Dead iMac Won’t Turn On? Fix for Black Screen & LED 1 Only | Logic Board Power MOSFET Repair

In this video, I repair a dead Apple iMac that won’t turn on or show anything on screen — only LED 1 blinking on the logic board (model 820-3588-A, iMac A1418 21.5-inch, 2013–2017).

This iMac no power problem is one of the most common faults and is often linked to a failure in the power-on sequence or standby voltage circuit on the logic board.

I’ll guide you step-by-step through the full diagnostic and repair process, explaining how to:
🔹 Interpret the iMac LED diagnostic codes
🔹 Check standby 12V and 3.3V power rails
🔹 Test S5 and SMC_ON signals
🔹 Locate and confirm the faulty MOSFET and driver circuit responsible for the failure

In this case, the fix came down to restoring the power sequence using an external MOSFET test setup — no logic board replacement needed!

This type of repair can save hundreds of euros and bring a “dead” iMac back to life without expensive Apple parts.

If you enjoy logic board repairs, microsoldering tutorials, or Apple motherboard diagnostics, you’ll find this video both practical and educational.
All measurements, schematics, and repair techniques apply to many other Mac computers that won’t power on.

🧰 Equipment Used

• Digital Multimeter
• Soldering Station
• Hot Air Rework Station
• Flux and Fine Tip Soldering Tools

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