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I've got several 27" iMac motherboards. I have at least 2 that power up and have the startup sound, however I don't have a working LCD. I have two external monitors and I have a display port to DVI cable but when I connect up the external monitor all I get is either a black screen or a "check signal" message.
What am I missing? I thought the display port should function regardless of the presence of the LCD panel or GPU?
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Not sure where you got the idea an iMac can function without an internal display. The issue is what is the default video port 0 or 1? The internal is 0 and is the default.
Now one can alter the output to go to both ports (mirroring) or extending the desktop space or even treat the output's discretely where you are presenting and need to see your notes Vs what the audience see's (mostly used with laptop setups). But that's not a test environment.
In your case testing logic boards one uses a holder to prop up the display to the side and using extension cables running between the main shell's mounted logic board. If you are working on the board alone I just used an oscilloscope to check the signals are present on the board not trying to see an image. But you could also use the extension cables as well here.
The last comment as far as a GPU being present, the CPU's Apple used in the Intel models all had built-in graphics services. Within the latter 21.5" models that's what you had, only in the 27" did you gain the dedicated AMD GPU. The older Intel models had a hybrid setup which included NVIDEA GPU's in the 20, 24 and early 21.5" models.
So in the case of the systems with MXM boards the system won't work without a working MXM board mounted. Only via the OS (technically the Firmware) can you alter which graphics engine to use. But you still need the MXM board present.
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I've got parts of 4 EMC2689 27" iMacs laying around here and I'm trying to see if I can get AT LEAST ONE to load / run.
ALL the LCDS are cracked.
On at least ONE board it sparked, shorted, and burned a bit of the board right near the LCD Cable connector.
2 boards have a startup chime but only LEDs 1 & 2 are lit.
Not sure if it means anything but my external monitor displays Check Signal Cable when the machine is off but all I get is a black screen when turned on. I've tried Resetting the PRAM, I've tried to enter target mode with Command-T and I've tried with and without all of the broken LCDS.
Electronics hobbiest / hoarding tendencies have me reluctant top possibly throw out a board that might work. But at this point I cant think of anything else to try.
@fxguy1 - Without a working internal display to connect via extension cables I don't know of any way to do what you want here.
Think of like an old fashion water pump... you need to prime it with water for it to pump the water in the well.
Here it's similar! As we need the internal to start the system, then it's possible to alter the output to a second display.
@danj is that after login?
@fxguy1 - That's later in the boot process! The hardware needs to work first.
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