Tektronix 070-8462-03, Инструкция по эксплуатации

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Theory of Operation
4–26
1735 HD High Definition Waveform Monitor
Shutdown Logic. Shutdown logic consists of three comparators that drive Q19.
When the output of one or more of the comparators goes negative, Q19 pulls pin
3 of U4 high and shuts off the controller. U5B, C, and D are all wired as
comparators.
U5D provides short-circuit protection. When the error voltage goes low, C56
starts to discharge through R99. If the error voltage stays low, the output of U5D
goes low and causes Q19 to turn off the controller. Once turned off, the supply
will attempt to restart at a repetition rate of about once or twice per second.
There will be an audible ticking sound when the supply attempts to start under
shorted conditions.
U5C is the low-line voltage disable. It outputs a low that turns off the controller
(through Q19) anytime the incoming line voltage drops below about 80 VAC.
U5B shuts down the power supply if the housekeeping supply falls below
approxi13 V.
Transformer Driver, Peak Detector, and Power Switch Receiver.
Secondary circuits
are isolated from the primary by T2 and T3. T2 is the power transformer and T3
couples needed feedback from the power supply secondary circuits to the
isolated primary side. The sawtooth waveform used for controller timing is
emitter-follower coupled to U5A, a square wave generator that drives Q18 with
an 80 kHz square wave. Current through Q18 is the primary current for T3. Q12,
across the secondary, is driven by the Error Amplifier. The signal voltage on the
primary is a reflection of the secondary loading, and is detected by the peak
detector.
The peak detector consists of CR23 and Q14. Its output is the negative feedback
for the controller output.
Q13 and Q15 form the power switch receiver. When the instrument POWER
switch is on, its contacts are open. With the switch contacts open, Q18 drives the
primary of T3 positive and allows the primary to “fly back.” When the primary
flies back, Q13 is turned on and its collector goes low, keeping Q15 turned off.
When the POWER switch is turned off (contacts closed), there is no signal
voltage developed across the transformer, which causes Q13 to turn off and Q15
to turn on. The collector of Q15 pulls down and turns on Q19, which pulls pin 3
of the controller high and turns it off.
Output Filters. Two of the three secondary windings produce a single voltage
level (+5 V and +40 V). The third winding is center tapped and o15 V
and 15 V. Each supply is rectified by a single diode (frequency is 80 kHz) and
smoothed by an LC pi filter, consisting of a single inductor and 2 filter capaci-
tors.