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iMac Intel 27" EMC 2309 (Late 2009, Core 2 Duo 3.06 or 3.33 GHz) ID iMac10,1, EMC 2374 (Late 2009, Core i5 2.66 GHz or Core i7 2.8 GHz) ID iMac11,1

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Built-in display works only if external monitor connected.

UPDATE1

Only diagnostic LEDs 1 & 2 are lit when the screen is black. The computer starts up with an external monitor .

The computer no longer works just by resetting PRAM, it requires an external monitor to be connected for the computer to boot (even to the chime). When an external monitor is connected all four diagnostic LEDs become lit.

As is the computer is now chiming every thirty or so seconds after having reset the PRAM once and without an external monitor plugged in.

Original post

Last week we reflowed the solder on graphics card and the iMac was working once again.

Two days ago we replaced the HDD with an SSD (860 Evo) and now the internal display only works if an external monitor is connected before start up (but can be removed straight away) or if we reset the PRAM every time we boot the machine.

We have replaced the Vsync cable as the connectors were damaged but that did not fix our problem.

The built-in display shows only a black screen if we boot it without resetting PRAM or plugging in an external monitor. The computer still chimes at boot.

New development:

Now when we boot the Mac with the original HDD put back inside the computer doesn't even chime. The screen is black for ~1 minute shortly before we get a crash screen saying it encountered a problem and shut down then goes to the Apple logo and starts up.

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Did you replace the PRAM battery when you took the logic board out? It’s one of the things I do on the older systems as they often need replacing in 8 or so year old systems. I’m thinking PRAM is loosing its settings.

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As far as replacing the HD with a SSD who’s did you use?

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The PRAM battery was replaced with a CR2032 (Duracell's DL2032). Which I believe should be okay in the machine.

SSD used is a Samsung 860 EVO, we've also tested with an 850 EVO which already had macOS installed on it, and then back to the stock HDD and still facing issues.

The battery replacement wasn't giving us issues until we replaced the HDD with an SSD though, we replaced the battery last week along with reflowing the solder of the graphics card and after it was working as expected.

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I should add that resetting the PRAM no longer lets the built-in display work, the computer no longer chimes when starting up unless an external display is connected.

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This is starting to sound like the GPU board needs replacing this time.

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