iMac genius required - black screen, but 4 LEDs lit

[edsup]

A1311 Late 2010 21.5" Core i3 3.2GHz EMC2389

i know my iMac is old and this looks like a common problem ... but I'd really appreciate a little help!
I have placed the correct brand new PRAM battery in
When the power socket is turned on I get No1 LED diagnostic light on the mother board
8 seconds after pressing the power button (a faint flash) I get the start up chime followed by the other 3 LEDs
So all hardware diagnostics passed (?!?!?) . ..
but then there's no back light, when shining a torch I can see the apple logo and the bootup progress bar
which stops half way ... then after 1min 5secs the imac shuts down and all diagnostic LEDs go out!

(since this post started, I have formatted the hard drive and the iMac boots up properly now)
I'm happy its not a Hard drive or GPU issue. So I think I'm down to it being the back light LEDs all going simultaneously or the LED Backlight board. There were no black areas on the screen previously so I've ordered a backlight board and a Mini DP cable to help trouble shooting... will report back in next few days. It may well be a lost cause ... but I have time and determination
to try to fix it

Thanks Ed

 

[ScreenSavers]

I had that same problem on a 27" 2009 iMac. It ended up being a bad power supply board. The LCD in those connects directly to it for backlight supply.

 

[edsup]

I had that same problem on a 27" 2009 iMac. It ended up being a bad power supply board. The LCD in those connects directly to it for backlight supply.

Thanks for the fast reply ... so do the above diagnostics rule out GPU card & hard drive failure ??
If you’re right - I don’t suppose there’s anyway of proving that without changing for a similar power board?
Would I have to get one from exactly the same model or are they interchangeable??

 

[ScreenSavers]

Thanks for the fast reply ... so do the above diagnostics rule out GPU card & hard drive failure ??
If you’re right - I don’t suppose there’s anyway of proving that without changing for a similar power board?
Would I have to get one from exactly the same model or are they interchangeable??

Hard drive failure wouldn’t cause the backlight to fail, so I doubt that’s the main issue. GPU failure is uncommon on 21.5” models, although maybe possible. My guess is it’s a Power Supply or Logic Board issue (if not the LCD itself.)

The power supply board has a part number on it you can type in on eBay or something like that. Wonder if there’s a way to test it...

What happens when you plug in an external monitor? That would rule out the GPU...

 

[edsup]

Hard drive failure wouldn’t cause the backlight to fail, so I doubt that’s the main issue. GPU failure is uncommon on 21.5” models, although maybe possible. My guess is it’s a Power Supply or Logic Board issue (if not the LCD itself.)

The power supply board has a part number on it you can type in on eBay or something like that. Wonder if there’s a way to test it...

What happens when you plug in an external monitor? That would rule out the GPU...

brilliant help already thx ....
I need to buy a mini display dv lead I think .... I assume that’s the only way I can link this to another screen? Seems strange to me that the power supply leaves the computer half booted up .... and will also stay powered for ages on the Mac Utilities page?

 

[ScreenSavers]

brilliant help already thx ....
I need to buy a mini display dv lead I think .... I assume that’s the only way I can link this to another screen? Seems strange to me that the power supply leaves the computer half booted up .... and will also stay powered for ages on the Mac Utilities page?

That is strange. Because it almost sounds like a GPU symptom. Yes, you would need a mini displayport plug for whatever monitor you use.

 

[edsup]

Thank you again for your help "Screensavers", I hope you're all doing ok over there in Georgia!!
I have some display port leads on the way now to diagnose the GPU - probably be a week until the post arrives in the current climate ....

Just as an added part of the diagnosis ... it won't boot up in safe mode (holding the shift button at power on)

Just how clever is the LED diagnostics??
I know that LED # 3 is Off: The video card and logic board are not communicating, but in my case all LEDs light up ... is it possible that they are communicating enough to light the LED but still cause problems with bootup/black screen??

 

[edsup]

Small break through - backlight suddenly started working ... and I get this in Diagnostic mode
No worries thinking the whole thing may be a software issue!!
Can I reinstall the os from the disc drive or so I have to use a flash usb drive??

 

[edsup]

And now this - I’m still out of my depth

 

[ScreenSavers]

And now this - I’m still out of my depth

It’s trying to do a verify/repair of the disk. Did you try erasing the disk?

