Tempest displays on scope, but not 6100

I have this Tempest PCB I've been working through. I have it displaying a self-test screen indicating some errors that need work, but I can only get the screen to come up on a scope. Because I have no Z channel and only have 10x probes, I can't read the error codes on the scope. When I hook the game to a K6100, I just get a spotkiller.

Has anyone seen this before?

I know it's not the monitor, because I'm testing in a working cabinet. I thought it might be the PCB adjustments, but I used the scope to calibrate them to the same levels as another PCB, which does display in this cabinet. I turned off & disconnected the scope, in case that was interfering with anything; no change. I'm hooking the scope to the X/Y out test loops on the board. I thought there might be a broken trace between there and the edge connector, but I checked and there is continuity. I turned the screen control up on the 6100 to bypass the spotkiller, and all I have is a dot in the center; no deflection at all, despite seeing a signal on the scope.

What am I missing here?

Background: Got the board resetting in game mode and blind in test. Cleaned & reseated ROMs, and got it playing blind as well. But it went back to resetting a day later, which indicates dodgy ROM sockets. I'd prefer not to replace all 10, so I decided to get self-test going so it would tell me which were faulty. Replacing the TL082s got me a box in self-test with no text, and swapping the ROM @N/P3 got me a reasonable looking test screen. But I can only see it on the scope.
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Received on Sat Jun 22 19:02:28 2013