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
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.