All replacement capacitors in iMac G5 PSU burned up

[Wizard]

Re: All replacement capacitors in iMac G5 PSU burned up

Look again, The partial schematic of this circuit is EXACTLY what a buck regulator looks like minus the transistor and feedback circuit. The pulses is square wave (appox) fed thru a forward biased diode to this circuit, the other diode is backbiased and forms part of the buck regulator circuit with this burnt coil.

You will may find that winding is same for 5V and 3.3V but 3.3V is through this buck regulator. This is reason for 3.3V/5V combined max wattage due to sharing same winding on the SMPS transformer.

Reason we need to know which output is monitored by feedback circuit back to the SMPS controller IC in primary side via the optisolator.

Cheers, Wizard

[PCBONEZ]

Re: All replacement capacitors in iMac G5 PSU burned up

You can tinker with rewinding the coil all you want. There are some exacting methods to insure this is done correctly.

It's not -THAT- 'exacting'.
Same core, number of turns, wire size, - is fine.
-
If you doubt that then go fine [say] 10 of the same motherboard and look at the same torOID on each one.
You won't need a micrometer to see they are all wrapped differently.

I'd like to see that coil measured with a proper inductance meter before it gets unwound.

It's shorted. - Remember?
That's the problem with it in the first place...

.

[Toasty]

Re: All replacement capacitors in iMac G5 PSU burned up

Well, I understand Wizard's post in theory, but I don't know why he posted it. He already said all that before. If it relates to rewinding or replacing the coil somehow, please explain, because I missed it. Some days I'm abnormally "thick". Please feel free to use a 2x4.

It's not -THAT- 'exacting' <snip> wrapped differently

I am certain of that, but they are "factory made" with assorted jigs to insure sameness and the tolerances and processes are well controlled. They may not look the same but they are the same.

Here we're dealing with (a) someone inexperienced in doing these things, (b) doing it for the first time, (c) doing it by remote control from our input, (d) doesn't even own a VOM.

The reason I question how exact, is because of Wizard's response to my previous question regarding using an OTS 3-5µH coil. I mean for $3 this could be done-done and back in service.

It's shorted. - Remember?...

Yes, but it is an intermittent short, per nwd's testing. It could be measured by you or I or anyone with the equipment and with that knowledge in hand.

Now, where the hell did I leave my coffee...

[PCBONEZ]

Re: All replacement capacitors in iMac G5 PSU burned up

They are made by hand wrapping in places like this.
http://www.adapter-transformer-charg...out-litone.htm
http://www.buildweb.com.tw/company-profile.htm
The only thing a jig does is space the windings. [IF they even use one.]
With only 9 turns it can be 'eyeballed' just fine.

Now, where the hell did I leave my coffee...

The cat shot it.
That's what you get for having a cup that says Fuhjyyu.
.

[Wizard]

Re: All replacement capacitors in iMac G5 PSU burned up

Okay. We needed to make sure to fix the CAUSE that killed the coil.

The reason we need to know which output is hooked to the feed back circuit beause we wanted to know if that bad capacitor (not replaced) is on that monitored output or not?

Also is there more small electrolytics in the primary side? Is that replaced also?

Cheers, Wizard

[PCBONEZ]

Re: All replacement capacitors in iMac G5 PSU burned up

Okay. We needed to make sure to fix the CAUSE that killed the coil.

I agree.
That's what I said 49 posts ago.

[Toasty]

Re: All replacement capacitors in iMac G5 PSU burned up

...Also is there more small electrolytics in the primary side? Is that replaced also?...

There are at least 4 or 5 that were not replaced. Four are on the primary side of the opto's, one is the unreplaced cap already discussed. The 2 mains caps have not been replaced. Eight in total have been replaced.

Toast

[ran]

Re: All replacement capacitors in iMac G5 PSU burned up

...If that cap has not been replaced, you're asking for trouble. See Wizard's post from 2 days ago. Probably the ENTIRE supply should have been recapped... <snip>

Do you really think the high-voltage caps need replacing?

as per my thread (again, not to hijack nwd's ) I just wound up ordering all the low-voltage caps for replacing, but since the high voltage/primaries look fine, I fig'd I'd skip those.

I guess for me, $15 total (from Mouser) and a half-day's w ork or so were worth trying to resurrect the PS, otherwise, I'll just get a new one.

