820-3302 won't turn on, only first led

Geof73

New member
Hello guys,

I have a imac 21.5 2012 (820-3302) with some weird issues, i will try to do my best to explain clearly (btw sorry in advance for my terrible english).
First, L9100 has blow up, i think because of the lcd cable that wasn't properly pluged (shorted, changed him).

I replaced L9100 but then, the fan was always at full speed enabled even if the imac was turn off. I read on the board that q7950 was probably the problem, i changed it and after the imac was ok.

I didn't used it only one day, i tried to turn him on but nothing, only led 1, i changed q7950 (q7950 not new) again and he managed to turn on, but after that, dead again, same thing, again after 1 day that i didn't used it.

Since, i again changed q7950 by another one but it never turned on again. Tried even a brand new q7950 but nothing, only led 1.

So i tried to change U7950, U7870, Q7870 (all are news), U7950 have a weird voltage, Q7870 and u7870 too :

U7950 :

Pin 1 = 12.16v
Pin 2 = 3.3v
Pin 3/4 = 0V
Pin 5 = 12.16v
Pin 6 = 19.20v
Pin 7 = 12.16v
Pin 8 = 12.00V

Q7950:

Pin 1, 2, 4, 8 = 12.16v
Pin 3, 5, 6 = 12.16v

U7870:

All good except
pin 7 = 19.22v
pin 8 = 3.8v

Q7870:

All good exept
Pin 4 = 19.22v

I also noticed that U5300 have pin 6 17v.

Big thanks in advance for your help.
 

2informaticos

Administrator
Staff member
First of all, welcome to the forum!

Keep in mind, N-channel MOSFET requires positive voltage between G and S to get ON.
Vgs=10-12V is normal value.
In your case, 7V (19.22-12.16) is a bit low; the MOSFET can overheat and die too soon.
I usually found 20-23V at Gate of these MOSFETs.

Please, count the pins correctly!
Pin 6 of U7950 and 3, 5, 6 of Q7950 are tied togeteher, check schematic.
You've mixed pins 6 & 7 of U7950 probably.

If you get stable LED501, 12V_S5 is present and U/Q7870 work both.

"I also noticed that U5300 have pin 6 17v."
This is absolutely abnormal; please use another multimeter.
Be sure to get correct readings...
 

Geof73

New member
First of all, welcome to the forum!

Thank you !

Keep in mind, N-channel MOSFET requires positive voltage between G and S to get ON.
Vgs=10-12V is normal value.
In your case, 7V (19.22-12.16) is a bit low; the MOSFET can overheat and die too soon.
I usually found 20-23V at Gate of these MOSFETs.

Very interesting, thanks for the advice. I bought these mosfets on aliexpress, very hard to find these days.

Please, count the pins correctly!
Pin 6 of U7950 and 3, 5, 6 of Q7950 are tied togeteher, check schematic.
You've mixed pins 6 & 7 of U7950 probably.

My multimeter is automatic, maybe it not work properly, i will check again.

If you get stable LED501, 12V_S5 is present and U/Q7870 work both.

So what can i check next ?

"I also noticed that U5300 have pin 6 17v."
This is absolutely abnormal; please use another multimeter.
Be sure to get correct readings...

OK i will check that too.
Thanks for your quick answer !
 

2informaticos

Administrator
Staff member
What is the actual state of the iMac, does it work?

BTW, write directly on the reply box; no need to quote an entire post, visible few centimeters above.
Please, maintain forum aesthetic...
 

Geof73

New member
No, i have only led 1. I've tested the button on/off and he is OK.
I will check again the voltage and i come back to you.

Sorry for that.
 

Geof73

New member
So i checked again with another multimeter :

U5300 :
pin 6 = 0V

U7870 :
pin 1 = 3.40V
pin 2 = 12.11V
pin 5 = 12.11V
pin 6 = 12.11V
pin 7 = 19.32V
pin 8 = 3.82V

Q7870 :
pin 1 2 3 = 12.16V
pin 4 = 19.20V
pin 5 6 7 8 = 12.16V

U7950 :
pin 1 = 12.11V
pin 2 = 3.31V
pin 3 4 = 0V
pin 5 = 12.11V
pin 6 7 8 = 0V

Q7950 :
pin 1 2 7 8 = 12.16V
pin 3 5 6 = 0V
 

2informaticos

Administrator
Staff member
No need to check again Q/U7870, I already explained why.

Q7950 will get on when board turns on.
U7950 should get 3V at pin 2 only then, and sends out 20V at pin 6.
Post diode mode to ground at 12V_S0_FET.
 

Geof73

New member
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