diode to pump it up then cap & resistor (makes delivery longer) pumps back around to the other diode to the ground. Which output is tapped for voltage feedback to the switching controller IC?

If the frequency or shape of pulses is wrong or or capacitor or resistor opens the coil fries because the core is oversatuated. This POINTS back to the feedback regulation or small electrolytic capacitors for the controller IC.

Shorted turn in the coil or transformer pops the transistors instantly.

Cheers, Wizard

[Toasty]

Re: All replacement capacitors in iMac G5 PSU burned up

Hmmmmm...

Wizard:
Then my hypothesis back some 26 posts ago ( #49 ) was on the right track. That if the frequency was off, possibly due to the unreplaced capacitor in the power supply throwing the supply to the controller off, the coil was saturating.

Which output is tapped for voltage feedback to the switching controller IC?

I am working only with the photos here. Between the pics of the topside that don't give me enough info, and getting my two rights confused when I flip back and forth from top to bottom pics, I am putting the info together as best I can.

I do not know which IC is the controller nor where the IC is so I cannot determine where the feedback comes from or goes to. There are no defining pics of that IC nor its surrounding circuitry.

Shorted turn in the coil or transformer pops the transistors instantly.

Yet the coil does show a short, and the supply runs cool with no load. It does run the system when connected, but then the coil overheats. Your thoughts?

Would a similar sized OTS coil, lets say between 3-6µH capable of 6A or better, be suitable here? Is the frequency in that area a concern as to the value of the coil's inductance?

nwd: See pic attached - Is the marked area a chip/ding in that disc capacitor or just a bit of that goo you had to remove that secured everything?


Toast

[Wizard]

Re: All replacement capacitors in iMac G5 PSU burned up

No no. Not in way sense in sense of a short for a coil or transformer.

When a short happens in a coil or transformer, since windings is close together, very rarely and it DOES HAPPEN is one turn shorts each other at one point forming a complete copper ring buried in a transformer or a coil. That ring is a brake on HARD to the forming and collapse of the magnetic field. The current ramp altered so dramatically that something had to give way and that's usually transistor(s) or a diode shorts or blow then fuse or a resistor finally fail. One cycle, POP. There was no time to tug plug out in that instant.

There are exceptions to this, especially if transformer have too high impedence like high voltage secondary shorted turn, all you get is heat or noise (squeal). Same with resistance too low on the output making PSU shut down with noise or tic tic noise.

You'll have to replace that coil for correct uH, that means raiding from another junk PSU.

What you do is find the controller IC by working backwards from there that drives totem pole transistors to the main transformer, from that IC trace back to the two isoptoslators, one will be power control to the pin that runs the main PSU or in standby and other one is voltage feedback, it will have bit of circuitry that tap into one of most important voltage outputs. Usually 3.3V or 5V.

If that capacitor is not replaced on the monitored output, that probably reason why.

Don't mistake the other taps on the many outputs that leads to the different ICs or transistors, this is for overvoltage monitoring and sometimes undervoltage. Most of SMPS especially high current ones have overcurrent circuit is monitored via a resistor current sense or a current transformer (like a GFCI concept) that trips the PSU into shutdown. Usually found on primary side and secondary side if using current transformer. Current transformer is a one thick loop (just one turn or less) through a 50-100 turns thin copper transformer.

Cheers, Wizard

[ran]

Re: All replacement capacitors in iMac G5 PSU burned up

Interesting. I think the same thing happened on my G5 Imac 1.6ghz machine.

No powerup, no nothing. HOwever, checking the diagnostic LED's on the back of the midplane does show the first LED lighting up, suggesting it is getting power from the PS when lit up. Tried resetting the SMU and using the midplane power-on button, no go.

Decided to open up the PS and noted about 7-8 caps on the low-voltage side that are bulging, but no blows or significant leaking yet.

Think it's worth replacing the caps anyway, or do you think it's the motherboard? I'm no stranger to replacing caps, did it on my G4 Snakebite PS (along with the PS fan) and it's been working like a champ for over a year now.

Was going to order from Mouser:
Nichicon Low Impedance Aluminum Electrolytic Capacitors - 105 Degree of the appropriate capacitance/voltage, but any suggestions of whether I should get:

UHM....
UHD....
UHE....

? Just a little confused as to the exact differences...

Thanks,

Ran

[i4004]

Re: All replacement capacitors in iMac G5 PSU burned up

>Shorted turn in the coil or transformer pops the transistors instantly.

this is waay to much generalizing, and in my opinion pretty misleading.
some of the reasons you gave in your next answer, but there are more(for example coil on secondary...how will it pop primary switchers, etc?)

also, why put thogether coil and trafo at all?

and, this coil has few turns and they're not close together at all.

so no, not any coil, or any trafo will pop the swichers, esp, if they're even not in the primary circuit...

[Toasty]

Re: All replacement capacitors in iMac G5 PSU burned up

..caps on the low-voltage side that are bulging, but no blows or significant leaking yet. Think it's worth replacing the caps...

Ran -
I think your best option is to start a new topic for your particular questions and problems. That way you get all the attention from the forum members who can best help you.

It also keeps responses directed at you and your topic and there is no question as to "whose problem am I answering or help solve" by those members that respond.

Just copy that entire post over as your opening for the thread. I'm sure someone like PCBONEZ will respond rather shortly (as soon as he gets his coffee).

Cheers!
Toast