Chosen Solution
It appears the display and/or logic board for my iMac has achieved terminal failure. This is not my screen, but the display exhibits these same symptoms.
The display is blocky and degraded even after wiping the HDD and installing fresh OSes (Yosemite through High Sierra) and when booting into the Utilities menu from an installation USB stick, as well as in screen sharing from remote desktop. Didn’t think it would happen there, but it does. It works flawlessly in target display mode, however, so I’m pretty sure the internals are roasted. As of May 2019, used Radeon 5750/6970 display cards are not cost effective to replace, so on to Plan B. I had been using the iMac in Target Display Mode exclusively, but even in TDM, having booted to a login screen on a vanilla system install, the fans constantly kick out processor heat. Not full-bore fan blast, but more than I’d expect for what should be idle load on the procs. It’s noisy and heats up the room, and I know it’s drawing more power than needed just to light the screen so on to Plan C. I got Apple’s shipping label for their free recycling program and am about to send the iMac to be shredded or whatever they do, but I need to open it to pull the HDD before packing it up. It occurred to me that while I’m in there I have an opportunity to disconnect any or all components not critical to target display mode, essentially up-cycling the iMac as a dedicated monitor. So here’s the question. Anyone know how target display mode works on the hardware/firmware level? Target display switches on instantly once a cord/signal is detected. A keypress is not required, nor does it need to switch over from the OS, so it does not appear to rely on software. Knowing nothing, I would think pulling the HDD, CPUs (if not soldered on) and disconnecting the fans should effectively make it a full-time monitor, but then again maybe not. This model has been around nearly ten years, and since there’s always someone on the internet who has already done what you’re about to do, I’m putting it out there: Who knows the what the deal is with target display mode? And is what I’m trying to do even possible?
Here’s my pictorial guide on doing this with a iMac 24”. Yes, it’s neat and a fun project. The parts are model specific, so do as Oldturkey suggests. How to build a display using a iMac
“up-cycling the iMac as a dedicated monitor. “ potentially possible but we would need the exact model number from your LCD panel. Ultimately if this is a panel that is supported you can eliminate 90% of the parts that are currently part of your iMac. This is not always an easy task but it is a fun project and will allow you to keep eWaste to a minimum. Let us know what your panel # is and we’ll see if we can hook you up.
Sadly you need the logic board as the display is still driven by it and you still need the power supply and the backlight driver board. So there’s nothing to bailout in the current setup. But … What you could get a LVDS to mini DP or VGA adapter board which could fit inside the case allowing you to pull most of the guts out. @oldturkey03 has much more experience on doing this than me. Lets see if he can offer some insight on what’s needed.
