It is important to protect CRT from phosphor burn by an
undeflected brite spot on screen under different fault
conditions. This can be done by cutting off electron
beam (2 amplifier) and also by allowing the CRT heater to
cool down before the cathode to G1 bias voltage
becomes small enough to turn on the electron beam on
screen depending upon different fault conditions. A
typical cathode to G1 voltage for a just visible display is
60 volts. D506 is an isolation diode to prevent discharge
of C504 through any resistance path in H.V. supply and
brightness control circuitry. Following are examples of
typical fault conditions.

A. On power down, 2 is cut of as Q503 and Q504 are not
    conducting, while D506 blocks discharge path of
    C504 long enough to hold cathode voltage until
    heater filament cools down.
B. On +90 volts failure, Q503 is turned off, which in
    turn cuts off Q504 while the cathode of the CRT will
    reach the 95V supply potential. Since Z is cut off and
    cathode is at 95V supply potential, this will eliminate
    any phosphor burn on CRT.
C. Spot killer circuit is also intended to protect CRT in
  case of X or Y or both amplifier circuits fail. Q500
  and 0501 are X and Y spot killer amplifiers. Since
  both work same way, only 4500 circuit has been
  analyzed here. Under normal operating condition
  feedback input signals of approximately 2.8V PIP
  from yoke are being rectified by D500, D501 to - 4.7
  volts. This will overcome comparatively small +ve
  bias from R506 and reverse bias Q500. Under fault
  condition, there will be no input signal and -ve supply
  on base of Q500. Positive bias will take over and
  turns on Q500, Q502. LED D504 will come on and
  indicate fault condition in amplifier circuit. Collector
  current of Q502 will develop sufficient voltage
  across R513 to reverse bias, the emitter base junction of
  Q504. With Q504 in cutoff, the cathode of the
  CRT will reach the 95V supply potential. Since Z is
  cut off and cathode is at 95V supply potential, this
  will eliminate phosphur burn on CRT.


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