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[help] iMac 27 dead! Strange behaviour

P

phantom

Member
Hi to All! I have an iMac 27 2019, motherboard 820-01236 Dead.

When i Connect power supply, I can barely see a light from the second led

Nothing happens if i try to to boot the system

I’ve Tried to Connect my power bench supply directly to the motherboard and I have this behaviour

Video 09-04-24, 21 43 55.mov

Shared with Dropbox
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What do you think
 
2informaticos

2informaticos

Administrator
Staff member
Very difficult to work on this; no schematic/boardview available.

First LED is steady on?
Check if symptom changes removing CMOS cell.
 
2informaticos

2informaticos

Administrator
Staff member
That circuit may be 12_S5_FET, or 12V_S0_FET control.
Check for short on all power rails.

Just in case, change both MOSFETs...
 
2informaticos

2informaticos

Administrator
Staff member
You can use another schematics.
Just search for mentioned circuits.
00034 and 00134 have similar circuits.

"i don't know anything abouth them"
These MOSFETs have marking codes on top.
 
P

phantom

Member
00134 seems most similar

Dropbox

www.dropbox.com www.dropbox.com

In the white circle There is not the component that I have in my motherboard.

I’m assuming it is q8490 or u8490…how to understand?

But now? What should I do?

The first component to become hot, should be an IC and it should be U8470

Than q8470 and q8490 should be mosfet “pulsing”

Dropbox

www.dropbox.com www.dropbox.com

Dropbox

www.dropbox.com www.dropbox.com


ps i know that they have marking codes...but if search for them on google, i cant fin anything
 
Last edited:
2informaticos

2informaticos

Administrator
Staff member
In these IC-MOSFET pairs, the MOSFET is the one failing.
But you should check first for short on all power rails.

One MOSFET is for 12V_S5 and should always let to pass the voltage.
The one for 12V_S0 (not SSD), also fails.
But these failures are known for older iMac boards; may happen here too, not sure.

Some schematics show codes for plastic package MOSFETs, easy to find.
You can compare with your and choose correct one.
 
P

phantom

Member
So in This picture

Foto 10-04-24, 11 11 16.jpg

Shared with Dropbox
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On the left I have the ic (and it should be u8470), on the right, I have the mosfet (it should be q8470)

On This picture

Foto 10-04-24, 11 11 21.jpg

Shared with Dropbox
www.dropbox.com www.dropbox.com

There is the third “thing” that become hot… and zooming with microscope, I’m sure that it is an ic and not a mosfet

So it COULD BE U8490?!! But I can’t find mosfet “in pair” around it…

I’m stuck! It is too complicated for me! I can’t find shorts…around

Please help
 
P

phantom

Member
ok! i have full short on c7620 pad ! it is pp3v3_s5_ssd!! if i inject voltage on this pad, i can clearly see the line near the ic that pulse....but i still have short even removing the ic!
 
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