About the "VDR":
The original Atari VDR (voltage dependent resistor) located on the board at location R24 was a custom made semiconductor. It was a specially doped silicon carbide wafer with leads attached to form an axially mounted resistor. This resistor has an unusual property in that its resistance value decreases exponentially as the voltage applied across the component increases. The non-linear response of the VDR was used to create the "linearity" adjustment that allowed the operator to calibrate the straightness of the diagonally drawn vectors. The VDR was used on the Space Duel and Tempest to Major Havoc converter boards and also on many of the game boards such as Space Duel, Black Widow, Gravitar, Major Havoc, Star Wars, and probably several others.
The original Atari VDR component is now very hard to come by outside of the game boards themselves. There is no modern equivelent that I am aware of. While there are plenty of so-called non-linear resistors available in the form of over-current and over-voltage protection devices (referred to as polyswitches, polyfuses, and MOV's), none of them lend themselves to the application of linearity adjustment. This is because they are all engineered specifically for over-voltage or over-current protection. Modern components are made from Zinc Oxide that has a very sharp turn-on response. Also, the turn-on response usually occurs at a voltage that is too high to be useful in the VDR application.
With no supply of original or equivelent VDR components available, I had to create a substitute circuit (BXVDR shown below) that provides the same resistive response as the original Atari part.