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display backlight dark after PRAM reset

R

rikers_mailbox

macrumors 6502a
Original poster
Sep 27, 2003
739
0
LA-la-land
In the process of troubleshooting a 20" iMac (Late 2006) I reset the PRAM. This somehow resulted in some strange LCD display behavior.

Now when I boot the machine the screen initially lights up and displays the grey screen, I hear the usual startup chime, but then after a few seconds the screen goes dark (just about the time the Apple logo appears). The machine otherwise starts up normally and I can see the boot finishing if I hold a flashlight up close to the screen.

The screen appears to work fine, the backlight comes on at startup but then shuts off. And it worked fine up to the point when I reset PRAM.

What happened when I reset PRAM that would cause the backlight to stop working? How can I restore PRAM display settings so the display will function normally again? What's wrong with my boot.efi?

Thanks in advance for any help.
 
justperry

justperry

macrumors G5
Aug 10, 2007
12,557
9,750
I'm a rolling stone.
rikers_mailbox said:
I have tried resetting PRAM again. No change.

Reset PRAM instructions say to hold until the 2nd chime. Why do you suggest the 3rd?
Click to expand...

Because that's what we are used to, but I think 1 should already do the trick.

What about a SMC reset, did you try that one, or try Apple Hardware Test and see if you get errors.

An SMC reset is more or less this, shut down the mac, unplug from power for minimum 10 seconds, plug it in again and wait for minimum 5 seconds(From Apple) and startup again.

I think though there is something going on with backlight, might be Hardware.
 
R

rikers_mailbox

macrumors 6502a
Original poster
Sep 27, 2003
739
0
LA-la-land
justperry said:
Because that's what we are used to, but I think 1 should already do the trick.

What about a SMC reset, did you try that one, or try Apple Hardware Test and see if you get errors.

An SMC reset is more or less this, shut down the mac, unplug from power for minimum 10 seconds, plug it in again and wait for minimum 5 seconds(From Apple) and startup again.

I think though there is something going on with backlight, might be Hardware.
Click to expand...

Tried SMC reset already. No luck. Another odd thing, the machine boots itself when plugged back in without pressing the power button.

If it's a hardware issue, why does the screen turn on initially at boot?
 
justperry

justperry

macrumors G5
Aug 10, 2007
12,557
9,750
I'm a rolling stone.
rikers_mailbox said:
Tried SMC reset already. No luck. Another odd thing, the machine boots itself when plugged back in without pressing the power button.

If it's a hardware issue, why does the screen turn on initially at boot?
Click to expand...

There are preferences to set the startup to be automatic when the power has been cut, in my System it's under the Tab Options in Energy saver but I am on an older Mac.

Do you have a bootable backup, if so try to boot from it and look if it happens there.
Or, hold the Command-R to get into recovery, not to recover but just to test if it happens there as well.

And, do the Apple hardware Test, hold D at startup.

Edit: Might be a loose connecter.
 
O

output555

macrumors member
Dec 17, 2006
65
14
justperry said:
Because that's what we are used to, but I think 1 should already do the trick.

What about a SMC reset, did you try that one, or try Apple Hardware Test and see if you get errors.

An SMC reset is more or less this, shut down the mac, unplug from power for minimum 10 seconds, plug it in again and wait for minimum 5 seconds(From Apple) and startup again.

I think though there is something going on with backlight, might be Hardware.
Click to expand...

You left out a key step: When the unit is unplugged, push the power button and hold it for 5 seconds, then plug it back in and power it up. Without pushing the power button unplugged there is no SMC reset.
 
justperry

justperry

macrumors G5
Aug 10, 2007
12,557
9,750
I'm a rolling stone.
output555 said:
You left out a key step: When the unit is unplugged, push the power button and hold it for 5 seconds, then plug it back in and power it up. Without pushing the power button unplugged there is no SMC reset.
Click to expand...

NOT
Since it's an iMac this goes, from Apple:

Intel-based Macs: Resetting the System Management Controller (SMC)

Resetting the System Management Controller (SMC)

Resetting the SMC for Mac Pro, Intel-based iMac, Intel-based Mac mini, or Intel-based Xserve

Shut down the computer.
Unplug the computer's power cord.
Wait fifteen seconds.
Attach the computer's power cord.
Wait five seconds, then press the power button to turn on the computer.
Click to expand...
 
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