Rev A-D iMac video failure: Connecting an external display

CRT iMacs - revisions A through D - sometimes exhibit a video display failure problem where after starting up the system, it powers down immediately after a few seconds, without even lighting the screen. The LED is observed to go from orange to green, and immediately shut down. Several MacFixIt readers have reported this behavior.

Guy Hemmings notes a work around that involves connecting an external display to the iMac's internal video out port:

"Take the base off the iMac case, unplug the video cable. Plug a Mac screen (or PC screen with video adapter) into the motherboard's video out socket, boot the iMac, and your external screen brings everything up as normal.

"You'll also discover by experimentation that the video will support up to 1600 x 1200 in millions of colors (dependent on VRAM configuration of the iMac in question) which makes a very nice graphics design solution."

UPDATE: Fixing the problem There appear to be three primary issues that cause the iMac video display problem:

The flyback transformer is faulty: This piece of hardware relays diagnostic information that allows the iMac to properly start up. Zach Koman writes "In the past, I have purchased 3 iMacs (Rev.A and B) with this problem and been able to fix them by replacing the flyback transformer on the video/analog board. A new flyback for this board is about $60-$90 depending on where you get it (NEI or Dalbani). Then it is just a matter of desoldering the old one and soldering in the new one. You have to discharge the CRT of course which is kind of dangerous."

The entire analog board needs to be replaced: Peter Hein writes "The fix was pretty easy, a new analog board. I was able to picked both of mine up on eBay for a decent price. There are also service provers that will fix your old one for $150.00 and warranty it for a year."

Outdated firmware: Apple released several firmware updates for the iMac, coming to rest at version 4.1.9 which was released late last year. Some of the updates "increase the reliability of iMac booting," and offer VRAM recognition enhancements. We have received at least one report indicating that connecting an external display, then updating to the latest firmware revision resolved the problem.

Insight on this issue? Drop us a line at late-breakers@macfixit.com.

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