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820-00292 - A1419 Late 2015 iMac - No LED Lights

M

mccafe

Member
Hello,

I have an iMac with a faulty motherboard it seems. None of the LED light indicators are on and I confirmed that 12V is being provided by the power supply. I also replaced the power supply to rule it out. Seems to be failing very early in the boot process and I am wondering how can I start with the troubleshooting.

Thank you.
 
2informaticos

2informaticos

Administrator
Staff member
You should get at least S5 LED on.
Check 12V_S5 FET voltages.
Let us know which schematic used, for more details.
 
M

mccafe

Member
I checked the voltages for Q8470.

FET_EN_P12V_S5_R = 21.6 volts (pin 4)
PP12V_S5 = 12.4 volts (pins 1, 2, 3)
PP12V_G3H = 12.4 volts (pin 5)

PP5V_S5 = 5 volts
PP3V3_S5 = 0 volts?

I am using 820-00134 schematic.

U7600
- PM_PGOOD_FET_P12V_S5_SSD_K -> EN1 = 4.4 volts (for 3.3 volt S5 enable)
- REG_VIN_U7600 = 12.44 volts
- REG_P3V3_S5_OUT = 0 volts (0.117 resistance to ground)
- PM_EN_REG_P5V_S4_R -> EN2 = 0.4 volts (for 5 volt S4 enable)
- REG_P5VS4_VOUT = 0 volts

U7620
- PP12V_S5_SSD = 12.44 volts
- PM_EN_FET_P3V3_S5_NON_SSD = 0 volts

Does this mean a faulty U7600? I hope this is not the case as I am not sure where to get an ISL6238 chip.
 
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M

mccafe

Member
I replaced the ISL62383C and I get 3.8 volts on the enable pin, but no REG_P3V3_S5_OUT. VIN = 12 volt still. I am not sure what I am missing, could there be something else? Perhaps Q7610 has failed as well? According to the spec it should be a positive voltage and I see about 1V.

Now I have an interesting issue. the PP5V_S5 is gone. I confirmed my soldering is ok so not sure what happened. Maybe the ISL62383C chip I soldered is faulty?
 
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M

mccafe

Member
I replaced the ISL62383C and it looks like LDO5 pin 18 is kind of back at 4.4 volts. VIN is no longer 12v but 4.4v. Am I just getting bad chips?

I also just re-read your reply. If diode mode is good (high resistance), then the chip is bad? It was a low resistance (0.070 or so) and thought chip was faulty so I replaced it. I misunderstood, but I am getting 4.4 volts now?

REG_P3V3S5_VOUT is 0.5 volts and resistance is 0.100. Is resistance normal?

EDIT: PP12V_S5 is now gone from the power supply - I think it just failed. Does the PSU have to be connected to the motherboard to provide 12V?
 
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2informaticos

2informaticos

Administrator
Staff member
You can use lab PSU to power the board in repair process.

12V_S5 comes from Q8470, not directly from the power supply.
 
M

mccafe

Member
After some troubleshooting, I found a short on Q7610. Replaced it and also replaced U7600 and now I have 12V back. I didn't get PP3V3_S5_SSD which = 0.6 v. Injected voltage and found short on U3781. Removed and short on PP3V3_S5_SSD is gone. However, I have a short on PP3V3_S5. Injecting voltage draws about 0.9-1 amp from the PSU however I am unable to find anything heated. Looks like a direct short to ground somewhere. Any advice?

I also removed Q8440 and Q8400 and confirmed that there is no short on PP3V3_S4 and PP3V3_S0 as well as the two FETS themselves.
 
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