Level 1

36 points

iMac not starting up (no chime and no USB power)

Hello,

My neighbor's computer is experiencing some problems: it does not start (the screen stays black, no startup chime, but the fans run, as does the hard disk. Also, neither the mouse or the keyboard seem to be powered (no light), so the computer doesn't even come to loading the EFI). The model is an iSight iMac G5 20'' (EMC=2082).

I have tried usual commands (reset PRAM and reset the SMC). Also tried with no device connected. (well, if something so trivial than all these tips did the trick, I wouldn't had to ask here ).

So, without keyboard, I couldn't even try special modes (safe, target disk, start from CD, etc), should the Mac be able to start in some mode. I have then decided to open the iMac (well, the warranty is way out, so nothing is lost). I have never seen a so-hard-to-open Mac, by the way (something was likely blocked).

I'm thinking of 4 possible causes for the problem:
-the hard disk could be damaged. It was malfunctioning some days ago. I assume it could come to a point where the logical board of the disk is failing;
-the RAM is an obvious possible cause;
-the CPU is also obvious;
-another component on the main board/logic board.
-I have forgotten a 5th point?

Well, I tried with unplugging the hard disk, and it had no effect, so it's not the HDD. I'm puzzled for the remaining ones (not knowing the iMac enough is a cause).

First, about the RAM, it seems the RAM slot is empty, so the actual RAM is somehow built-in (I haven't seen something seeming like usual RAM bars), so, if I can't remove the RAM, how can I test whether it's damaged or not?

Then, the CPU. There are 2 of them. So I thought I may take one out (and then the other one) and see whether the Mac starts. Well, I can't figure out how to remove the CPU cooling piece (there are just 4 "buttons" that can be pressed but nothing happen). I prefer asking here first, not removing the entire main board without advices.

Lastly, if it's the main board, I know I won't be able to do much. I have read there are LEDs to possibly diagnose power issues, but I don't see them (no LED is showing light, it seems). I'd like to test my other assumptions before thinking the main board is the cause.

Else, is there something I could try I have forgotten?

P.S: sorry for my (possibly) bad english, and the troubles of explanation; it's not as easy when english isn't the native language. I just wanted to mention that I have already repaired computers, so don't hesitate to tell me complicated answers.

Thanks in advance for your help.

MacBook Pro, Mac OS X (10.5.1)

Posted on Feb 12, 2010 9:49 AM

Top-ranking reply

Anic264b Author

Level 1

36 points

Posted on Feb 13, 2010 9:18 AM

spudnuty wrote:
Anic,

Also, neither the mouse or the keyboard seem to be powered (no light),

First you might try booting without any USB devices connected.

Well, we sort of tried that (while the Mac was open, with no device connected, we switched it on several times: it didn't started up either).

I tried with unplugging the hard disk, and it had no effect

So you already have it open.

Yes, hardly, but yes. I already did what I learnt in "high school", except the RAM which seems to be hardly testable.

the CPU. There are 2 of them.

Hmm there's only one actually under the large shroud and heat sink with the large fan and a small one at bottom.

So I thought I may take one out (and then the other one) and see whether the Mac starts

Well don't bother since it's not removable without professional rework equipment.

So it seems nothing can be tested on that model? You can't remove the CPU nor remove/replace the RAM… Really a bad model! (still, I like Macs, but that's amazing…)

I have read there are LEDs to possibly diagnose power issues, but I don't see them (no LED is showing light, it seems).

OK the LEDs are under tape. From the service manual:
"13. Locate and peel up the small piece of black tape near the bottom of the logic board.
14.Under the black tape you’ll see the four labelled diagnostic LEDs...
LED 1: Indicates that trickle voltage from the AC power inlet to the power supply has been detected. This LED will be ON when the computer is turned off and your power supply is working correctly.
LED 2: Indicates that the main logic board has detected proper power from the power supply when the computer is turned on. This LED will be ON when the computer is turned on and the power supply is working correctly.
LED 3: Indicates that the main logic board has established communication with the LCD display.
LED 4: Indicates a processor over-temperature condition. The system will shut down when this LED turns on."

Ah, yes, I've discovered them.LED1 and 2 are correct (LED1 is on when the computer is plugged and LED2 when the power button has initiated the start up). LED3 is always off, which reveals there's something wrong beyond just the display. LED4 is off, expectedly.

Be very careful when working with the front off since the power supply is exposed and high voltages are present.

Thanks. I indeed never touch a board when the computer is plugged.

There are two elements to the power supply: the supply itself and the DC - DC board."

Thanks, I have located them.

Email me if you need more info.

