iMac 820-3126 Half Fan Spin

mushr00m

Member
Hello

This iMac is with white screen of death and I just bought a K2100 to solve this issue

However when dissambly I break 1 cap (C3634) and had to put back R3604.

Not sure if I damage anyother thing. However I did a GOOD visual inspection and didn't find anything

Now it only has 1 LED light on the motherboard and half fan spin
Any idea where to look for the problem.

Can I use my lab PSU to inject voltage on the main rail? 12V?
 

2informaticos

Administrator
Staff member
For repair purpose, you can use 12 lab PSU.
Removing CMOS battery will force board turn on when power is applied.

Just in case, check if starts in SMC bypass mode.
 

mushr00m

Member
Removed the CMOS!

Still not turning on and HALF FAN SPIN
SMC BYPASS mode try to start but only gives HALF SPIN


INJECTING 12V @ PP12V_G3H

NO FAN SPIN AT ALL
REMOVED THE CMOS = WHICH I UNDERSTAND = AUTO TURN ON

Should I turn on the board with a tweezer?
If so please confirm wich is the right pad/component to touch with the tweezer?

Below are the measurements on the board after injecting 12V on PP12V_G3H (C610)

LED 1 = WORKING
LED 2 = OFF
LED 3 = OFF
LED 4 = OFF

PP12V_S5 = 12V
PP12V_S0 = 0V
PP3V42 = 3.40V
PP5V_S5 = 5V
PP3V3_S5 = 3.30V
PP5V_S0 = 0V
PP3V3_S3 = 3.30V (Measured on R502 right besides LED 2 which still OFF)
PP5V_S3 = 5V
PP3V3_S0 = 0V
 

mushr00m

Member
The only thing which I do not think is important to inform you. However I am giving you the image was the J4521 connector which is connected to the DVD/ROM it broke when I was removing the cable. However do not think is bad or critical for the board to work normally

image attached
 

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2informaticos

Administrator
Staff member
J4521 is not important, if doesn't cause any short.

As you get S3 voltages, board started power on sequence.
Don't need to trigger SMC_ONOFF_L; S5010 pads can be used, if really need that.

Do you get 12V_S0?
 

mushr00m

Member
What do you mean by as you get S3 voltages. board power on sequence?

Sorry did not understand

After giving full power rails info. Where would you recommend me going to check?
 

mushr00m

Member
I also tried to power on the board with a TWEEZER on S5010 (PIN 1 AND PIN GROUND) and didn't happen anything.

Only still LED 1 ON

I am bit lost where to go on this one
 

2informaticos

Administrator
Staff member
Mmmmh, this old machine has 12V_S0 FET placed into its PSU.
Injecting 12V from lab PSU directly doesn't help.
You can try to run a wire between 12V_G3H and 12V_S0, for this purpose.
Be sure first no short there.

I recommend to use original known good PSU.
 

mushr00m

Member
Is there a test to check if original PSU is good? It came working normally. The problem was white screen of death VGA problem.
 

2informaticos

Administrator
Staff member
Be aware, working with high voltage device is dangerous!!!

According with schematic, iMac PSU can be tested forcing PS_ON signal; it should bring on 12V_S0 output.
 

mushr00m

Member
Percect!

What means FORCING PS_ON? Can you give me more details?

12V_S0 is not shorted to ground. It DOES not beep to ground in continuity
 

2informaticos

Administrator
Staff member
I don't recommend to work with PSU alone.
If PSU is connected to MLB, trigger SMC_ONOFF_L; PS_ON should change its status.
 

mushr00m

Member
The iMac original PSU does not make any sense to be BAD.

It came as white screen of death. Turning ON and OFF normally.

PAY ATTENTION to this

I assemble this board into another working machine with a known good PSU

It did same thing

With pressing power HALF SPIN stop HALF SPIN again still ONLY LED 1 OK
With SMC BYPASS samething however HALF SPIN try to start stronger ONLY LED 1 OK

HOWEVER ALL OF SUDDEN

Twice it turn on the second LED.

I turn off from the power cord assemble the screen and back to the same behavior


With pressing power HALF SPIN stop HALF SPIN again still ONLY LED 1 OK
With SMC BYPASS samething however HALF SPIN try to start stronger ONLY LED 1 OK


ANDDDD NOWWWW

Assembling it back to the original PSU it appears the LED 1 for a second and then turn off.

Now with its OWN psu I do not have LED anymore.
 

mushr00m

Member
Replaced U7700 with a U7600 from a 820-4668 which according to the schematic is the same IC.

Same behavior with PSU lab

Only LED 1 active
0.050a on the PSU lab

Tried to trigger ON C5014 with a twezzer and still only LED 1 active
 

2informaticos

Administrator
Staff member
As I said, 12V_S0 is provided from iMac PSU.
If you inject 12V with lab PSU just into 12V_G3H rail, board will never start!
 
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