Ran

[Wizard]

Re: All replacement capacitors in iMac G5 PSU burned up

Actually must. Due to this tight confines and heat, these small capacitors needs to be replaced in primary side. Very important as this is the "protection" against your investment in effort.

When these small caps degrades, the signals for the main transistors distorts and weakened, will cause transistors to go "UNDERDRIVEN" and create much more heat and can do much worse damage.

And the controller IC that runs these transistors and monitors the feedback needs clean power that small caps provides and supplies energy that IC can't without the capacitors (as surge capacity) for the DRIVE.
Also I don't know if this IC needs other caps for the oscillation/ramp circuitry that might be one of reasons the "on" period got too long or too short and higher frequency and burned the coil.

SMPS usually start up high in frequency and descend or start very short pulses and widen the on period of PWM and till voltage comes up and come into regulation.

On periods too long or too low frequency burns things and goes over voltages. And in some dumb designs like this PSU, this can happen in strange way.

Cheers, Wizard

[Toasty]

Re: All replacement capacitors in iMac G5 PSU burned up

My work ethics are based on the old adage "If you don't take the time to do it right the first time, will you have the time to do it right the second time?". It's something I learned very early on, and has been one of my mantras since.

I figure I'm already there. The other 2 caps are gonna cost me $5 each and 45 minutes time - cut them out (the goop), desolder and remove them, solder the new ones in place, and re-goop them. At minimum wage that's about $5 (at average repair shop rates of $60-$100/hr that's $45 to $75) so for $15 I am confident that any more problems that the supply has are not because I didn't replace the mains caps.

Wizard has the technical reasons for this, I have the financial reasons. IMO, together they make a sound argument for replacement.

Toast

[Toasty]

Topic Status Update

A brief update as to the status of this topic.

I contacted NWD via PM about 6 days ago and offered to recap the balance of the PSU and repair/replace the burned toroid at no charge to him except for the postage and parts. I sensed a level of frustration on his part. He has no test equipment and must run to different places to beg or borrow something everytime we ask a technical question.

His response was positive but, he informed me that he had ordered a used replacement off of eBay. We agreed that he could send both for recap and that this was an added bonus as I would be able to measure and scope the untouched unit before recap. { I could see Wizard dancing. }

NWD compiled an order to duplicate the replacement caps for the "new" supply, using the old supply as a reference. I asked him to run his choices past PCBONEZ for his opinion and recommendations. His response was thorough, descriptive, and very informative.

I asked NWD to (once he received the new one) please test it in the computer for a few days to confirm it was working. I dreaded the thought of receiving an untested unit, finding it dead, and the ensuing battle with the seller on eBay.

Shortly thereafter, he received the new unit. Alas, it is different than the original. Grrrrr. I commented that it is probably the replacement that Apple issued after the fiascos with the PSU's and Logic boards. So, can the order for the caps.

He did install it and it runs the system fine. He found that the system seemed to be running hotter, but after some browsing through Mac forums, the temps seem inline with what others experience. (CPU running 65°C-70°C)

The offer still stood and I still wanted the old PSU for me to investigate. I was willing to cover the expenses of getting the old one to me just so I can have a go at it. After some investigation of the new PSU, NWD decided that the caps match some of the failures reported in the forums - "Mostly G-Luxon caps, 1 Nichicon, and 3 KZE". He decided to have the new one recapped while we are at it. I concur, and my offer still stood/stands.

As of today, the new caps are ordered and enroute to me from Digi-Key. (Panasonic FC, 1 LXZ, and 2 big Panasonics snap-ins(mains) ) They will recap the new supply and NWD was kind enough to acquire the remaining, unreplaced caps for the old PSU for me.

The 2 PSU's are also enroute and I should have everything in hand by the end of this week or Monday of next.

I will be posting updates and information as I obtain it. If anyone has questions or measurements that they would like to see done, please don't hesitate to ask.

The primary task will be to measure & test the new PSU, get it recapped, tested, and back to NWD as quick as I can. After that, I'll move on to the burnt unit.

Whew!

Cheers!
Toast

[Wizard]

Re: All replacement capacitors in iMac G5 PSU burned up

Yes!! Keep us updated! (Toasty, you picture myself right, clapping and dancing in the head hehe).

