Tektronix WFM 601A, Service Manual

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Maintenance
6–30
WFM 601A, WFM 601E, & WFM 601M Service Manual
e. Use the oscilloscope to observe the signal at the anode of CR26. It
should be a 100 kHz square wave, switching between 0 V and 12 V. The
fall time is 0.2
m
s and the rise time is 1
m
s.
f. Remove the short between pins 1 and 2 of U6.
5. Power Switch and Shutdown Logic
a. Connect the negative output from the +20 V
DC
Power Supply to TP3.
Connect the positive output to a 0.25 W, 150
W
resistor and connect the
other end of the resistor to the left end of R119 or R121. This connection
should provide 6 V at 10 mA to run the switching circuit.
b. Connect the oscilloscope probe ground to TP3.
c. Listen for the Maglatch to click when J3 is shorted and unshorted.
d. Using the oscilloscope, observe the signal at the collector of Q13 with J3
unshorted. This point should have a 1 V, 170 kHz sine wave riding on 5
V
DC
. With J3 shorted this point should be at 5 V
DC
.
e. Short pin 2 of U6 to GND (TP3).
f. Check that pin 6 of U9 is high.
6. Error Amplifier, Voltage Reference and Over Voltage Check
a. Set the variable Power Supply to 4.6 V
DC
. Connect the negative lead to
TP1 and the positive output to TP4 (+5 V).
b. Connect the digital multimeter common lead to TP1.
c. Using the digital multimeter measure the voltage at U8, pin 6. It should
be at TTL high.
d. Using the digital multimeter, measure the voltage at U8, pin 3. It should
be 2.5 V.
e. Set the variable Power Supply to 5.4 V.
f. Using the digital multimeter, measure the voltage at U8, pin 6. It should
be 0 V.
g. Slowly increase the voltage of the variable Power Supply. Before the
voltage reaches 6 V, the over-voltage protection circuit should fire and
pull it below 1 V.