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Old 10-11-2021, 05:07 PM   #1
Crystaleyes
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Default iMac A1224 PSU, 614-0421 - no standby voltages

Hi chaps.

Having recently picking up two dead 2008 iMacs, one is now working, however there is still one to repair.

I was told that they both had failed power supplies, and seeing how I intend to crack SMPS repairs, it was a challenge not to ignored.

Anyway, this PSU in question had already had someone having a pop at soldering.

The current problem is (at the very least) in the primary side.

Fuse is ok
MOV is ok
NTC is ok
Bridge rectifier is ok (~380vDC)
Fat filter cap is ok
Transformer FET is ok with high volt DC on Drain

What is missing are both the standby voltages, which are normally found on pins 5 and 9 of the output cable connector. Pin five is supposed to be 3.something volts, and pin 9 is normally 12v

I am suspecting that the inverter section would fire with these missing voltages.

My question as a learner, who still hasn't quite grasped standby circuitry is, "where can I trace these voltages back to, and is there any component or area in particular which should be checked?"

I'll include more photos as I take them. These are of an identical unit.
Attached Images
File Type: png Underside.png (790.5 KB, 4 views)
File Type: jpg PSU top.jpg (41.3 KB, 8 views)
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Old 10-11-2021, 06:57 PM   #2
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Default Re: iMac A1224 PSU, 614-0421 - no standby voltages

From my limited experience, I would have expected to see a secondary transformer for the standby.

Perhaps I am just missing it?

Apart from the main transformer, appears to be only inductors.

Do they sometimes combine the standby windings in the main tranny?
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Old 10-12-2021, 09:24 AM   #3
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Default Re: iMac A1224 PSU, 614-0421 - no standby voltages

So, poking around with a DMM appears to show short across the four secondary terminals of the transformer.

Apart from the FET linked to the PFC chip, there are four more FET's, which when pulled, three tested bad using diode test option. Not short, but steadily rising to OL between Source and Drain.

Starting to wonder if this PSU wasn't zapped in a lightning storm or something similar?
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Old 10-12-2021, 10:22 AM   #4
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Default Re: iMac A1224 PSU, 614-0421 - no standby voltages

fets can be deceptive unless you use a component tester
a lot of multimeters cant get clean readings from schottky diodes either.

so lets talk basics about switching psu's
the chip needs power,
the power comes from the transformer when it's running,
but to get it started you have some high value resistors in series that go through a diode and charge a small electrolytic - usually 10-220 uf

once the electrolytic charges, the psu starts and runs,
we call that the startup-cap

this dries out pretty fast because of it's size and use, i always change them on sight.
also on older stuff using chips like the 3842's they can sometimes charge a bit fast and degrade the feed resistors - so check they meter the same as they are marked.

Last edited by stj; 10-12-2021 at 10:24 AM..
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Old 10-12-2021, 01:11 PM   #5
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Default Re: iMac A1224 PSU, 614-0421 - no standby voltages

Do actually have a cheap MK 328 component tester and the three 'bad' FETs tested ok.


I'm gonna try to describe as much as I can...

The two chips on the pcb side are a 16-pin L6599 HV resonant controller, and an 8-pin 1PCS02G, PFC controller (.pdf's attached)

On the top-side, coming off of the 380vdc positive of the bridge rectifier, there are two 24k 1W(?) resistors in series, followed by a bulbous diode, an NTC and a TO-220 diode before arriving at the 'Drain' pin of a FET, whose 'Gate' pin is connected to pin-8 of the PFC controller and whose 'Source' connects directly to the primary windings of the transformer.

What is confusing me, is that this 'Source' pin is also directly connecting to the negative terminal of the bridge rectifier. So if the FET gets triggered it would create a direct 380vdc short across the bridge rectifier, no?



Back to the start-up cap..

Other than the main filter cap, there are three small electrolytics in the primary section on the topside of the PSU, however I can't see how or where they connect to the resistors and diodes in series?

Anyway, they are;

1 x 50v 22F and 2 x 16v 100F.

All three test fine in circuit. Neither short nor open.


