Isn't this the overvoltage circuit clamping the edge of the vectors? Does your deflection board have the extra circuit board added on top towards the back of the board?
You could change the position and size of your game with the pots but I think you see clamping on Tempest when the extra circuit board is installed on the deflection board.
I have a Tempest I picked up with no overvoltage board...the whole screen gets a bit bright from the overdriving of the vectors during the zooming part.