Tektronix WFM601, Service Manual

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Theory of Operation
3–28
WFM 601 - Service Manual
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 is an LED that is internal to the instrument. It is lighted 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 sinewave 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.
C26 and Q8 are a start delay circuit that holds the error amplifier output low,
through CR12, until C26 is charged. Delaying the start of the high voltage
oscillator allows the Low Voltage Power Supply to start, unencumbered by the
load from the high voltage oscillator.
CR7 is the high voltage rectifier. Filter capacitors C6 and C7 work with CR7 to
provide –2750 V to the CRT cathode. U1 is a four-times multiplier providing
+11 kV to the CRT anode.
Q1 and Q2 form an operational amplifier that sets the voltage at the bottom of
the focus divider. The front-panel FOCUS pot determines what that voltage will
be. The Center Focus control, R9, is set for optimum beam focus, as viewed on
Over Voltage Protection
HV OSC and Error Amp
Power Supply Outputs
Focus Amplifier