Last week we reflowed the solder on graphics card and the iMac was working once again.
Two days ago we replaced the HDD with an SSD (860 Evo) and now the internal display only works if an external monitor is connected before start up (but can be removed straight away) or if we reset the PRAM every time we boot the machine.
We have replaced the Vsync cable as the connectors were damaged but that did not fix our problem.
The built-in display shows only a black screen if we boot it without resetting PRAM (each boot) or plugging in an external monitor (each boot). The computer still chimes at boot.
New development:
Now when we boot the Mac with the original HDD put back inside the computer doesn't even chime. The screen is black for ~1 minute shortly before we get a crash screen saying it encountered a problem and shut down then goes to the Apple logo and starts up.
Two days ago we replaced the HDD with an SSD (860 Evo) and now the internal display only works if an external monitor is connected before start up (but can be removed straight away) or if we reset the PRAM every time we boot the machine.
We have replaced the Vsync cable as the connectors were damaged but that did not fix our problem.
The built-in display shows only a black screen if we boot it without resetting PRAM (each boot) or plugging in an external monitor (each boot). The computer still chimes at boot.
New development:
Now when we boot the Mac with the original HDD put back inside the computer doesn't even chime. The screen is black for ~1 minute shortly before we get a crash screen saying it encountered a problem and shut down then goes to the Apple logo and starts up.