Imac won't recognize external monitor anymore

[Tgraykansas]

My iMac is old. It's an iMac core2 duo 1.83 that was introduced in 2006. I've had it since it was new. About a year ago I had the display only showing hundreds of colorful vertical lines and so I bought a display adapter and hooked it up to an old external display I had laying around. Everything worked great until a week ago. This time when hitting the power button on my iMac I heard the startup chime but noticed that the external display was staying black. I still see the same vertical lines on the iMac display and can hear it's running but my external display stays black. I tried hooking up my external display to a completely different computer and the display works fine. It's as if there's no connection between my iMac and the external display anymore. Any ideas?

 

[chscag]

It sounds like the graphics card in your iMac may have died. Did you try another cable to make sure the one you're using is OK? Since the display works with another computer, it's probably your iMac or the cable that went south.

 

[DeltaMac]

If the display tests OK on a different computer, then I would suspect that you either have a bad video adapter, or the video output has simply died.
Try to find a different mini-DVI adapter. If another adapter does not work with an external display, then your external video output is likely dead, and the logic board needs to be replaced.

If you actually have the Core2Duo iMac, then that one has the integrated GMA 950 chip, and the video memory is shared with main memory. You COULD try reseating the RAM sticks, and be sure to try an SMC reset, which is simply shutting your iMac off, unplugging ALL cables, including power, from the back. Wait for 15 seconds, then plug back in. I would then try a PRAM reset (iMac off, then press and release the power button, then press and hold Option-Command-P-R. You will hear the boot chime sound. Continue to hold the same 4 keys until you hear the boot chime two more times, then release the keys.) The SMC reset, followed by the PRAM reset may fix you up - - or not change anything at all.