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820-2784

I

itstallion

New member
Hi I've got an 820-2784-A out of a mid 2011 21.5 iMac. Only have #1 LED lit. hitting the power button spins up the hdd but no video, no chime, no fans. I've checked all S5 rails they are there. Checked all S3 rails they are there. I've checked all S0 Rails and the only discrepancy I'm seeing is PPVCORE_S0_CPU = .882 v according to schematic should be 1.1 v,
PP1V05_S0 = .008 v according to schematic should be 1.05 v. I pulled L7614 to check ohms on both sides on pin 1 i'm getting around 33-34 K Ohms, on Pin 2 i'm getting 21.6 Ohms. What do you think?

Thanks
Mark
 
larossmann

larossmann

Administrator
Staff member
Does it light up a USB mouse? vcore is supposed to move up and down depending on what it is doing so that is normal.
 
I

itstallion

New member
Yeah I saw your video the other day on that. no light on my mouse. I've checked for an enable signal on the 1.05v reg and I've also checked for 5v and 12v going to it. They're all there.
 
I

itstallion

New member
Yeah unfortunately I don't have a known good video card to swap out with. I did pull the video card and observe the same behavior.
 
D

dukefawks

Administrator
It needs the power good signal from the video card.
Check on U6900 if MXM_PGOOD is present, video card must be installed.
 
I

itstallion

New member
Video card installed.

Here's what I saw on U6900

Pin 14 = 3.306
Pin 13 = 0
Pin 12 = .023 video card uinstalled = 3.303 all other pins stayed the same
Pin 11 = 0
Pin 10 = 3.285
Pin 9 = 3.302
Pin 8 = 3.306
Pin 7 = 0
Pin 6 = 0
Pin 5 = 0
Pin 4 = 0
Pin 3 = 3.306
Pin 2 = 3.304
Pin 1 = 3.294
 
I

itstallion

New member
And U7000

Pin 14 = 3.305
Pin 13 = 3.302
Pin 12 = 0
Pin 11 = 0
Pin 10 =3.302
Pin 9 = 0
Pin 8 = 0
Pin 7 = 0
Pin 6 = 3.305
Pin 5 = 3.302
Pin 4 = 3.304
Pin 3 = 0
Pin 2 = 0
Pin 1 = 3.305
 
Last edited:
D

dukefawks

Administrator
Pin 12 on U6900 is the PGOOD signal of the video card. It's missing with the card installed so the card is bad for a start. The machine should however chime without the video card and USB mouse should light.
 
I

itstallion

New member
Yeah I'm getting neither of those with the video card installed or uninstalled. Since the PP1V05 seems to be missiing and that's killing the PCH what's the next step I should take. I've been checking all the components around that regulator. Start swapping with known good components?
 
D

dukefawks

Administrator
Is enable present on pin 1 of U7600? If it is present check for a short to GND on L7614. If there is none, replace U7600.
 
I

itstallion

New member
Ok so I replaced U7600 and I now have Led #2 but no boot no chime still. I do have fan spin now. Voltage on PP1V05_S0 is reading .390 v now. Reading .006 v drop from L7614 pin 2 to ground. Time to inject some power and see what get's hot? I've also replaced C7607 and 7608 with new ones. I went back and checked U6900 and I'm getting 3.3 v on pin 12 with the video card installed now. I had the temperature sensor unplugged before when I measured it if that makes a difference.
 
Last edited:
D

dukefawks

Administrator
If the output of L7600 is at 0.39V there is no way you can get a PGOOD signal on PCHCORE_REG_PGOOD. Check if that signal is missing else U7600 may be bad soldered and not pulling down the PGOOD signal.
 
I

itstallion

New member
Ok went back and resoldered. Back to where I started. No LED #2, .007 v on PP1V05_S0. Now what? inject power on L7614 pin 2 side and see what get's hot?
 
I

itstallion

New member
Yeah the only thing exhibiting heat is the PCH. My power supply showed .356 amps being used by the PCH. I applied some heat to the PCH and the usage went up well above an amp. I checked for voltage drop from pin 2 of L7614 with the PCH heated up and it showed .000. I'm calling this a PCH.
 
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