@jaysee Unfortunately it isn't the cause - I broke this backlight board by mistake. I have seen this issue occur with quite a few 27-inch iMacs though and hope to probe around further for the root cause once I can get my hands on one nearby for testing. The last one I worked on was fixed with an LCD replacement, so I believe whatever the issue is, it's in the panel itself rather than an issue on the backlight board. If I figure it out I'll report back here.
@cosmichobo I do believe so. For service purposes Apple doesn't manufacture new boards, instead they rework the old ones. In some ways it's a good thing, repurposing components instead of wasting resources on manufacturing new ones, but it also has its drawbacks with reliability and rework quality sometimes, as owners of 2011 MacBook Pros have recently discovered when their GPUs fail a second and sometimes third time.
@Oldmacs A screen connector? Ouch. I haven't soldered down a screen connector before and would probably need to source a hot air rework station before I could do that type of repair. I have a couple of boards myself that need new LVDS connectors so this could be on the cards soon.
Is it the cable connector or the connector on the logic board itself? I think I saw a spare logic board for this model in spares recently so I'll have another look and see if it works.