Service manual

Wells-Garnder Color Vector Monitor Guide
Page 59 of 73
SPOT KILLER
The purpose of the spot killer is to turn off the video intensity amplifiers when deflection is not
occurring. Failure of the spot killer can cause the phosphor coating on the picture tube to become
burned.
The two signal input voltages to the spot killer are taken from resistors R610 and R710 in the X
and Y deflection circuits. Diodes D801 through D804 and Capacitors C800 through C803 form
two separate voltage doublers. The output of the voltage doublers are applied to the bases of
transistors Q801 and Q802.
When either of the deflection amplifiers is not driving current through the deflection coils, and
then either transistor Q801 or Q802 becomes biased so that it conducts, which turns on transistor
Q800 and the LED D800 in its collector circuit. When transistor Q800 is conducting, then
transistor Q503 in the neck PCB is cut off, forcing the red, green and blue amplifiers to turn off
their electron beams.
HIGH VOLTAGE POWER SUPPLY
Integrated circuit IC901 is a timer circuit that produces a 20 KHz output, which drives transistors
Q904 and Q905. These transistors are current amplifiers that drive the primary winding of the
Step down transformer T901. The output of T901 is used to turn on the main driver transistor
Q906 that in turn drives the High Voltage Step-up transformer T900. The output of the
secondary winding is applied to the picture tube at the focus and intensity grids and the 19.5KV
anode.
Diode D901 allows capacitor C910 to charge to +180V during the discharge of the primary’s
magnetic field. Transistors Q900 through Q903 are error amplifiers that regulate the +180V
video B+. Pot R918 provides an adjustment to the video B+ and hence the high voltage. To
adjust the high voltage, you will need a high voltage probe, a voltmeter, and an insulated
screwdriver tipped adjustment tool. Turn off power to the display. Connect the high-voltage
probe to the voltmeter, and insert the tip of the probe under the high voltage anode rubber shield.
Insert the insulated screwdriver tipped adjustment tool through the top of the high voltage cage,
making contact with potentiometer R918. Turn on the display and adjust the high voltage to
19.5KV.
NOTE: Mark Spaeth has written a very informative description of the Wells-Gardner 6100
vector monitor and the input protections described in this document. Download your copy from
http://rgvac.978.org