iMac P/S posts then shuts down

[Toasty]

Re: iMac P/S posts then shuts down

Motherboard (midplane in Apple-speak) and power supply caps are primary suspects.

[trex66]

Re: iMac P/S posts then shuts down

thanks, Toasty - I pulled the PSU and I posted another thread, 'odd voltages' I'm getting 45.7V on the 3.3V pins when it is plugged in but not switched on. When I jumper it on, it falls to 3.3V BUT, the other voltages that turn on 5/12V pins start at 5.17V and 12.19V , then climb higher and higher. Afetr a few min I the 12V pins were 12.75V so I shut it off. I'm thinking when the voltages get too high for the midplane to filter, it's making the Imac shut off?

[Toasty]

Re: iMac P/S posts then shuts down

I suggest that you stay in the "odd voltages" thread. Until your PSU problem is corrected, posting here and there will just be confusing.

https://www.badcaps.net/forum/showthread.php?t=13163

nntr

Toast

[sylphhawkins]

Re: iMac P/S posts then shuts down

Hi guys, this is my first post (after a year or 2 of reading posts on badcaps, I finally registered because I have a problem that I haven't been able to fix from hunting down anyone else's posts, =p), and I'm aware that this may not the best thread to put it up in, so please let me know if I should start a new thread elsewhere, =)

Basically, I have one of the aluminium intel iMac 7,1 (mid 2007) 20' 2.4ghz models, with a 614-0403 power supply.

The imac has a problem where 90% of the time, it will power up (3 of the 4 diag LED's light up on the motherboard), with no chime, DVD and HD spins can be heard, and fan's power on, then it will shut down (usually after only 3 or 4 seconds), (diag LED's will return to just LED#1 on, others off), and then it starts up again, (same symptoms, DVD and HD spins, fans spin), and remain in that state with no change, no chime, no boot, no display.

Having repaired iMac G5's before, I immediately checked the PSU for cap issues, and they look fine, and measure fairly close to their specs.

Going over the board, the only thing noticeable is a yellow "burnout" looking flux residue on the underside, by Q33, D41 and D45.

I've checked the voltages on the PSU pinouts, and the only strange one that jumps out at me, is a 2.6v reading when in "standby-power off" mode, on PIN 5.
However, the voltages specified by a post over at ifixit.com:

(http://www.ifixit.com/Guide/Repair/i.../2740/2#s13220)

state that it should be reading 3.8v, and that it is acting as the PSU-enable pin?

Does anyone have any ideas, or experience with these PS units?,
I've replaced the caps that connect directly to PIN 5 from the 12v source, but the voltage has stayed at around 2.6v?

Thanks everyone!, any help will be super appreciated. ^_^

- Sylph

[godonr]

Re: iMac P/S posts then shuts down

Sylph... have not had the pleasure of messing with your model but make sure it's not a bad harddrive load causing the problem. Booting single and running fsck has cured the issue you describe on G5s I've worked.

[sylphhawkins]

Re: iMac P/S posts then shuts down

Hi godonr, =)

Thanks for the idea, i thought that perhaps there was a HD issue, so I pulled the HD out, and the problem remains the same, i've also loaded the HD in an iMac G5 that i fixed, and it's working perfectly, =p, for now, the G5 has also inherited the bluetooth card from the C2D until I can get it working, ^_^

The strange thing though, is that I have been able to get the 20' iMac to chime and boot, with perfect display and everything working great, but only after some random trials, and it only remained on for a few mins before freezing up. (Before I pulled the HD)

I was once able to get it to boot by leaving it running in it's "no boot, no chime" mode for a few hours, =p, with the CPU fan on max (by removing the LCD temp sensor), and after that, the next power off, power on, it chimed and booted!, I was shocked!, but it was only short lived, as it lasted for a few mins, then froze, and after a few restarts and freezing at various times, it was back to the same state as before, no boot, no chime.

I suppose that lead me to thinking that it might be an overheating problem?, and that the extra heat generated over the few hours with the power on, had helped to "reseat" something, or some component was able to operate somewhat?,

I also notice that there is probably a problem with the CPU temp sensor, as the CPU fan comes up to full blast after 1 min of being powered on in its no-boot state, and when I was able to get it boot a week ago, istat showed the CPU temp sensor as sitting at around 21*C, even after a few reboots,

Does anyone think that the CPU temp sensor could be stopping the iMac from even getting to Post?, disconnecting it doesn't change the problem?, but makes the fans run flat out from the word go.
^_^

[Toasty]

Re: iMac P/S posts then shuts down

This just got confusing. This is a G5, correct? We are 65+ posts deep in this thread....

[sylphhawkins]

Re: iMac P/S posts then shuts down

Woops, Point taken, ^_^

I am talking about an Aluminium Intel iMac (mid 2007 model), but it is exhibiting exactly the same problem as godonr, so I thought that maybe it would possibly be linked the same problem. =)

I'll make a new post and ask the question again, =p

- Sylph