 

[edsup]

yep - erased disk ... now trying to work a way of reinstalling an OS from a USB .....
meantime the backlight has failed again and I've changed nothing ... so maybe it is a hardware issue after all

 

[edsup]

I think it’s all fixed .. thank you Screen saver .... details are at the top ... I edited the first post

 

[ScreenSavers]

Well that's new. Never seen no backlight being a software issue- especially on a mac, too. As far as baking, it's the same as reflowing, which is sometimes short term but does work. BAking the GPU is doable- don't bake the logic board though!

 

[Georgio]

I’m going exactly the same issues with my late 2012 A1419 with an image viewable with a torch but no backlights.
Getting four LED’s and hooking up an external monitor the display is perfect.
Doesn’t want to start in safe mode for some reason, but I suspect either the power supply to the backlights has failed or a toasted LED on the backlight strips knocking them all out.

Does anyone know the pin out configuration on the LED power supply connector?
It’s the black plastic one to the right of the metal display connector.
I’d like to verify there is power out at this connector before I start splitting the display panel itself to get at the LED strips.
Thanks in advance...👍

 

[edsup]

Yep ... you were right screensavers ... problem is back ..... fairly happy GPU is ok though ... but will be able to prove it with Mini Display port cable in next few days, although with torch I can see al graphics and no strips etc.
I'm wondering whether it is the LED Backlight Board now?? I've tried the test points with the LED screen off and have got no voltage registering at POINT 6 & 13 ... but maybe this is because the LED screen is disconnected??
.... any ideas of other tests to work out whether it is the LED Backlight board or not??

 

[Georgio]

Yep ... you were right screensavers ... problem is back ..... fairly happy GPU is ok though ... but will be able to prove it with Mini Display port cable in next few days, although with torch I can see al graphics and no strips etc.
I'm wondering whether it is the LED Backlight Board now?? I've tried the test points with the LED screen off and have got no voltage registering at POINT 6 & 13 ... but maybe this is because the LED screen is disconnected??
.... any ideas of other tests to work out whether it is the LED Backlight board or not??

If you can see an image on the LCD with a torch as I can then the GPU and PSU are fine. I suspected the the capacitors to the right of the screen power connector, so the black plastic clip, but now I think it's a bad LED on the strips knocking them all out.
Unfortunately the only way to verify this is to split the screen and inspect the LED strips. The good news however is taht these strips are realtively inexpensive at about £16 for a pair and at that price it's worth replacing all four at once.

 

[ScreenSavers]

But the PSU provides power the the backlight. The LVDS data comes from the logic board.

 

[Georgio]

But the PSU provides power the the backlight. The LVDS data comes from the logic board.

Except that on his Mac it has a separate power board for the backlights which is replaceable.
On my Mac the power to the backlight is incorporated into the logic board, top right by the backlight power connection to the screen.

 

[edsup]

If you can see an image on the LCD with a torch as I can then the GPU and PSU are fine. I suspected the the capacitors to the right of the screen power connector, so the black plastic clip, but now I think it's a bad LED on the strips knocking them all out.
Unfortunately the only way to verify this is to split the screen and inspect the LED strips. The good news however is taht these strips are realtively inexpensive at about £16 for a pair and at that price it's worth replacing all four at once.

wow - thanks Georgio ... any ideas where to find the LED strips from? Thats great information. it is a minefield to pick thru at first! I wish I had the knowledge to help you with your problem.

 

[Georgio]

wow - thanks Georgio ... any ideas where to find the LED strips from? Thats great information. it is a minefield to pick thru at first! I wish I had the knowledge to help you with your problem.

I found a supplier in HK who can supply. The screen module uses the same adhesive strips as for the screen to body on the IMac so it’s pretty easy to split.

 

[edsup]

Yep - I've seen a good youtube video on pull apart and rebuild an LCD screen, but I'm reticent to start until I can find any replacement parts .... I just an't find any LG backlight replacement strips that say iMac 21.5" ... I know it has to be white LED for this mid2010 model, but ebay shows these: https://www.ebay.co.uk/sch/i.html?_...=0&_sop=15&_osacat=0&_odkw=lg+led+replacement
And I don't know whether any are the correct size or will work? You don't happen to have the name of your supplier from HK do you?