Service manual

Wells-Garnder Color Vector Monitor Guide
Page 33 of 73
FINE-TUNING THE PICTURE
Now that you have a working monitor (and game board), it is time to fine-tune your picture.
Before making any adjustments, turn your game on and let it warm up for at least 10 minutes.
R918 is the HV adjust and unless you have an HV probe, you probably should not mess with it.
If you have an HV probe, set the HV for 19.5 kV with zero beam current (i.e., with the game
boards disconnected from the machine so that no input signals are being supplied).
Here is a trick for adjusting R918 without an HV probe. Hook up power to the monitor (but
without any input signals from the game board) and let it warm up for about 10 minutes. Now
measure the Video B+ voltage either at pin 5 of P900 on the HV board, or at J501 on the neck
board (refer to your schematics). Adjust the HV so the B+ is 181v DC. Since the Video B+ and
the 19.5 kV outputs are linked together, you will get the HV pretty close to perfect.
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.