Cheers, Wizard

[Toasty]

Re: All replacement capacitors in iMac G5 PSU burned up

...and the mystery coil izzzzz...................

Drum roll please...........

*Boom!*

Tah-Dah

6.5µH

Digi-Key: M1411-ND

[iointerrupt]

Re: All replacement capacitors in iMac G5 PSU burned up

Toasty -

Thanks much for finding that coil part number.

I have a PSU that seems to have failed the same as NWD's, 6 bad caps and a rather sad looking coil. I figured it was hopeless when I saw that, but now maybe there is an outside chance for it.

Just to be clear, the final recommendation for fixing NWD's PSU seems to be to replace every primary and secondary cap, 15 in all?

[Toasty]

Re: All replacement capacitors in iMac G5 PSU burned up

Yes. Absolutely. There is no sense in replacing just some. You're there already, why not?

Wizard explained it from one point of view and I from another 4 & 5 posts ago.

The one that nwd did not replace originally because of it's difficult access is a TEAPO SC series, 330uF @ 35v, 10 x 20mm. According to the SC datasheet it should have an ESR of 0.044.

Out of circuit it reads 0.47 ohms on the ESR meter @ 320uF. Capacitance is okay (although I like them a little high 5-15%), but ESR is nearly 12x over spec.

That should say it all.

[nwd]

Re: All replacement capacitors in iMac G5 PSU burned up

Crazy. More strong support for replacing all caps whether they look fine or not - looks mean nothing! My tech said he had tested the Teapo cap and it looked okay. Now I wonder if he did test it and if so, what he tested for. Disappointing, as many people have told me he is the go to guy in this area.

It sounds like Toasty figuring out the specs on that coil may help many people with the same issue. Thank you!

[Toasty]

Re: All replacement capacitors in iMac G5 PSU burned up

>My tech said he had tested the Teapo cap and it looked okay. Now I wonder if he did test it and if so, what he tested for.<<

Many techs are not aware of how much the ESR factors into handling the high ripple current experienced in these PSU's and Mobo's. He probably tested it just to see that it was not shorted and that it responded to the Ohmmeter test. There's little else he could have done with it in-circuit. Capacitance tests are useless in this section of the PSU because there are 2 other caps, several diodes, and a transformer directly connected to it.

@iointerrupt
You will have to bend that piece of copper heatsink over the cap up a bit to remove the cap. The leads are just a bit too long to clear the board and allow easy removal.

To minimize the possibility of damage to the circuit board, make sure you use a high wattage/heat soldering iron to remove these. Their holes are thru-plated with heavy traces on both sides of the circuit board connecting them. I find a wide tip that will bridge both leads to be very effective. Use a solder sucker or a heavy pin (solder doesn't stick to them) later on to clear them for the new ones.

[PCBONEZ]

Re: All replacement capacitors in iMac G5 PSU burned up

- looks mean nothing! My tech said he had tested the Teapo cap and it looked okay. Now I wonder if he did test it and if so, what he tested for. Disappointing, as many people have told me he is the go to guy in this area.

It's probably not your tech's fault.
Was probably only taught the check caps for bloating, leakage, and with an ohm meter for charge/discharge/shorting and nothing else.
- Unfortunately even in electronics courses that's about all they teach.
The importance of ESR and how to check for it isn't mentioned (or is barely mentioned). You learn about it later from 'the old guys' when your specific job requires it. In PC repair most of 'the old guys' don't even know about it. PC repair courses generally teach TS&R to main assembly (motherboard, PSU, drive) only. They don't teach how to FIX motherboards, PSUs, drives, only how to tell if they are bad.

The poor training is in part a result of instructors going to school to be instructors and being allowed to teach without having any real world experience under their belt.

.

[Wizard]

Re: All replacement capacitors in iMac G5 PSU burned up

True.

My co-worker who was teaching me early on almost 7 years ago, had to ask for my help to fix a toshiba or hitachi tv with vertical issues and it was not vertical IC being bad. My bob parker's ESR meter spotted a small badcap after a minute of looking in a deflection processor circuit that uses this cap as a R-C ramp generator nearby.

Cheers, Wizard

[RJARRRPCGP]

Re: All replacement capacitors in iMac G5 PSU burned up

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

LOL Dr. Phil, just like the octuplet.