Chosen Solution
The previous owner said it just turned off in the middle of doing something. He removed the HDD and sold it to me. I removed the LCD and noticed the LVDS cable was damaged (suppose this has been done by the previous owner while removing the HDD). Ordered a new one, waiting to arrive. When I start the iMac I can hear the chime, 3 diagnosis lights lite up, the 4th is off since the LCD panel is not connected. Hooked up an external monitor using a mini display to HDMI cable. At this point I would expect some image on the ext. monitor, but there is nothing. Tried a different GPU and on first boot it produced output (missing boot drive icon) on the ext. monitor, however all other attempts were unsuccessful. Do I have somehow to force the signal to be send to the external monitor (CTRL+F1 does not work) ? Is my second GPU now defective(all 3 diagnostic diodes still lite up)? Thank you!
Using a USB keyboard lets see if you can get something more on the display (either). Your Mac has two graphics services the intergraded one within the CPU and the dedicated GPU. When you start your system it goes through a process looking first to see if the SMC has latched into fail state. If that is set the system won’t go any further until the condition is lost (SMC reset). The next step is to go through POST. But just before that there is one exit which is when you have pressed the keyboard T key which tells the system you want access to the internal HDD in Target Disk Mode. At this point your intergraded GPU will run off of the EFI driver. You should see either a FireWire or Thunderbolt Icon jumping around on the screen! Are you able to see it? You may need to reset the SMC first! References: Mac startup key combinationsHow to reset the SMC of your Mac If you do then you know the display and LVDS cable is working correctly. That only leaves the dedicated GPU as being bad or the wrong board for the system. Apple has custom firmware on there MXM boards and the version of board needs to be one that the system can support (what it came with).
