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iMac works but no back light

Hi, I have an A1311 iMac that works but has no backlight. When you push the power on button the screen flashes up a whitish yellow colour for a second then goes black. The power on chime sounds and it loads up. If you push the power button for a couple of seconds the screen displays for 2 seconds and goes off. When it goes off you can see the image when shining a flash light into it.

I’ve replaced the backlight board but it’s still the same. Replaced the PRAM battery and done a reset, still the same. I checked all connections and con checked the v ribbon cable which is fine.

whilst replacing the backlight board I noticed a couple of components on the power supply board look dodgy (TVS131 & TVS132) see pic. What do these control and could they be the culprit?

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What year is your system? 2009, 2010 or 2011

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It turned out to be a bad lcd assembly. Replaced and now working fine

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It looks like TVS131 and TVS132 are busted, not sure if they’re diodes. Maybe even that cap C142. Check the fuse R177 for continuity. Those components you show are most likely the culprit. I checked duckduckgo for your motherboard, the A1311, download here. Check the PDF for those components which I mentioned. Replace them. If you find the *.BDR file, then you can use Openbroardview from https://pldaniels.com/ . Check through the whole motherboard, reverse side as well, for other strange looking components, as well as the screen connector for corrosion or ugliness in some form. If you find no more, then clean the motherboard thoroughly with as pure alcohol as possible and a short, small brush, then rinse again with alcohol and let it dry well. Please let us know if this helps or not.

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Thanks for your help, it turned out to be a bad LCD or a component within it. I replaced the screen assembly and it now works perfectly

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@Daniel Heathcote Glad you fixed it! Thanks for letting us know!

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Daniel Heathcote sera éternellement reconnaissant.
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