This can also be due to the sync switch being in the wrong position, or if you're using RGB, try combining the H and V sync cables and plug the one cable into the H/V connector. You need a BNC 'T' connector to combine the two BNC lines.
if it is "hooking"at the top you may have run the raster size adjustment out to the max. The adjustment is on the green deflection board below the green CRT. It will be labeled " height" adjustment. Adjust this down a bit and see if the hooking goes away. Be careful not to short anything out and wrap the screwdriver shaft with tape.
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