On a slightly different note, there are two (what appear to be) 2-pin ceramic TO-220 components on heat sinks which have no clear markings.
I have never seen these before, so not really sure how to measure them although the one on the primary measures 1.5kΩ, or between 2 to 2.2 volts on diode test, with the one on the secondary measuring 9.5kΩ, or 1.256v both ways.
Attached Files
File Type: pdf 1PCS02.pdf (396.8 KB, 0 views)
File Type: pdf L6599_datasheet.pdf (635.9 KB, 0 views)
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Old 10-12-2021, 01:32 PM   #6
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Default Re: iMac A1224 PSU, 614-0421 - no standby voltages

it does not matter that the caps dont look shorted or open, what matters is they still have the ability to store enough mA of current.
do an ESR test.

as for PFC - it's a fucked up concept - yes, it shorts the supply by design to discharge the inductor.
i think thats the trick anyway.
it beats the hell out of the big mains smoothing caps!
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Old 10-12-2021, 01:57 PM   #7
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Default Re: iMac A1224 PSU, 614-0421 - no standby voltages

Thanks for the input.

Interesting. When tested the other day the 22F was reading 25 and now it is over 100, and tested various times.
Gonna pull it out but need to go out now.

I'll report back later.

Here are the readings...

C108 - 100V 22F

ESR = 0.63Ω
V Loss = 12%
Captnce = 114.5F



C130 - 16V 100F

ESR = 0.21Ω
V Loss =0.9%
Captnce = 101.1F



C38 - 16v 100F

ESR = 0.23Ω
V Loss = 6.9%
Captnce = 107.8F
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Old 10-12-2021, 02:13 PM   #8
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Default Re: iMac A1224 PSU, 614-0421 - no standby voltages

the 22uf cap has internal leakage fooling the tester - it's fucked as expected.

maybe you should update the tester firmware and change the crystal too.
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Old 10-12-2021, 02:28 PM   #9
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Default Re: iMac A1224 PSU, 614-0421 - no standby voltages

Maybe I should?

It wasn't something that I was aware of...

I have a second one which is intermittent which would be great to resurrect
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Old 10-12-2021, 03:43 PM   #10
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Default Re: iMac A1224 PSU, 614-0421 - no standby voltages

post pictures of them - so i know the hardware they are based on.
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Old 10-12-2021, 08:42 PM   #11
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Default Re: iMac A1224 PSU, 614-0421 - no standby voltages

Two images attached.

Pulled the 22F cap and it reads

ESR = 0.55Ω
V Loss = 1.2%
Captnce = 21.20F

Had hoped it would be clearly damaged.
Attached Images
File Type: jpg P1010036.JPG (2.05 MB, 5 views)
File Type: jpg P1010035.JPG (2.76 MB, 4 views)
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Old 10-13-2021, 04:14 AM   #12
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Default Re: iMac A1224 PSU, 614-0421 - no standby voltages

test the big mains caps - as it has pfc

the yellow tester in the acrylic is a T4
not sure about the other one - never seen that before.
wanna take internal pcb pics?
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Old 10-13-2021, 07:09 AM   #13
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Default Re: iMac A1224 PSU, 614-0421 - no standby voltages

The larger tester has 'MK' 328 on the case, although it displays 'GM' 328 when switched on.

Just noticed all that goop below the on/off button, which may be why it has been problematic..?

I'll have to test the cap later.


So do you not think reading short across the transformer secondary terminals is a problem?
Attached Images
File Type: jpg P101034.JPG (2.78 MB, 3 views)
File Type: jpg P101035.JPG (2.56 MB, 3 views)
File Type: jpg P101036.JPG (2.26 MB, 3 views)
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Old 10-13-2021, 07:12 AM   #14
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Default Re: iMac A1224 PSU, 614-0421 - no standby voltages

Unless you have a shorted rectifier on the output etc, I've generally found in switch modes it's a smaller cap on the primary side, like 1uf near the PWM controller. There's a couple of resistors and a cap that provide a kick start to the controller. The bigger value caps on them rarely fail. If you do replace any, use good quality low ESR caps.

My experiences with SMPS have been mixed over the years. Often the components used are obscure and hard to source. I've had a few in years gone past where you go through, replace all the bits that are blown, only for it to blow everything again on power up. My technique for these is to replace every semiconductor on the primary side in that event (FET, diodes, IC) but I've only ever done that when I can't get the supply as a spare. Anything too complicated, it goes in the bin. The worst one I ever had was a self oscillating SMPS in a Barco projector back in the 90's. Gee we blew up some semi's on that one, and we ultimately replaced it. I learnt the hard way on not to spend too much time on them.

The proper way to test these would be to construct a proper component tester I guess (check out Mr Carlson's lab for some good tips on building one). But the old adage of "If in doubt, rip it out" applies if you have a big enough store of spares.

