iMac 'black screen' Where to go from here?

[flyinhigh]

I did post yesterday but go no responses, thought I'd try again. Yesterday, while on line it went black, like to sleep mode but couldn't bring it out. I've tried all the trouble shooting in the manual to restart but couldn't get it past a few seconds of just a white screen, no chime. Today (after trying to use apple harware test) when I power up, I get the start up chime (didn't get it yesterday) get the white screen, the apple logo and the starting up circle for a few seconds and it goes to black but I can hear that it is powered on. (putting my ear to it is only way of knowing if it is off/on). Sometimes (not every time) when I hold the start up button gently I get a second or two of my home screen, where all looks normal. Strange thing is, it's not the log on screen. I bought it second hand, two weeks ago, took the chance. Was working wonderfully and as expected was on it for most of the two weeks! But alas.
Trying to get my head around where to go from here. I have read many threads about the black screen but not quite the scenario I've got.
imac 27" late 2009 core 2 duo 3.06

 

[DeltaMac]

You did have two responses at your other thread, so you could have simply posted back there.
https://forums.macrumors.com/threads/1697563/

Anyway, check that there's any trace of video on the screen by shining a bright light at various angles into the screen. Particularly look at the top edge for menu items, and the bottom for the dock with its icons. I always will look for the clock in the top right corner.
If you have no reason to expect the Mac to log in automatically and go straight to the desktop, then you may have a login window in the center of the screen.

By your comments, Do you have the service manual for your iMac? Check for the diagnostic LED's visible through the bottom vents. You have to tip the iMac back to view them more easily. If you see all 4 LEDs after it has been powered on for a couple of minutes, and no video on the screen, then the backlight board might have failed.

If you have only had this iMac a short time, might be a Good Thing™ to contact the previous owner (if possible) to ask if they had any video issues….

 

[flyinhigh]

My apologies DeltaMac, I missed those responses yesterday, I wasn't thinking very straight yesterday, wasn't looking in the right place for responses, was in distress/panic mode. Thanks for pointing it out to me and for your advice, very much appreciated. I don't have a service manual for the imac guess I can get it online? Spent much of yesterday getting my v old emac up and online to access help online. (I'd already passed my G5) I will follow up on all yours and the other posts advice today and I guess in the end it will be a matter of off to the shop for a diagnostic. (I had so many exciting plans for this computer). thanks.

 

[DeltaMac]

Sorry, I was just making assumptions (again) with your comments about "trying all the troubleshooting in the manual..", so I was going by that…

Be sure to try the checking for video on the screen, even though it's dark, there still may be video present if you look carefully.
PRAM reset
SMC reset,
then, reseat all the memory sticks, which are available inside the access panel on the bottom edge of your iMac.
Might help?

 

[Brian Y]

Do you get video output to a second screen?
Is it connecting to a network (via cable?)?
Is bluetooth working? (e.g. is the wireless keyboard connected/does the caps lock key light up?).

Diagnostic LEDs are the way to go from there. If you have 2 lights on, then it's the graphics card, if you have 3, then it's the LCD panel, if you have 4, it's probably the backlight.

You can always take it to an apple store for diagnosis. It might be something silly, like a loose backlight cable or something.

 

[flyinhigh]

I've made a list of all the recommendations made and will go thru them all step by step. Probably won't stick all day on it today (weary from it, got to get out)
-Re the diagnostic LED lights, three green lights are showing after power up. Brian Y says this indicates LCD panel issues. Any further insight on this?
-I haven't yet shone the bright light in to the screen but yesterday I was getting momentary 5 second flashes of my home screen (holding start button gently), couldn't see that video was playing but will try the bright light. I do have my setup at the moment going automatically to desktop.
-I'm trying to find a service manual on line but links posted in this forum that I've come across are broken. I only have the basic 'Everything Mac' manual that came with the computer and have tried everything suggested in it.
Thanks again, anything else to add appreciated and will let you know how I go.

 

[flyinhigh]

Update

After moving my computer around to see LED lights (3) and moving it back into position and trying a PRAM reset I'm getting a longer period of time on the white load up screen than before but still goes to black. However a gentle hold on the startup button while on is more regularly and slightly for a longer period getting me a view of my desktop. All is there, no videos running, just finder and apple store apps running. Within the short period I get to see the desktop screen I can confirm that the computer responds as it should. I got to close the apple store app. Clock is good, right time etc., The scenario is like it's going in to sleep mode and I can't get it out of it. Hopefully I'm not dealing with as major issue as I first thought but then again... Thanks team for the support. At least I'll have known I've done what I could before going for a diagnostic repair.
Any feedback from this appreciated.

 

[DeltaMac]

I think that getting at least a little video when you gently hold the power button may mean a loose connection. So, could be a simple (read: low cost?) fix of opening up the iMac, and checking/reseating all connections to and from the screen. Keep in mind that it may also mean that the "loose connection" is simply a coincidence, reseating the connections changes nothing, and the LCD panel may be failing. I hope the fault is somewhere in between. Best to have this done by someone who can give you a good diagnosis (or a quick reseat ) - Apple "genius", or a local reseller, if you still have a good one.

 

[flyinhigh]

Thanks DeltaMac. Time now to start researching where to go locally for a diagnosis/quote. Will take your advice and best stick with Apple approved resellers, not just any old computer repair shop. Hoping to avoid the $Apple shop. Crossing fingers the matter is at least somewhere in between as you say. Feelin better about the prospect but cross fingers same time.

 

[flyinhigh]

Reasonable price for LCD screen replacement?

I've been making online enquiries re having my imac diagnosed/repaired. I've had one reply so far from a local apple reseller. Free diagnosis and quote offered and a written quote that if at worst and is the problem LCD screen replacement would be AUD$550 (all inclusive). Seem on par as to be expected? Responses appreciated. I'm working out who to go with cost and outcome effectively. Thanks.

imac late 09 27"/3.06/2dc/1TB