Chosen Solution

After system update (High Sierra) I get a black screen and the start up chime continuously loops. Keyboard commands do not work neither does putting in an OS X disk as the disk drive. I also tried to plug in an external display (MacBook) using a Mini Display Port to Thunderbolt cable and I’ve got no signal. Removed all RAM (4x2GB) got error beeps, put 2 back on and got the chime loop with the black screen. Diagnostics LEDs: #1 is always on, #2 and #3 are on when the iMac is started, #4 is off. What could be the cause of this issue? The computer has been working perfectly until now.

Your MacBook won’t work as an external display. You can hook it up via the Fireport and try to access the iMac’s hard drive using Target mode. But my bet would be on a failed GPU. It will probably need to be replaced. I have also had about a 70% success rate with doing a hot air workstation reflow. iMac Intel 27" (Late 2009 or Mid 2010) Graphics Card Replacement

@alrazule - I ended up fixing it by baking the GPU and that fixed it for now. A longer term fix will be to replace the GPU but as it’s already 10 years old I’m not sure it’s worth the investment. I did follow along a thread about replacing with a modern GPU so that it can run Catalina but again I’m not sure that’s worth the investment. If you’re interested in this route the thread has a wealth of information: https://forums.macrumors.com/threads/201… Good luck with your iMac!

Hi both; I ran out of options, so I decided to bake the Graphics Card - And this did the trick. The iMac has been running flawlessly for another 2 years since it happened.

Hi @fourcandles , I have the same issue, same LEDs as you. (4th is off). The only way I get something different is

  1. Remove ram which gives me the RAM error code And
  2. Removing GPU, which causes it to only have first two LEDs on as expected. Fans spin to mac but no sounds at all. Other things I tried: Recovery mode, safe mode, diagnostic boot, usb boot, new RAM sticks, no hdd, hdd with preloaded OS, replacing the CR3202 battery, reseated the CPU and GPU (also reapplied thermal paste since I was already in there). All these had no effect. I am trying 2 things next unless you figured out the problem: A) Tonight, 2020-09-17, try external display with and w/o GPU, his is very likely not the issue but just want to cross it off None worked. Same Chime loop B) Tonight, 2020-09-17, test LCD panel and lvds with an adapter board. This is very likely not the issue but just want to cross it off My adapter was for the 21.5” model >.< Didn’t have the right power connector C) Tomorrow, 2020-09-18, oven bake the GPU since I’m pretty much out of options. If none work, I’ll just gut it, toss a Raspberry pi in there with the LCD board and if it recognizes panel, use it for retro games or maybe just as an external monitor for my P I don’t know why I didnt find this other post before: Reviving iMac 2010 with looping chime on booting, VIDEOcard was faulty He had the SAME observations as us. He replaced it the inferior card from the i3/21” model and it booted. Given this, have high hopes for tomorrow as I’ve read it fixes GPU issues although not as precise/efficient as hot flow but I don’t have a station and at my job there are plenty of ovens we use that aren’t used for food so I feel OK baking electronics in there. If it doesn’t work still, I’ll hunt for a cheap card (hopefully under $30), if it works, I’ll upgrade it to a gtx 765m or 770m which are reportedly better/less prone to failure and keep the cheapo as backup or for lending it to people local to me to troubleshoot their own iMac.

UPDATE: in the meantime, i upgraded my 2010 27” iMac with a Nvidia GTX780M and successfully got Catalina to work on it (not easy, many sleepless nights, but well worth it!). Worked seemlessly for months! Now - only 8 months later - it looks like that 780M is broken. Computer crashing, and back in Chime Loop I am :-( Probably going to try and bake this card as well. Any other suggestions?

Update: I did all the above and it breathed life back into this imac! I was able to successfully flash the Nvidia 860M 4gb to work and it worked well but unfortunately it only lasted 1.5 years until the gpu failed again. This couldve been avoided by sanding/working down part of the heat sink since it didn’t match perfectly with the 860M. I already tried baking this one but no go unfortunately. @virv , I would go ahead and do it, mine was irrecoverable but I think its b/c of the poor match I had before that really messed it up. I had obtained the 860M for about $50 back in 2020 but prices are a bit crazy nowadays (its going for almost $180-200 used!) so I’ll just be getting a new mac with an M1 core. Good luck for anyone who comes across this later!