And Google says:
HV Over-Voltage Protection Circuit
The later revision HV supplies (P329) have a circuit called the HV over-voltage protection
circuit. It monitors the voltage of the focus assembly in the secondary circuit of T900. If the
high- voltage at the anode of the picture tube increases beyond the threshold set by H.V. TRIP
adjustment R930, this circuit shuts off the timer of IC901 and LED D903 turns on to indicate the
over-voltage condition. Presumably, this was added to help discourage Tempest from eating HV
transformers, as it is prone to do since they are by far the most expensive component in the
game. If you know your HV is good, adjust R930 so that it is just beyond the point where LED
D903 lights (so that the LED is off).
NOTE: The following adjustments are a 2-person job (unless you are very handy with mirrors)
because it is nearly impossible to view the screen and turn the pots at the same time. When
adjusting your monitor, all adjustments should be made exactly in the order as presented below
as many of the adjustments have effects on other qualities and will cause you to have to go back
and redo them if you change the order.
On the outside of the HV assembly are the focus and brightness adjustments. Adjust the focus
until the picture is sharp then adjust the brightness just under the point where the dot in the center
starts to show or just under the point where you can see the connecting lines between objects. Be
careful not to get carried away with the brightness as you can do permanent damage to the
phosphors in the tube.