iMac no longer boots. Only startup sound

Do you have a second Mac? You could try accessing the system via Target Disk Mode

Otherwise you’ll need to debug the logic board which likely failed.

Your iMac Intel 27" EMC 2390 (2010 model) exhibiting a black screen with only the startup sound indicates a failure in either the **GPU, display, or logic board**. Here’s how you can diagnose and potentially fix the issue:

1. **1. Check for External Display Output**

- **Connect an external monitor via Mini DisplayPort or Thunderbolt.**

- If the external display works, your internal display or backlight may have failed.

- If nothing appears on the external display, the issue is likely with the GPU or logic board.

1. **2. Backlight vs. Display Issue**

- **Shine a flashlight at an angle on the screen** after hearing the startup chime.

- If you faintly see the macOS login screen, the **backlight has failed** (possible LED driver board or power supply issue).

- If you see nothing, the display itself or GPU may be at fault.

1. **3. GPU Failure**

- The 2010 iMacs are known for **ATI Radeon GPU failures**.

@tonicatany couple of things you can still check.

If the computer detects out-of-specification or no SDRAM, the screen will remain black but the computer will beep. This error condition may be due to physically damaged RAM, installing the incorrect type of RAM, poor connection, or not having RAM installed.

First re-seat the memory and test the computer again. If the memory fails the POST test again, remove all installed memory and test by installing one by one each memory that has been verified to work correctly on another system (i.e., “known-good”) or order new memory.

A sequence of tones heard at startup or a no video symptom may also be fixed by temporarily removing/replacing the backup battery.

Removing the Battery, Measuring DC Voltage

1. Unplug the computer. Allow several minutes for power supply to discharge.

2. Remove logic board.

3. Remove coin battery for 1-2 minutes. The coin battery is located on the back side of the logic board (see graphic below).
4. Measure DC voltage on battery touching battery with red probe, and grounding with black probe. If voltage is 2.7v or less, replace battery.

5. Reinstall the battery and logic board. This will reset the logic board.

6. Power on computer.

7. If computer boots, check for and install all software and firmware updates

I would also suggest that you do go ahead and check the diagnostic LEDs and see if and which ones light up. The LEDs are located under the mylar tape, on the bottom, left corner of the logic board. They can only be seen when looking through the lower vents of rear housing

Let us know what you find out.