This is a series regulator with Q900 being the control ele-
ment, Q901 a driver, and Q902 an error amp. ZD900 forms
the emitter reference voltage source. Since the generated
high voltage and other voltages are linked by means of
the magnetic field of T900, any change in H.V. will be
reflected back to all of the other voltages. This H.V. fluc-
tuation is sensed on the 90V line and is used to control
Q902. R906 and R914 are used to limit the range of H.V.
adjustment by R905 to roughly 13.5 to 15.5 KV @ 0 beam
current. ZD901 and C905 provide a means of dropping a
large DC component without attentuating the error
voltage that is needed to stabilize H.V. line. R904 pro-
vides a means of biasing ZD900 to insure a stable
reference voltage for the error amp to work against. R902
and R903 is the collector load resistor for Q902 and
biases Q900 and Q901 in the absence of an error signal.

The error amp. Q902, can only divert some of the base
current from Q901 and shut the regulator down. It can not
increase the conduction of the regulator above a level
determined primarily by R902 and R903 and the current
gain of Q900 and Q901. R901 is a buffer resistor to pro-
tect Q901.

C903 and C902 are used to suppress any tendency for
high frequency oscillation that could generate radio fre-
quency interference. C900, C901, C904 and R911 are
filters and damper to minimize ripple on power lines as
well as to decouple 25 KHz pulses that are impressed on-
to the + 38V line by a normally operating H.V. generator
and prevent interference with other circuitry. L901 is a
small high frequency filter choke to decouple the H.V.
generator from the +VE supply rail and minimize line
conducted radiation.

Q903, R907, R910, C906 and T900 form a free running
Hartley Oscillator circuit that operates at approximately
25 KHz. The operating frequency is primarily determined
by transformer and transistor characteristics. Feedback
is established by the proper phasing of primary to feed-
back winding. AC feedback is determined by the turns
ratio of primary to feedback winding as well as R908 and
C906. The transistor must also be forward biased suffi-
ciently to where oscillation can start when power is ap-
plied. This is controlled by R907 and R910. L900 is not
essential to circuit operation but helps smooth out the
AC waveform generated into a reasonable approximation
of a sine wave. R915 and R916 are Safety resistors that
will burn out, and prevent damage to Q903, should the
output circuitry come under excessive current drain.
0900 is a surge diode.

There are two secondary windings. The H.V. winding
delivers sufficient AC to the H.V. rectifier D903 to
generate 14.5 KV anode voltage at a maximum current of
100 ua, and tapped lower voltage winding that is used to
generate focus voltage of 0 to 410 VDC, and G2 voltage of
+ 410 VDC, and + 90 volts for operation of the 2 amp cir-
cuitry. D900, 901 and 902 are fast recovery diodes to
operate @ high frequency of 25 KHz.

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