820-3478 1st led, noise HDD nothing more

raainxx

Member
Hi,
i have this board 820-3478 (clean), only 1 led with iMac supply.
I use an external power supply, I inject 12V on PP12V_ACDC, the current is 0,77A
I think it's too much, I have U2800 is very hot, PP1V05_TBTLC=0,3V, I suspect U2800. PP3V3_TBTLC=3,3V.
have you an idea?
 

2informaticos

Administrator
Staff member
Did you inject directly on the J6900 pins?
If you get S5 LED on with iMac PSU, should get it with lab PSU too.
Test again with original PSU.

Q8450/70 fails a lot on this board.
Q8470 should be good now, if you get steady 12V_S5.
 

raainxx

Member
hi,
Q8470 is good, I have 12V_S5. and 8470 too.
PP3V42_G3H=3,41V, S3,S4,S5 is OK.
I have PM_EN_REG_P1V05_S0=0V why? need the real iMac power supply?
thanks
 

raainxx

Member
PM_PGOOD_REG_GPU_VDDQ_S0 (pin 10) = 3,3V
PM_PGOOD_FET_REG_P1V5_S0 = 3,3V
PM_EN_REG_P1V05_S0_R = 0V
I try to find why PM_PGOOD_REG_GPU_VDDQ_S0 at 0.

I check that, I change U7700 but always same:
PP1V5R1V35_S0_GPU_REG = 0V
PM_EN_REG_GPU_VDDQ_S0 = 3,3V
PP5V_S0_REG_FBVDDQ=5V
 

raainxx

Member
Hi,
sorry no I have 3,3V on pin 9 and 0V on pin 10.
I check that for the pin 10:
PM_EN_REG_GPU_VDDQ_S0 = 3,3V
PM_PGOOD_REG_FBVDDQ_S0 = PM_PGOOD_REG_FBVDDQ_S0 = 0V
PP5V_S0_REG_FBVDDQ=5V
I change U7750 but always same
 

2informaticos

Administrator
Staff member
Check resistance in ohm mode, instead of diode mode; preferred method for low values.

"P5V_S0_REG_FBVD=5V"
Looks like broken trace to 5V_S0.

Be sure no short to ground at pins 19/20 of U7750.
 

raainxx

Member
L7710 = 0,0084V in diode mode, i compare with another same board (0,450V).
can I inject power? to find short? how can I do?
pins 19/20 of U7750 are OK (0,5V in diode mode)
 

2informaticos

Administrator
Staff member
I said to post ohm value.
If you have around 10 ohm there, voltage injection will not help probably.

What about missing 5V at U7750, don't you want to fix that first?
 

raainxx

Member
i have the 5V, why do you tell me about 5V.
I think it's the GPU because he heat when I inject a small current (1V 0,8A)
 

2informaticos

Administrator
Staff member
My error, I was thinking to miss voltage there...

If ohm resistance to ground is such low, comparing with good board, then you have bad GPU...
 
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