Last edited by reformatt; 10-13-2021 at 07:15 AM..
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Old 10-13-2021, 07:47 AM   #15
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Default Re: iMac A1224 PSU, 614-0421 - no standby voltages

transformer seconary before diodes or after?
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Old 10-13-2021, 02:15 PM   #16
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Default Re: iMac A1224 PSU, 614-0421 - no standby voltages

Quote:
Originally Posted by reformatt View Post
Unless you have a shorted rectifier on the output etc, I've generally found in switch modes it's a smaller cap on the primary side, like 1uf near the PWM controller.
Would it be correct to assume that the PWM controller is the L6599, and therefore any ~1f cap would be an smd?

Quote:
Originally Posted by reformatt View Post
Anything too complicated, it goes in the bin.
It is true that it would be easier to buy a replacement, however, the only reason I bought this dead iMac was to further my understanding of SMPS design and repair.
I am qualified with many years experience in electrical engineering/construction and although I could swear that each time I look in the mirror I am getting younger, my body refuses to play the same game, therefore I fancy a future which is electronics repair based, as I've been playing around with tronics for a fair while...
So, I've been reading plenty on SMPS's but now need the hands-on experience, as well as those with experience who can help fill in the missing pieces.

Quote:
Originally Posted by reformatt View Post
The proper way to test these would be to construct a proper component tester I guess (check out Mr Carlson's lab for some good tips on building one).
Didn't realise he had tips on offer but shall have a look.

Thank you
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Old 10-13-2021, 02:44 PM   #17
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Default Re: iMac A1224 PSU, 614-0421 - no standby voltages

Quote:
Originally Posted by stj View Post
transformer secondary before diodes or after?
The only diodes I can see after the transformer are SMD's, so I presume it is not them you are asking about...
Are there diodes hidden somewhere which I am unaware of?




There is only one big mains cap, it is a 150f 420v

and reads...

ESR = 0.14Ω
V Loss = 1.2%
Capacitance = 135f
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Old 10-13-2021, 05:30 PM   #18
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Default Re: iMac A1224 PSU, 614-0421 - no standby voltages

Most SMPS incorporate either overvoltage or overcurrent protection, and will shut down in that event. So a shorted rectifier diode on the secondary side is often the cause of a shut down event. Another technique is to put a crowbar circuit on the main output. This is an arrangement of a zener diode linked to an SCR. The zener voltage is selected to be just above that of the rail. If the voltage goes high, the zener will conduct, it triggers the gate of the SCR, it turns on, shorts the output, then the supply shuts down. I used to get these protection circuits firing incorrectly, often due to a bad zener.

BTW, if you are working on these things live, an isolation transformer is absolutely necessary. The primary side is floating otherwise. Some supplies incorporate a bleeder resistor on the main caps too to ensure that the 400V or so will discharge when the supply is off. Otherwise you should short the caps before doing any passive tests. Personally I'm a bit of a pussy now compared to my younger days so I usually replace them if I have to do any serious primary side troubleshooting.

So you need to determine what is the nature of the failure for your troubleshooting. Is it a catastrophic failure on the primary side (blown fuse, shorted switching FET's, blown snubber caps/diodes etc), overcurrent or overvoltage protection, or just not starting up?

If you don't have a schematic, my general rules of thumb are these for SMPS:

1. For lead free designs, go over the bigger components (FET's, TX, Diodes) with leaded solder and a big tipped iron.
2. Physically inspect all components under a microscope for holes/burns etc.
3. Check/replace the smaller value electrolytics, mainly on the primary side near the controller (like a TL494). Electro's go low in value with heat/age and their value usually rises when they warm up. This is why old TV's used to come good after they got warm! Tantalums in my experience usually go short rather than change value.
4. For catastrophic failures, check snubber components in addition to all the blown semiconductors. These will be ceramics and diodes around the main FET(s). If in doubt, rip it out, and uprate any ceramics to 2kV. If the PWM controller is direct coupled to the switching stage, then I'd be replacing that too.
5. Zeners are difficult to test passively. They can measure ok in diode mode, but that's not how they're used is it? So in the event of catastrophic failure, I would replace any on the primary side on spec.
6. Transformers can also have shorted coils, bad laminations etc. So if you suspect that, you replace the supply.
7. I don't know if it's a thing these days, but back in the 80's and 90's, manufacturers (like Sony) used to use a glue on the PCB's to secure components in place prior to wave soldering. This glue would go black and conductive after a few years. Sony subsequently bought out new glues that fixed that. They were special types if I recall that didn't use acetic acid for curing. So any black glue gets removed first.

