Model A1311 / Mid 2010 / 3.06 & 3.2 GHz Core i3 or 3.6 GHz Core i5 Processor

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Troubleshooting no video issue

Hi all,

There are many threads on this specific issue, but alas not too many solutions.

Situation is as follows:

iMac as stated, no video. External monitor does work.

Nothing on lcd, not even very faint.

I have tried to replace the lcd, but no dice.

I have measured the contacts on the driver board that are supposed to drive the back light leds of the lcd and none of them have any voltage over them.

When measuring other components on the driver board, I measure 12V, so it's not dead.

I then delved into the service manual of this iMac and measured the voltage of the test pins on the motherboard. Pin 4 and 13 (to test lcd related functions) give 0 Volt, all other pins have the correct voltage.

According to the service manual I should replace GPU first and if that doesn't solve the problem, the motherboard.

Am I correct in presuming that the GPU is cleared since external monitor is working, or could this still be a GPU-related issue?

Funnily enough during the troubleshooting flow in the service manual they do use external video working/not working as a condition for diagnosing, but not to differentiate at the end between GPU and motherboard.

So to me it's unclear at the moment if my tests clear the GPU completely or not.

I also have to mention that of the 4 test leds only the first 2 light up. Nr 3 and 4 do not. Led nr 3 is supposed to light up when there is communication with the GPU. But since the test is performed without an lcd, possibly the led isn't on because of that?

Should it go on when I connect an external display (which works)?

My gut feeling says it's the motherboard that's faulty, but I don't really have the luxury to order both parts.

Any insight much appreciated.

Answered! View the answer I have this problem too

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Chosen Solution

Since it works with an external monitor, that clears the logic board and the GPU. I'd replace the LED driver board: iMac Intel 21.5" EMC 2389 LED Driver Board Replacement

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Comments:

Hi mayer and thanks for the reply.

It certainly does not clear the logic board.

I've read up on many of the threads with similar or exact issues and often the driver board was replaced, but it did not solve the problem.

If it wasn't for the multiple threads and the iMac service manual, logically I would say this part is the culprit, yes.

However, based on the info I have, it is almost certainly not.

Also, it states in the official service manual if there is no voltage measured between the test points, as mentioned in my post, there are two possible causes, namely the GPU or the motherboard - the driver board is more or less ruled out at this point.

My question is if the fact that external video works, rules out the GPU as a possible cause, that is all.

The service manual is fairly ambigious when it comes to this specifically.

And to clarify: the test points on the motherboard do not test the driver board itself, but the part of the motherboard that's supposed to control the driver board.

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