iMac 2011 820-2828 - LED #1 lits for a second then goes off

[tom.tom]

Hi all,

Got a 27 2011 iMac that wont boot. In the beginning, when connected to power, the LED #1 was lit and it went off after pressing the power button. I removed the MoBo battery, did SMC reset and now, when plugging the power cable LED #1 lits for a second and then goes off.

Checked with two other different PSUs - same thing.
Checked the MoBo with everything disconnected except PSU - same thing.

Till now have checked PP12V_G3H, PP3V42_G3H in a couple of TPs and adequate voltage is there. Any thoughts what shall I check next?

Edit1: SCM_RESET_L is 3.55V so the SMC is not in the Reset mode...

Thanks in advance,
Tom

[piernov]

Re: iMac 2011 820-2828 - LED #1 lits for a second then goes off

There may be a short to ground on a secondary power rail causing the PSU to enter overcurrent protection.
Go through the list of power rails and measure resistance to ground for each.

[tom.tom]

Re: iMac 2011 820-2828 - LED #1 lits for a second then goes off

Just to make sure I understand the power rails descendancy - this is described on the page #3 of the schematics, right?

So we have:

1) PP12V_S5 -> PP5V_S3_REG -> PP5V_S0 -> PP1V8_S0_REG
2) -> PP3V3_S5_REG -> PP3V3_S3 -> PP3V3_S3_MINI, PP3V3_S3_ENET
3) -> PP3V3_S0 -> PP1V0_FWPHY, PP3V3_S0_SD
4) -> PP1V5_S3_REG -> PP1V5_S0, PP0V75_S0

Is that correct?

Do I also need to track PP12V_S0?

Since LED#1 is on the PP3V3_S5, shall I suspect just this line: PP3V3_S5_REG -> PP3V3_S3 -> PP3V3_S3_MINI, PP3V3_S3_ENET?

Also - how do I read page 98 of the schematics, what does "I235 --> POWER -> POWER -> 3.3V -> PP3V3_S5" mean? What's the "I235"?

[piernov]

Re: iMac 2011 820-2828 - LED #1 lits for a second then goes off

The list of power rails is on page 6 "Power Alias", everything to the left of the '=' signs.
Page 98 doesn't matter, just some design rules for PCB routing.

[tom.tom]

Re: iMac 2011 820-2828 - LED #1 lits for a second then goes off

Resistance of the PP3V3_S5_REG_R line measured on the C7790 pin 1 and the GND gives me around 100ohms - is that OK or shall this be considered as a short?

I'm getting around 3V for a short while on the PP3V3_S5_REG_R line just after connecting the PSU. Does that mean ISL62383 (U7700) is OK and I should look further into PP3V3_S3 and PP3V3_S0 lines or is it too far, because they are powered only after pressing the power button?

If that's the case, the potential problem should be closer to the U7700?

Am I right or am I missing something?

[piernov]

Re: iMac 2011 820-2828 - LED #1 lits for a second then goes off

100 ohms to ground on PP3V3_S5 is probably too low but check resistance to ground on every power rails as I already said, there may be a problem elsewhere.

[tom.tom]

Re: iMac 2011 820-2828 - LED #1 lits for a second then goes off

Hi @piernov,

Had time only today to continue on that. With removed CPU, the LED#1 stays on. Reviewed the CPU socked, removed some strange small stuff from two places, but after putting the CPU back, the LED #1 goes off after a second again.

Any hints where to look at futher?

[tom.tom]

Re: iMac 2011 820-2828 - LED #1 lits for a second then goes off

...of course tried with another CPU - same thing