Chosen Solution
We baked the old card and reinstalled and now the computer is booting up. I saw a flicker at one point so no telling how long it will work. Unless I discover some issue-fix re the 5750s, I am going to return the cards. Subsequent searches suggest maybe they are not compatible. TMI out there on the WWW, it’s hard to separate the grains of pertinent truth.
To locate the diagnostic LED’s you’ll need to pull off the black tape that covers them
And here’s what it looks like underneath
And lastly, here’s the decoder ring! LED 1 - Indicates that the trickle voltage from the power supply is detected by the main logic board. This LED will remain ON while the iMac is connected to the AC power. The LED will remain on even when the computer has been shut down or put to sleep. The LED will turn off only if the AC power is disconnected or the power supply is faulty. LED 2 - Indicates that the main logic board has detected proper power from the power supply when the computer is turned on. This LED will be ON when the computer is turned on and the power supply is 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 display 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 LCD display panel, the LCD display panel or inverter might be installed incorrectly or need replacement. Let us know what they tell you
