MCR2 to jamma - sync combiner problem

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I've just made up a jamma adaptor so I can test a couple of my MCR2
boardsets - trouble is that I'm getting a lot of curl and tearing
along the 'top' of my monitor image. I usually rectify this by
adjusting a 10K pot that I have attached to my jamma rig's sync line
but in this case I can't adjust it enough to eliminate the curl.

I initially tried using a 74LS86 for my sync combiner but this just
displayed a broken up out of sync image. So I tried a 74LS08 instead
and got a much better and steady in sync display but with the
aforementioned display curling/tearing at one edge.

Any thoughts please?

Should I be using another TTL to do the combining?