Tektronix 2215, Инструкция по эксплуатации

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differential-mode transformer T907, and capacitors C901, C903,
C904, and C905. Capacitors C907, C908, and C910 form a
high-frequency bypass network to prevent the diodes in CR904 from
generating EMI.
The Preregulator provides a regulated dc-output voltage for
use by the Inverter circuitry.
When the instrument is turned on, voltage developed across
C909 will charge C913 through R911. When the voltage has risen
to a level high enough that U920 can reliably drive Q933, U920
will receive its Vcc voltage through Q915. This level is set by
zener diode VR917 in the emitter circuit of Q917 and by the
voltage divider consisting of R912 and R913. The zener diode
will keep Q917 off until the voltage at its base reaches
approximately 6.9 V. Then Q917 will be biased into its active
region and the resulting collector current will cause a voltage
drop across R916. This voltage drop will bias on Q915, and the
positive feedback through R914 will reinforce the turn on of
Q917. Thus Q915 and Q917 will drive each other into saturation
very quickly. Once Q915 is on, U920 will begin to function.
Pulse-width modulator IC U920 controls the output voltage of
the Preregulator by regulating the duty cycle of the pulse
applied to the gate of Q933. It utilizes an oscillator whose
frequency is determined by R920 and C920 (approximately 40 kHz)
and whose output at pin 5 is a sawtooth voltage. An internal
comparator compares this sawtooth voltage with the output voltage
produced by the two error amplifiers. Whenever the sawtooth
voltage is greater than the error-amplifier output voltage, Q933
is biased on to supply current to both C934 and the rest of the
circuitry. The two error amplifiers are used to maintain a
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