Quadraprong video issue

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Hey all,

I picked up a Quadrapong machine at the Pinball at the Zoo show today. I'm trying to diagnose the problem it's having and I'm not 100% certain if it's the board or monitor. Basically I've got an image that isn't stable vertically and usually it will need tweaking horiztonally when first fired up. You can make out the game playing a little, but that's about it. My gut feeling is the Zenith 19EB12 b&w tv has issues but of course I want to make sure. Has anyone tried using another TV to test the output from those early Atari Pong games? I can probably easily find one to use for testing and that would rule out the original Zenith TV as the culprit.
 
I found a cheapo tv with UHF and VHF connectors on the back. Hooking it up I can see some kind of signal from the board, but it's not as distinguishable as the image from the cabinet's tv. Anyone know if there's some mods done to the tv's used in early Atari games? There is a white wire coming from inside this tv's case that was hooked to one of the terminals but I unhooked it when testing the signal from the Quadrapong pcb.
 
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