Ok, let me clarify a bit.
Initial issue:
Stock Late 2012 iMac 21.5". It was working fine, then one day, I get the "No RAM Installed" beep code. (1 beep, 5 second silence, repeat)
With the screen removed, logic board lights #1 & #2 illuminate, then beep codes begin. Main cooling fan spins up. Nothing in display, apparently not even backlight.
Attempts to fix:
Phase 1:
I removed one stick of ram. Issue remained.
I removed 2nd stick, replaced it with the first stick. Issue remained.
I installed RAM into a PC, ran MemTest86+. 30 min deep test revealed no errors.
Phase 2:
I got a used logic board from ebay. Decided to replace the CPU with an i7-3770S while I was at it. May as well.
Installed the new logic board with new CPU. Surgery was not all that bad. Much easier than many ultrabooks I have replaced screens and such.
Issue remained.
Phase 3:
I installed new RAM. Crucial CT8G3S160BM. 8GB DDR3L-1600. Issue remained.
Removed one of the sticks. Issue remained.
Phase 4:
I replaced the power supply. Issue remained.
Removed Hard drive and installed a new blank one. Issue remained.
So, I have basically replaced everything but the case and screen. I'm not going any further. Either I get it working with what I have, or I bail and return most of what I have purchased.
Worthy of note, the original i5-3550s cpu was installed in a PC, and works fine.
At this point, the only thing that makes logical sense is that the logic board I purchased used was also faulty in the same manner. It also boots with only two of the indicator lights lit, and goes into the same tone code.
What I know, or am 90% sure of:
1. All the RAM I have is good. Both the original, and the New Crucial RAM.
2. The power supply must be good. Light #1 on the logic board indicates power supply is functioning. Changing it did not provide any different behavior. Plus, the issue did not put the power supply in suspect, but it was cheap(er) so I went ahead and replaced it just to rule it out.
Any other ideas?