Cause of blank screen, otherwise normal operation?

I suspect the display has died but does anyone have any suggestions on things to check to fix an imac screen?

The diagnostic LEDs are showing

1. Lit when connected to the mains

2. Lit when machine is running

3. Lit when machine is running

4. Not lit when on or off.

The machine starts and runs and outputs to an external monitor but its internal display is completely dead. No backlight or anything.

This occurred after a failed hard disk was DIY replaced, the mainboard to display cable was damaged and replaced during this operation - is there a way to test the display or confirm that a particular component is defective in the chain? The mainboard display connector seems ok, all its pins look visually ok, the vsync lead is ok, the temp lead also ok and the power (I think) lead from the video adapter inverter is also ok.

Be good to know where to investigate even if its only to know which bits cannot be sold on as spares. I'm perplexed as to the display led #3 reporting that its ok even though its not showing a picture, unless the picture is being sent but the display isn't powering up to show it?

I'm testing it without a hard drive in case this makes a difference. I do get the start up 'chime' and I've reset the pram and pulled the cmos battery (2.9v) and left overnight. But to no, unsurprising, effect. Its an a1311 model.

Any pointers appreciated,

Answered! View the answer I have this problem too

Is this a good question?

Score 0

Comments:

Hi Dan,

No display at all, I had thought of waiting till tonight and seeing if there is any image at all when its pitch black. The screen gives no indication of life at all though, no flicker or anything. the damaged cable is at work, pretty much mangled the logic board end connector, the socket does seem ok though. I'd half expect the backlight to be on and just mess on the screen if this connection was bad though? re: shorting the cable, the whole thing had been powered down and was unplugged when we attempted the repair, I'm not sure we could have caused this level of broken?

cheers,

toby

by

Hi machead3,

Ah sorry, I've read quite a few guides on this sort of imac and have probably gotten the diagnostic arrangement muddled with something else. If #4 is the screen one then it does confirm the OBD knows somethings up. The invertor board (besides PSU) seems ok, nothing burnt and the sockets and cabling seem fine. does it give an indication that its alright via some onboard surface mount LED or does it just not work? I don't really see how the inverter could be incorrectly installed though :)

would you suggest that one is more suspect that the other - given there is no backlight and no image does that point more at the invertor no supplying the screen or the screen just being completely dead?

cheers

toby

by

I'm getting ~7v (DC) across the two pairs running from the inverter to the display in case that's important?

by

Add a comment

3 Answers

Chosen Solution

LED 4 - Indicates that the computer and the LCD display panel are communicating. This LED will be ON when the computer is turned on and video signal is being generated (it will be off when the machine is off). If the LED is ON and there is no image on the LCD display panel, the LCD display panel or inverter might be installed incorrectly or need replacement.

Recheck your connectors - examine cables for cuts/nicks. If the connectors and cables are good the LCD is all that's left.

If this answer is acceptable please remember to return and mark it.

Was this answer helpful?

Score 1
Add a comment
Most Helpful Answer

With the system running use a flashlight at an angle at the display do you see a faint image of the desktop on the screen (poor man's backlight)?

As to test the display alone, unless you have a second system to swap out the display not possible. But you could have burnt the fuses on the logic board if the cable shorted out when you damaged it. These are very small 2 pin SMT devices often you'll see them physically burnt. You could have also damaged the display its self.

Can you show us the damaged cable damage so we can figure things out?

Was this answer helpful?

Score 2
Add a comment

Do you think it could be a faulty p/s? If I hit the power button the screen comes back for seconds.

Was this answer helpful?

Score 0
Add a comment

Add your answer

toby will be eternally grateful.
View Statistics:

Past 24 Hours: 0

Past 7 Days: 2

Past 30 Days: 7

All Time: 1,062