I've attached a schematic of a Delta supply I've found. This is a very complicated supply so it's not the best one to start on. But it gets the general gist across with standby voltages, power good signals etc.
Attached Files
File Type: pdf delta_dps200pb-59h_rev.00_atx_power_supply.pdf (431.3 KB, 2 views)

Last edited by reformatt; 10-13-2021 at 05:39 PM..
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Old 10-13-2021, 07:33 PM   #19
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Default Re: iMac A1224 PSU, 614-0421 - no standby voltages

Thanks for the schematic.

It is similar to some of the other ones I have been looking at.

A basic summary of my limited understanding on SMPS's (in general) is that AC mains comes in. After passing through a few inductors and caps, it is rectified to high voltage DC before further smoothing by the large filter caps.
From here, it supplies any standby transformer and subsequent circuit, which once activated, supplies power (perhaps via a relay) to the PWM to trigger the FETs which switch the direction of the rectified DC at a high frequency across the primary of the transformer, (which may have various windings, per output voltage).

Into the secondary area or the PSU, the now alternating current is once again rectified and smoothed using diodes, inductors and capacitors. It wouldn't surprise me to see voltage dividers sometimes too.


As I said, my understanding is limited, yet it is considerably greater than even one month ago.

I do get it that frequencies can be generated using crystal oscillators or small value capacitors on the input of an IC.

Overvoltage or overcurrent protection only enters my reality at the moment in the form of MOV's and NTC's, but I am up for learning

As of right now I have no isolation transformer, however I am extremely aware of the dangers of DC in this voltage range, but thank you for pointing that out.




Some of the things different from other SMPS's which have stood out with this Mac PSU are;

.. that there is seemingly no standby transformer.
..There are no large diodes on the output of the transformer
.. There are no large inductors on the secondary section

There was nothing visually out of the ordinary when I got this PSU. Under a microscope, nothing is clearly burnt, exploded nor dry connected. The fuse, MOV and NTC are all fine, as are all the fets and, resistors and caps which I have tested to date.

One thing that I would like to ask regarding this board, is, "what are these two TO-220 ceramics on the heatsinks, which you were possibly referring to as being part of the 'snubber' circuit (whatever a snubber is)?"

They have TH131 and TH151 written next to them...

Last edited by Crystaleyes; 10-13-2021 at 07:56 PM..
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Old 10-14-2021, 04:59 AM   #20
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Default Re: iMac A1224 PSU, 614-0421 - no standby voltages

The TO-220 packages are likely the rectifiers (2 diodes in each one, with centre pin being common). They would likely be Shottky types; fast recovery and low voltage drop.

A conventional linear power supply running at AC 50/60hz requires a bulky transformer and we're only dealing with pure SIN waves. They're quite shitty for efficiency, especially at higher power deliveries. They're big, bulky, inefficient and generate lots of heat.

Basically what a SMPS does is switch at a much higher frequency. This allows the transformer to be much smaller as a result. Rather than using SIN waves like a linear supply, they use on/off switching (square wave) instead. The switching element (BJT or FET) has to get from fully off to fully on as fast as possible in order to reduce losses in it, and to also to prevent it's own destruction too. The duty cycle is varied by the controlling PWM IC, using feedback of the voltage output provided from the secondary side. To maintain electrical isolation, this is usually done with an optoisolator, or via a transformer in some cases.

Re snubbing, you'll need to brush up on back EMF with inductors/transformers to understand that. But essentially, when you pass current through the coil, then stop it, the voltage across the coil reverses in order to maintain the current through the coil. In my TV days, this was called 'flyback'.

This voltage can be much, much higher than your 400V DC from the caps. If left as is, it will exceed that maximum voltage for the switching FET and destroy it. The snubber is formed via the diode in reverse across the FET/coil (some FET's have this inbuilt into them), along with a filter network of cap/resistor to suppress that flyback voltage. It's all Fourier stuff with harmonic suppression etc, but suffice to say, if you have issues with the diode or caps of a snubber network, you ultimately have destruction of your switching device pretty quick.

To demonstrate how important the snubbers are, I recall this from many years ago. The line output stage of an CRT TV works on similar lines to a SMPS. I recall reading about one of these snubbing caps going open circuit on an old Philips TV years back. The EHT stage is supposed to run at 10KV, then stepped up to 30KV via a tripler (network of caps and diodes). The cap being open cct caused the output to rise to 100KV and blew the yoke of the TV through the top of the cabinet!

Last edited by reformatt; 10-14-2021 at 05:02 AM..
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