Actually, it's the info about which info I need that I'm looking for 😉
After having tried what I could, the remaining bits seem locked (unlike other models).
Thank you for your suggestion.

Oh did you try an external display?

Well, my neighbor does not seem to have the external screen adaptor. Perhaps it was optional in the purchase? (if it wasn't optional, then she does not know where it is).

I have a question regarding this: I guess the LED3 is concerning the whole graphic card, not just the built-in output, right (in other words, not just about the "main" display)? So I doubt an external screen would make any difference since the LED3 is off (tell me if I'm wrong).

It could also be a GPU chip problem. I've got a 20" iSight here taken apart. It has a bad GPU chip and the logic board is currently out for a swap of that chip.

Sadly, I have learnt computers (hardware parts) on PCs (I had not choice of learning on Macs, except at home). Now, that's true there are differences.

Two questions come from your phrase:
1: should a bad GPU not prevent just the graphics to stay black (so to say, the keyboard would still power on and the computer start anyway)?
2: how do you diagnose a dead GPU if the Mac can't even start the Apple Hardware Test? Do you actually measure the voltage on the GPU's connectors to see it's wrong? (I have no idea which connectors and values I should measure).

Thanks for your answer. I'm always happy to learn things!

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19 replies

spudnuty

Level 5

7,128 points

Feb 17, 2010 7:53 AM in response to Anic264b

Anic,

I wondered if the speakers could have been deactivated (like for instance, on some models, if you turn off the sound volume while using the computer normally, the startup chimes also deactivates).

Yes that is a possibility. Unlike PCs where after you reinstall a generic Windows XP you also have to find the sound card drivers and you can't do that until you find the ethernet card drivers and get it working. Uhhh sorry.

no CD available only to a subset of people

There would have been an "Apple Hardware Test" disk w/ the computer when it was purchased.

Yes, so it's not a problem specific to the screen cables or graphics card.

Right so that's why I hook up an external monitor first thing.
At this point it might be worthwhile to go inside the iMac. Email me and I can point you to instructions.
Richard

Anic264b Author

Level 1

36 points

Feb 17, 2010 11:32 AM in response to spudnuty

spudnuty wrote:
Anic,

I wondered if the speakers could have been deactivated (like for instance, on some models, if you turn off the sound volume while using the computer normally, the startup chimes also deactivates).

Yes that is a possibility. Unlike PCs where after you reinstall a generic Windows XP you also have to find the sound card drivers and you can't do that until you find the ethernet card drivers and get it working. Uhhh sorry.

Well, as long as I don't get either a video signal or a keyboard light, I'll still consider the iMac does has a problem other than a video one (and exclude a simple muted sound problem). Also, the disk does not make noise in order to boot.

no CD available only to a subset of people

There would have been an "Apple Hardware Test" disk w/ the computer when it was purchased.

Well, of course, but you mentioned a CD not available to everyone. It's a shame we can't even buy it.

Yes, so it's not a problem specific to the screen cables or graphics card.

Right so that's why I hook up an external monitor first thing.
At this point it might be worthwhile to go inside the iMac. Email me and I can point you to instructions.

The iMac is currently open (I have kept it open since 8 days ago to try various things). I'm going to email you soon.

Thanks

spudnuty

Level 5

7,128 points

Feb 17, 2010 8:30 PM in response to Anic264b

Anic,

the disk does not make noise in order to boot.

So that's the hard drive? Hmm did you try disconnecting it? and booting. Reset PRAM.

It's a shame we can't even buy it.

Well you could but you'd have to become a certified Mac tech, actually it's free.

With two of the LEDs lit that's saying its fine up to the point where it communicates with the display.
That's why you've got to get an external display and adapter to test.
Richard

Anic264b Author

Level 1

36 points

Feb 20, 2010 5:43 AM in response to spudnuty

spudnuty wrote:
Anic,

the disk does not make noise in order to boot.

So that's the hard drive? Hmm did you try disconnecting it? and booting. Reset PRAM.

Greetings,
No, it's not the disk. I just meant the disk does not make "enough" noise to indicate a boot (the disk spins up fine, and is on, but if the computer actually booted, the disk would make noise during about a minute or so, but it does not).

It's a shame we can't even buy it.

Well you could but you'd have to become a certified Mac tech, actually it's free.

Seriously? You certainly have to subscribe somewhere, don't you?

With two of the LEDs lit that's saying its fine up to the point where it communicates with the display.
That's why you've got to get an external display and adapter to test.

I thought it was the third LED that indicated the communicating with the display, isn't it? The first LED for powered mainboard, the 2nd one for turned-on mainboard, but I don't have the 3rd LED on (so no communication with the display).

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iMac not starting up (no chime and no USB power)