Late 2012 imac "No RAM Installed" startup beep

Perhaps I was not specific enough. This is not the result of a RAM upgrade. The unit was functioning properly, and then not. It was not a result of any hardware tinkering. When opened, it was quite clear that the RAM was properly seated, and retaining clips in proper placement. Also, my process of elimination exercise by its very nature included several re-settings of the RAM. I am a service tech, so I can recognize a proper seating. This possibility has been eliminated.

Excuse my miss-use of terminology. I have changed main-boards of Macs before. Its not a problem for me. I just dont want to do it if there is a possibility that is something less expensive, like the power supply, but I'm already 90% sure its the main-board if there is not some "typical" issue known in this particular model.

Symptoms are a black screen with single tone, then 5 seconds of silence, repeating.

This is a Late 2012 imac 21.5". A1418

Lastly, I am quite a distance from a mac dealer. I find that I get much better help from forums on these matters anyway, especially since I can perform the labor for such repairs myself.

Lastly, this was my first time opening this era of imac. Is there a favorite adhesive used for resetting the screen?

I look forward to your replies.

Thank you.

Thanks OT,

I think I will risk it with an eBay mainboard. I have done this with 3 other iMacs with success. A $120 spot with return policy is much more pallatable than a $650 outlay for a refurbished of the same model - give or take. I'll worry about the expense details, those are concrete and I will balance that in my consideration.

I will attempt safe mode. I will report back the findings. This mac was used in a fairly controlled environment with limited and experienced users. It wasnt my first instinct to think something half baked was installed on the thing, but its certainly worth the effort.

I am curious about the "user upgradeable" detail. I did see that in the serial number details I looked up. I know Apple is not very keen on people opening their products. I half-way ignored it. I'm not the typical Mac person. I'm a tech. The fact that the RAM is not convenient to get too (sandwiched between the back of the main-board and the back of the case) was my belief on why it was not user "serviceable". Crucial site had sticks for up to 32GB total discreetly stated for this model. I can always return it if it does not work, but I was not thinking Apple would actually limit RAM in firmware based on physical ease of replacement. The capabilities of the CPU, and supporting chipsets clearly support more ram than 8GB. But, it would not be the worst thing Apple has done to make things inconvenient.

Thanks again.

Yea, Ive been working on stuff like that since the 80s. I'm not afraid of a soldering iron, etc. My limit was found when working ipods when they started getting stupid small. No longer would I work on small electronics. Phones either. I dont touch them. I can rip open a PC or laptop anytime. Thanks for your comments.

Well, I replaced the logic board. The system exhibits the same problem. Single beep every 5 seconds. They only thing left is the power supply. Is this a plausible issue? There isnt anything left, really.

That was the first thing I did. I even tried different RAM. It cannot be the RAM. I ran the RAM successfully in a PC, and ran MemTEST86+ on it, and passed completely.

I'm with you. I dont know how it can be a power supply, but there isnt anything else to try.

I did not fully assemble the mac yet. It is still open, and I can do about anything.