Tektronix 1740A Series, Руководство пользователя

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Circuit Theory
1740A/1750A/1760 – Series Service Manual
3–35
U7 is a quad comparator, whose outputs are open collectors. All four comparator
outputs are connected in parallel and under normal operating conditions are high.
Whenever the output of any one of the comparators goes low, Q15 will turn off
and the pulse width modulator current sense line will go high and shut down the
power supply.
U7B is the comparator for the +16 V supply. U7C is the line voltage compara-
tor, sensing the rectified ac primary. If either output is low, Q15 turns off to shut
down the power supply. U7A senses the power switch status from the Q output
of U8A. Whenever the output of Q8A is low, the output of U7A will also go
low.
U7D prevents the power supply from running on in the event of a +5 V supply
short. Shorting the 5 V supply disables the optoisolator, which causes the error
voltage to fall below 2.5 V. After a short period of time C65 discharges and
causes the output of U7D to go low and shut down the power supply.
Q11 is a silicon-controlled rectifier (SCR) that is triggered if the +5 V output
rises above approximately 5.5 V. If the SCR triggers the +5 V is shorted to
ground and the supply shuts down and waits a few milliseconds before attempt-
ing to restart. Over voltage shutdown can be tested by shorting R74 and R78
together.
DS6, which is an LED that is internal to the instrument, is lit whenever the +5 V
supply is running. This is simply a servicing aid making it possible to determine
if the power supply is operating without having to look at the front panel.
Diagram 22 High Voltage Power Supply
The High Voltage Power Supply is generated by a sine-wave oscillator and
step-up transformer. Q7 and T1 are the principal elements of an Armstrong
oscillator running at about 22 kHz. Error amplifier U2 regulates the +100 V
output and keeps the High Voltage Power Supply constant under varying load
conditions by controlling the base current to Q7. The +100 V output is regulated
directly, while the High Voltage Power Supply is indirectly regulated through a
current feedback circuit.
R40, C15, R66, and R61 form the High Voltage Power Supply current feedback
circuit. As the current from the High Voltage Power Supply is increased, the
voltage to the + side of the error amplifier (U2) increases, which increases the
base drive to Q7, the HV Osc. This current feedback compromises the regula-
tion of the +100 V supply to keep the high voltage constant with varying
intensities.
Shutdown Logic
Over Voltage Protection
HV Osc and Error Amp
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