IMAC 27" Backlight Problem

[Griffinballs]

Hi all
Got an IMAC 27" with a backlight fault.
Screen always lights up perfect for a few seconds on boot. After a few seconds, backlight goes off, machine is still running fine, you can see it if you look close enough.

Now I've read about a common fault with these where half the backlight goes off and flickers and what not and this is due to a connector on the panel itself. But this fault looks a little different than what I'm getting. Here is a vid showing that fault and the cause.

My fault I am currently leaning towards this being a LED driver board problem, specifically the caps on that board. Here is a pic of the inverter board http://www.macplus24.de/media/catal...db753da6cd7499147ede0ca58848/d/s/dsc_7630.jpg
There are 4 electrolytic caps on that board. The ones on the one I'm repairing don't look bulging or anything, although on the board, their solder joints, some look a little oxidised and there appears to be what looks a bit like flux residue around them. Is this a sign they have blown?
I'm thinking it couldn't hurt to replace those, my theory being, the caps allow the backlight to come on but due to their problem, don't allow it to stay on. And hence why turning it off and on again resolves it for a few seconds i.e. the cap gets drained and is able to perform ok again for initial start up or something like that?.

The main PSU caps all look good really, nothing worth comment that I can tell.

Thanks for any help guys

 

[Martaine2005]

Hi Griffinbakks,
Funny that!
I was going to link that exact same video. Gerry Sweeney.
In an ideal world we would have two of everything to swap boards around..

Also, in an ideal world, everybody attempting repairs should have an ESR meter.
The flux residue wont have anything to do with your fault. But it wont hurt to replace them if you suspect otherwise.
Your theory on the caps is plausible although the opposite is normally true.

I would be looking for something heating up and thermally shutting down.
Or a dry joint/cracked solder joint.
Have a good look around and re-solder the dodgy looking joints and replace the caps if you really want to.
It is going to be a process of elimination.

Martin

 

[Griffinballs]

Hi Martin
Thanks for the response. Totally agree on the two of everything remark. I work in desktop support daily (this ones a homer) and laptops in work, we literally do have many of each model so as you say, we often swap parts around for testing.
I've only recently started messing about with macs and as such, haven't even got a mac keyboard, let alone any other spares. And have no personal or work macs lying around for testing

On the positive side though, scrap my whole post lol, today upon booting the mac, only one side of the backlight was out and if I put pressure on the area of the connector, it came back sure as the youtube vids show. So as it happens, it is exactly the same fault, to the letter. Awesome, always nice when a fault you have is heavily documented and with repair videos up

So now I'm about to strip the mac and get ready for soldering.

One piece of advice I would like though. In that video by Gerry, he solders in wires to the pins coming from the back of the board, he doesn't acually strip down to the board, I can see why and I like the idea, but I have a concern that over time, those pins/wires could end up shorting. I prefer the idea of removing the board or at least gaining proper access and reattaching the offending part. This does indeed look a bit more involved and personally, never stripped the stickies off of an LCD panel, a little apprehensive about damaging the thing taking it apart.
My soldering is up to scratch though at least so I've no doubt I'll be able to do the fix exactly as Gerry. Its just the potential for later shorts worrying me.
What are your thoughts? How would you personally tackle the issue?

Thanks for your help as always

One last thing, what exactly is an ESR meter and how does it help because if I need one I'll get one. My electronics knowledge is all self taught and not particularly extensive to say the least lol.
Completely off topic, do you run a data recovery company? Completely random I know but I know a man called Martin who does and he is also extremely helpful and knowledgeable about electronics.

 

[Martaine2005]

Yes, I agree with you. I would carefully strip it right back and re-attach at the board itself.
In Gerry's defence, he did say it would take a while and didn't want to spend too much time on it.
However, for me time is not an issue for my own goods. I would prefer the proper strip down and fix.
I am sure if you take your time and do things methodically, you will have a better outcome.

Regarding the pins shorting over time, I doubt that. They are reasonably rigid and can't move. But you could always use heat shrink or the like for each pin.
This is a known fault that Apple still wont accept. So there will be plenty more to practice on if all goes tits up.
I would lay a carpet off-cut on the floor or bench and strip it carefully and slowly.
You will never know unless you try. Got to be cheaper than a £400 new screen.

I say go for it....Keep us posted on the results. I am sure with your soldering skills, everything will be ok.

Martin