iMac power supply short-circuit diode to ground

[pcardoso73]

Hi,
I have an iMac with the power supply reference: ADP-200DFB.

This power supply is not working and when trying to debug the problem with a multimeter, I found what I thought to be a short circuited diode (the short-circuit is to ground).

With the continuity test it beeps both ways, so doesn't block current flow at all then reverse biased. But after removing the diode the beep continues and the diode seems healthy. It seems to exist a real short-circuit.

As I have another ADP-200DFB power supply (faulty one as well), I have checked the same diode and it seems to be short-circuited as the first.

I don't know if you agree with me, but there should be another component which is connected to this diode, which is faulty and creating this short-circuit to ground.

Any ideas on how to debug this? Where should I start?

Thanks in advance.
Best regards,
Pedro

[sam_sam_sam]

Re: iMac power supply short-circuit diode to ground

Ok
Is the short on the secondary side of this switching power supply
If so start removing parts one at a time until the short is gone

If it is on the primary side of the switching power supply then you for a part with 5 or more pins that the data sheet says that it is a regular of some type and check it for short

[pcardoso73]

Re: iMac power supply short-circuit diode to ground

Ok
Is the short on the secondary side of this switching power supply
If so start removing parts one at a time until the short is gone

If it is on the primary side of the switching power supply then you for a part with 5 or more pins that the data sheet says that it is a regular of some type and check it for short

Hi,
Thanks for your reply.
This diode is on the primary side, but I could not really understand your last sentence. Could you, please, re-phrase it for me?

Cheers,
Pedro

[sam_sam_sam]

Re: iMac power supply short-circuit diode to ground

You should look and if you find an ic chip that the part description says that it is some type of online switcher or off line switcher or some type of PWM controller chip for switching power supply

Here is an example “TOP246” or TOP247

https://www.digchip.com/datasheets/p...TOP247-pdf.php

[pcardoso73]

Re: iMac power supply short-circuit diode to ground

You should look and if you find an ic chip that the part description says that it is some type of online switcher or off line switcher or some type of PWM controller chip for switching power supply

Here is an example “TOP246” or TOP247

https://www.digchip.com/datasheets/p...TOP247-pdf.php

Hi,
The ICs nearby the diode are the DAP017-AH, for which I cannot find a Datasheet and I don't even know what it is, and the DAP015-AD (some kind of oscillator/pwm generator).

Could one of these be the culprits?

Kind regards,
Pedro

[sam_sam_sam]

Re: iMac power supply short-circuit diode to ground

Hi,
The ICs nearby the diode are the DAP017-AH, for which I cannot find a Datasheet and I don't even know what it is, and the DAP015-AD (some kind of oscillator/pwm generator).

Could one of these be the culprits?

Kind regards,
Pedro

It look like the data sheets for this device has been pulled for some reason

The part can be found but I do not know if it is becoming obsolete and that why you can find the data sheet

Is where the short is coming from if so just replace it and see if the short is not there any more

Can you please show a clear picture of this part so the information can be read

Thanks

[pcardoso73]

Re: iMac power supply short-circuit diode to ground

Can you please show a clear picture of this part so the information can be read

Thanks

Please find the picture attached.
As I have 2 PSUs with exactly the same problem, maybe this IC had a fundamental problem and was removed from the market.

Pedro

[budm]

Re: iMac power supply short-circuit diode to ground

Can we see good clear straight shot pictures of the whole top and bottom side of the board, pictures must be clear enough to see the traces?
That diode may be snubber Diode that is connected in parallel with the primary driving winding of the power transformer so that is why it shows low resistance reading, we need pictures to verify since it can also be connected in parallel with low Ohm resistor.

[pcardoso73]

Re: iMac power supply short-circuit diode to ground

Can we see good clear straight shot pictures of the whole top and bottom side of the board, pictures must be clear enough to see the traces?
That diode may be snubber Diode that is connected in parallel with the primary driving winding of the power transformer so that is why it shows low resistance reading, we need pictures to verify since it can also be connected in parallel with low Ohm resistor.

Hi,
Thanks for your reply. I've taken some pictures and I hope they are good enough.

Cheers,
Pedro

[pcardoso73]

Re: iMac power supply short-circuit diode to ground

Forgot this picture.

[budm]

Re: iMac power supply short-circuit diode to ground

I look at the bottom side of the board but unable to trace out the diode connections due to the black solder mask, I cannot see the traces.

[pcardoso73]

Re: iMac power supply short-circuit diode to ground

Hi,
I am sorry for not replying.
I bought another power supply, which works. As I have 2 power supplies from DELTA, this time I used another brand (ACBEL) and it works fine.

However, I really want to fix the other 2 or at least one of them.
I can try to clean it, so you can see the tracks, but even looking at it in my hand I cannot see much.
Maybe this PCB is multi-layer, since there are few tracks visible.

I am feeling tempted to sacrifice one of them, removing all the components, and try to draw a schematic from there, so I can fix the other.
I would like to take the components off the sacrificed board and design a PSU myself. In principle, I should have good quality components there.
Cheers,
Pedro

[sam_sam_sam]

Re: iMac power supply short-circuit diode to ground

This will be something that will be very time consuming
At the same time it might be a good learning experience for you

The two power supply are they the exactly the same or are they little different

[pcardoso73]

Re: iMac power supply short-circuit diode to ground

Hi
The power supplies are exactly the same.
Pedro

[Per Hansson]

Re: iMac power supply short-circuit diode to ground

I can try to clean it, so you can see the tracks, but even looking at it in my hand I cannot see much.
Maybe this PCB is multi-layer, since there are few tracks visible.

Most likely it is just a dual layer PCB, the problem is the black solder mask.
Try to get sunlight through the PCB, or a really strong artificial light.
That may reveal the tracks...
Example:
https://www.overclock.net/photopost/...4744-Crop.jpeg
https://www.overclock.net/photopost/...4741-Crop.jpeg

[pcardoso73]

Re: iMac power supply short-circuit diode to ground

Hi,
I tried both sun and a lamp, but I can't see anything.
The board is very populated, so it is difficult for the light to pass. I think it will be possible to do that if I start desoldering heatsinks, transformers, inductors and large components.
As I have 2 identical power supplies maybe I will do that to one of them.
However I think it is a multi-layer PCB and I will tell you why. Maybe I am wrong.

When I test the diode, as referred in the beginning of this thread, the multimeter shows a short circuit. After removing the diode, the diode is ok and the short circuit remains.

When I put the test probes in the hole where the diode was, the multimeter beeps but I am touching the inside of the hole.

You see, my soldering iron is not very good. It is a 15w one, which has no thermal capacity to unsolder thick solder balls. To desolder the diode I had to keep the iron there for a long time and apply lots of pressure. With that, the pads lifted from the pcb. So, now the only tracks I have are possibly tracks inside that hole. I don't know it is just a thought.

Cheers,
Pedro