Normal startup chimes then black screen keys work enough to get error

jack start off with something like this:

Power On Self Test (POST)

Intel-based Mac computers such as the iMac rely on a combination of tones and blinking LEDs to display Power On Self Test (POST) error codes.

• 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.

• Some RAM may appear to pass the Power-On-Self-Test (POST) but still cannot be used by the operating system. In this case, the computer will display a gray screen, sound three tones and repeat the tones until the computer is turned off.

• The solution to both of these situations is to 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.

Resetting the System Management Controller (SMC)

The System Management Controller (SMC) is a chip on logic board that controls all power functions. If computer is experiencing any power issue, such as not starting up, not displaying video, sleep issues, battery management, or fan noise issues, resetting SMC may resolve it. To reset SMC on an iMac:

1. Shut down the computer.

2. Unplug all cables from computer, including power cord.

3. Press and hold the power button for 5 seconds.

4. Release the power button.

5. Attach the computer’s power cable.

Note: If you press the power button while inserting the power cord, the iMac will enter a mode in which the fans run at full speed.

6. Press the power button to turn on the computer.

Resetting Parameter RAM (PRAM)

PRAM stores certain system and device settings in a location that Mac OS X can access quickly.

Exactly which settings are stored in the computer’s PRAM varies depending on the type of computer as well as the types of devices and drives connected. To reset PRAM:

1. Shut down the computer.

2. Locate the following keys on keyboard: Command, Option, P, and R. You will need to hold these keys down simultaneously in Step 4.

Note: If the keyboard does not have an Option key, use the Alt key instead.

3. Press power button.

4. Immediately press and hold Command-Option-P-R keys.

Important: You must press this key combination before the gray screen appears.

5. Hold down keys until the computer restarts, and you hear the startup chime a second time.

6. Release keys.

Diagnostic LEDs

The iMac (27-inch, Mid 2010) computer has four built-in diagnostic LEDs on the main logic board that can help you to troubleshoot the computer. The LEDs are located on the bottom left edge of the logic board (under a piece of black mylar tape) and can only be seen when looking through the lower vents of the enclosure. Refer to the next page for a close up graphic of the troubleshooting LEDs.

Location of Diagnostic LEDs

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 lower vents of rear housing

LED #1

• Indicates that trickle voltage from power supply has been detected by logic board. This LED will remain ON whenever the iMac is connected to a working AC power source. The LED will remain ON even when computer has been shut down or put to sleep. The LED will remain OFF only if the AC power source is missing or disconnected , if the logic board is disconnected from the power supply, or if the power supply board is faulty.

LED #2

• Indicates that the main logic board has detected proper power from the power supply when the computer is turned on and that logic board voltage regulators are functioning properly. This LED will be ON when the computer is turned on and the power supply should be working correctly.

LED #3

• Indicates that the computer and the video card are communicating. This LED will be ON when the computer is communicating properly with the video card. If LEDs 1 and 2 are ON and you heard the startup sound, but LED 3 is OFF, then the video card might be installed incorrectly or need replacement.

LED #4

• Indicates that the computer and the LCD panel are communicating. This LED will be ON when the computer is turned on and video signal is being generated. If the LED is ON and there is no image on the display, then the LCD panel, the LED backlight board, or the cables between might be installed incorrectly or need replacement.

Let us know what you get after this.