Troubleshooting guide

Cinematronics Vector Monitor Repair Guide v.1.0
Page 13 of 53
Spot Killer Protection Circuitry
Transistor Q2 can be considered as the master intensity control switch. When it is deprived of its
base drive, the collector-emitter junction opens and eliminates the ground path for the intensity
channels, cutting off the beam. The game logic board can fail in such a way as to cause the beam to
remain on continuously, burning the CRT phosphor.
R25 and C24 form an R/C time constant of about 6.8ms. Since most of the vectors on the display
require only microseconds of beam time, any turn on pulse from the CCPU, which is 6.8ms or
wider constitutes a failure mode. The outputs of IC7 (pins 2 and 4) are sampled by Q6, which
inverts the pulses and feeds them back into IC7 on pin 11.
When the input is high, the output transistor turns on and essentially shorts pin 10 to ground. When
the input is low, the transistor turns off and opens the circuits. So, for example, a high pulse at IC7,
pin 4 of 10ms duration becomes a low pulse at the collector of Q6. This causes IC7 pin 10 to open
the circuit, allowing C24 to charge. Since the pulse width is greater than 6.8ms, C24 has ample
time to charge and turns on Q7, which shorts the base of the master intensity switch Q2, killing the
display.
An additional protection circuit involves R20, R21 and pins 9 and 8 of IC7. Pin 9 is normally held
negative by the voltage divider of R20 and R21. Should the circuit breakers blow and we lose -25
volts, pin 9 of IC7 goes positive, causing pin 8 to short the base of Q2, again killing the display.
SPOT KILLER PROTECTION CIRCUIT