4 diagnostic leds are on, but no boot chime or picture on the screen

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iMac Intel 21.5" EMC 2428

Model A1311 / Mid 2011 / 2.5 & 2.7 GHz Core i5 or 2.8 GHz Core i7 Processor

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Pieter @fixingthemac

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Postado: 10 de jul de 2020

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4 diagnostic leds are on, but no boot chime or picture on the screen

If I turn on the iMac, all four leds are on, but I hear no boot chime. The screen flashes a single time white and that’s it. I have connected the iMac with an external display, but nothing appears on the external screen.

I know for sure that the speakers work, because the iMac beeps if I remove the ram.

Can somebody explain me what’s wrong with the iMac or does anybody have a suggestion that might fix my iMac?

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What happens when you shine a flashlight into the display? Do you see the faint outline of your desktop and its icons?

10 de jul de 2020 por Dan

@danj thank you so much, now I see it. A flashing icon when i shine my flashlight on the screen. I have to replace the inverter board I guess? I have tried two inverter boards, coincidentally both are bad I guess.

10 de jul de 2020 por Pieter

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What happens when you shine a flashlight into the display? Do you see the faint outline of your desktop and its icons? — Dan
@danj thank you so much, now I see it. A flashing icon when i shine my flashlight on the screen. I have to replace the inverter board I guess? I have tried two inverter boards, coincidentally both are bad I guess. — Pieter

Dan @danj Rep: 480,1 mil Self employed contract worker 35 329 971 · Answer 1 · 2020-07-10T08:01:38-07:00

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Dan @danj

Rep: 480,1 mil

Self employed contract worker

35

329

971

Postado: 10 de jul de 2020

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Good! So we know the logic board is good then. So the focus needs to be within the full backlight logic path, from the inverter to the display, and then onto the LED’s within the display assembly its self. I would start at the display end making sure the LED strip is working correctly. Test the resistance of the LED backlight power cable as well as the vertical sync cable ends disconnected from the LED Driver Board. I would also check the voltages with the cables connected to the LED Driver Board for both.

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I have ordered a new inverter board. I think the screen is fine, because when I press the power button the hole screen turns white for a second and then it is really dim. I will see what the result is and keep you posted.

10 de jul de 2020 por Pieter

I have received a refurbished inverter board. I have more succes now, but the screen is flickering. So am I unlucky with receiving a bad inverter board or is it something else. Here is a video of the screen flickering: https://youtu.be/34DfAyCueLY

11 de jul de 2020 por Pieter

@fixingthemac It looks out of focus! Are you able to get a clean imaged after logging in?

11 de jul de 2020 por Dan

That is just the blur of macOS when you log in. After I logged in it was as clear as it used to be but still the flickering.

11 de jul de 2020 por Pieter

Yea, I know I've seen issues where the focus didn't get better which is why I asked. I would check the voltage at the LED Driver Board. You may have a display issue within the cable and/or the LED board inside the display.

11 de jul de 2020 por Dan

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iMac Intel 21.5" EMC 2428

4 diagnostic leds are on, but no boot chime or picture on the screen

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