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The regulator circuit can be shut down by turning on Q3, which shunts the source current to
ground.
-15 V Regulator (B)
The -15 V Regulator is a tracking supply, holding the -15 V supply to the same magnitude
(inverted polarity) as the +15 V supply, through divider resistors R19 and R20. Overvoltage
and current limiting circuits are identical to those of the +15 V supply.
Thermal Shutdown (C)
Thermistor R46 (negative temperature coefficient) is mounted to the heat sink of Q5 and Q9.
As the heat sink temperature increases to approximately 8O"C, the resistance decreases to
less than 107 R, and the voltage across the bridge circuit in which the thermistor is located
changes polarity, causing U4 to go high. This turns on transistors Q3 and AlA7Q2, to
shut down all power supplies, and transistor AlAlQ2, to light the front-panel STANDBY
indicator. Current feedback through CR6 holds the bridge circuit in an unbalanced condition
until the temperature drops to about 55°C when the increased thermistor resistance again
switches the bridge polarity. QlO turns off the supplies by unbalancing the bridge when its
input (HPON) is less than about +2.0 V.
High-Voltage Oscillator (D)
The collector of AlA6Q7 is connected to the primary winding of transformer AlA3T1, and
a feedback winding is connected to the base of Q7. Positive feedback from the feedback
winding causes the circuit to oscillate at a frequency (approximately 25 to 30 kHz) determined
primarily by the inductance and capacitance of Tl. Q7 operates as a Class C amplifier,
supplying a large current (approximately 2 A peak) over a conduction period of less than
one-half cycle.
Oscillator Level Control (E)
Amplifier U3 regulates the dc level of the high-voltage CRT cathode voltage by controlling the
base drive to Q7 through the feedback winding. The cathode voltage is sampled via current
through AlA3R4, which is compared with a reference current through AlA6R31, R32, and
R50 (referenced to the +lOO V supply) at the input to U3. The output of U3 then holds the
base drive at the level necessary to maintain about -4000 V dc at the cathode of AlVl.
Amplifier open loop gain is set by R35, with R34, R36, and Cl4 limiting the control loop
bandwidth to approximately 7 Hz.
Cl5 supplies surge current for Q7, and L2 controls the conduction time of Q7. CR15 and
CR16 allow U3 to generate self-bias sufficient to maintain level control if the -15 V supply
should fail.
controlled from A4A9 IF Control, which may be addressed via the Instrument Bus.
2 A1A6/A1A7
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