LC Video Sync Issue

[warmech]

Alrighty, I'm stumped with this one. I've been using an Apple Multisync Display with an LC that I just recapped that, up until earlier today, was fine. All of a sudden, I get this:



Having worked with arcade monitors before, I suspect this is a sync issue. The thing is, if I use a video adapter dongle to connect to VGA, it works fine:



Am I just missing something, or did something die on this LC board, but isn't dead enough to not display anything?

EDIT: Also, other Macs work with this display with no issue, so I'm leaning toward the LC being the problem, just not what could be wrong with it.

 

[warmech]

It took a minute of head-scratching, but it occurred to me that the LC probably wasn't actually the problem. I cracked open the monitor and had a go at the horizontal hold potentiometers; a teeny tiny little twist of HHOLD2 (it barely even turned) snapped the image into alignment. I'm sill a bit flummoxed as to how it could have gone out of sync, but, then again, it's a 30 year old display. I have newer RGB arcade monitors that have been recapped more than once - this thing is probably due for new caps itself by now.