iMac backlight problem

[ComputerClinic]

Customer brings in iMac (this model)and says the screen goes black intermittently and can sometimes be restored by hitting power button twice (in and out of sleep). Letting it run idle for a few minutes reproduces the problem and shining a flashlight on the LCD reveals that video is still being sent but the backlights aren't on. Just to be sure this was a hardware problem I boot up to an ubuntu live CD and sure enough the screen goes black after a while.

My next step was an SMC reset, which made no difference. Since there was no flickering and this LCD has more than one backlight I assumed the inverter board is to blame. After replacing the inverter board I leave the PC on overnight w/ no issue. Everything seems OK so the PC is returned to the customer.

A few days later the customer calls back and says the problem remains. I am able to reproduce the problem w/ the new inverter board as well. I had a slightly different inverter board (ordered wrong part accidentally); the only difference being a different connector for the vertical-sync cable. I installed this board (leaving v-sync uplugged) and was able to reproduce the problem again.

So I doubt that 3 inverter boards have the same problem and now I'm thinking the LCD needs to be replaced. Does this sound like the most likely solution to you guys or am I missing something?

 

[madcow007]

I would use a ccfl tester to check the lcd. If you dont have one check ebay they start at around $5.

//edit: just realized this is led not ccfl you can still find a tester but it will probably be more expensive

 

[anonymous Mac Tech]

My next step was an SMC reset, which made no difference. Since there was no flickering and this LCD has more than one backlight I assumed the inverter board is to blame. After replacing the inverter board I leave the PC on overnight w/ no issue. Everything seems OK so the PC is returned to the customer.

A few days later the customer calls back and says the problem remains. I am able to reproduce the problem w/ the new inverter board as well. I had a slightly different inverter board (ordered wrong part accidentally); the only difference being a different connector for the vertical-sync cable. I installed this board (leaving v-sync uplugged) and was able to reproduce the problem again.

So I doubt that 3 inverter boards have the same problem and now I'm thinking the LCD needs to be replaced. Does this sound like the most likely solution to you guys or am I missing something?

I see you did a SMC reset. But what about zapping PRAM? Could be as simple as that. Just because it does the same thing booted to another environment just rules out software. Still could be something hung up in the video RAM. But I've seen a fair amount of backlight problems with these models, so it probably won't be as simple as a PRAM reset. But if it's the only thing you didn't try, it could turn out to be the solution.

By inverter board, I assume you mean the backlight board? More times than not it turns out to be display panel, but I've seen a backlight board fix the issue before, so I wouldn't assume just because you tried the incorrect one the backlight board is ruled out. Sometimes with certain models, if you order a display assembly from Apple it comes with another backlight board so they are very much paired up for a reason (maybe one blows and the other follows?).

 

[ComputerClinic]

Well it looks like what I did was an PMU reset (command+option+p+r) and I got that mixed up with an SMC reset. I went ahead and reset the SMC and so far the backlight has remained on. I'll leave it running for a day or two though considering how intermittent the problem can be. If it does return, is it true that the only remaining solution would be an LCD replacement?

And yes I did mean LED backlight board when I said inverter board.

 

[anonymous Mac Tech]

Well it looks like what I did was an PMU reset (command+option+p+r) and I got that mixed up with an SMC reset. I went ahead and reset the SMC and so far the backlight has remained on. I'll leave it running for a day or two though considering how intermittent the problem can be. If it does return, is it true that the only remaining solution would be an LCD replacement?

And yes I did mean LED backlight board when I said inverter board.

No, you did a PRAM reset (command+option+p+r). A PMU reset is basically what an SMC reset is on PowerPC Macs. So essentially, the PMU reset is gone with the PPC Macs.

As far as if its true, that the only other remaining solution would be display panel assy, read my previous comment regarding backlight boards and display assemblies. Based on the information given and my personal experience working on several of these machines, I'd say it's more likely it needs a new display assy or both display and backlight board for that matter. Also, what isn't ruled out is the logic board. Long story short, you'll have to swap parts until you find what works.

 

[sal13]

That model has a mini display out. It might be worth it to hook up an external display and see if you experience any of the same issues on it.