sounds like it could be that.
I've not worked on ADM3 or ADM3A, but it was a popular terminal in the day.
It should be a 15KHz monochrome video setup.
I'd take and take the metal panel off with the 1 screw, use a null modem cable, set it to something simple like 9600N81 no flow control, use a null modem cable to another terminal or a computer with a terminal program using the same settings, and see if characters typed on the terminal make it to the computer.
If they do, problem is probably mostly video, and probably no harder than finding/fixing a problem in an arcade monitor.... if you can't establish communication, there may be more problems involved.
Working or not, it may have some value on eBay as well. Like us arcade nuts, there are people that collect old computer stuff (like myself). (Though I don't really collect old terminals.... but I do have a VT52 and a couple of VT220s

).
Shipping would suck though !
I wouldn't mind finding an ADDS580.... I have fond memories of using one in the mid 80s to connect to a DECsystem 10 (wouldn't mind finding one of those too

). Built like Sherman tanks (and probably weight about as much one too).
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