The case of the iMac screen that mysteriously blacks out – Macworld

Cecil Usher wants to give his granddaughter a 2010-era 27-inch iMac. However, there’s a problem.

The screen goes completely black and stays that way for no apparent reason, sometimes shortly after turning the mac on and sometimes not all. Sometimes the screen simply never turns on and stays completely black. When the screen stays on, it looks perfect.

Cecil took the iMac to an Apple-authorized service center that couldn’t replicate the problem and their diagnostics found nothing wrong. However:

We took the machine home and the screen came on at startup and within a minute went blank. This has been happening for over a year.

The iMac works with an external display just fine, whether or not the internal display is blank.

Without laying hands on the computer, my guess is this is an electrical fault that has a thermal component. While I haven’t seen this recently, in my youth—when I was more hands on with soldering irons and circuits—it was relatively common to have faults that only materialized under certain circumstances in which heating or cooling caused expansion or contraction that caused a temporary gap in whatever conductive material was passing electricity.

(As a child, my family had a color TV set, but when it heated up, it shifted to black and white. We had to bang it to get color back. Banging the set jarred the discontinuity in the solid-state circuit that did color decoding. Yes, I’m that old.)

With modern manufacture, that sort of nonsense is much less likely, but given that moving the iMac into a different location made the problem impossible to replicate lends credence. In movement, it might have been jarred, or the repair facility might be heavily air conditioned or not at all, while Cecil’s home is the opposite.

I suspect the repair shop only ran hardware diagnostics via software, which wouldn’t reveal this, and didn’t open it up. It might cost $100 or more to have someone qualified crack the case and look for signs of failure, at which point unless they’re handy with a soldering iron and it’s something that can be fixed with molten tin and lead, it won’t be worth repairing.

Just to on the safe side, I always suggest the following as part of diagnosing video issues:


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