High Voltage Regulator

Feedback from the CRT cathode —2450 volt supply is applied to the base of Q1404 through R1431D. Any change in the level at the base of Q1404 produces an error signal at the collector of Q1404, which is amplified by Q1408 and Q1416 and applied to the base of Q1418 through the feedback winding of T1420. Regulation occurs as follows.

If the output voltage at the -2450 volt test point starts to go positive (less negative), this positive-going change is applied to the base of Q1404. Q1404 conducts harder, which in turn causes Q1408 and Q1416 to conduct harder. This results in greater bias current to the base of Q1418 through the feedback winding of T1420. Now, Q1418 is biased closer to its conduction level so that it comes into conduction sooner to produce a larger induced voltage in the secondary of T1420. This increased voltage appears as a more negative voltage at the —2450 volt test point to correct the original positive-going change. By sampling the output from the CRT cathode supply in this manner, the total output of the High-Voltage Supply is held relatively constant.

The output voltage levels of the High-Voltage Supply are controlled by the High Voltage Adjustment R1400 in the base circuit of Q1404. This adjustment sets the conduction of Q1404 to a level that establishes a -2450 volt operating potential at the CRT cathode.

Q1413 and Q1414 compose an overvoltage protection circuit. In the event the regulating action of the circuit should cause the CRT cathode supply to approach approximately —3000 volts, the voltage level at the emitter of Q1416 will be very close to -6 volts. Normally Q1413 and Q1414 are biased off and do not conduct. When the voltage level at the emitter of Q1416 reaches approximately —6 volts, Q1413 is biased into conduction which in turn biases Q1414 on. Q1414 now starts to reduce the base drive applied to Q1418 and prevents the amplitude of oscillations from increasing. This prevents the CRT cathode supply from going more negative than approximately —3000 volts.

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