Tektronix WFM601, Service Manual

Page: 142 / 304

Maintenance
6–10
WFM 601 - Service Manual
Table 6–4: Control Circuit Test Points
Circuit Location Signal
U6, pin 1 Approximately 5 Vdc
U6, pin 2 Approximately 1 to 2 Vdc
U6, pin 3 0 V
U6, pin 4 80 kHz triangle wave, 2 V p-p
U6, pin 6 80 kHz square wave, 18 V p-p
c. Use the oscilloscope to observe the signal at the anode of CR26. It
should be an 80 kHz square wave, switching between –1 and 12 volts.
The fall time is 0.2
m
s and the rise time is 1
m
s.
d. Remove the short from across Q15.
5. Transformer Driver Check
a. Connect the negative output from the 20 Vdc Power Supply to TP3.
Connect the positive output to the cathode of CR22.
b. Connect the negative output from the 5 Vdc Power Supply to TP3.
Connect the positive output to the cathode of VR3.
c. Connect the oscilloscope probe ground to TP3.
d. Using the oscilloscope, observe the signal at U8, pin 5. It should toggle
between 0 and 5 Vdc when J3 is shorted and unshorted. If the pulse is
not present, continue with the check.
e. Using the oscilloscope, observe the signal at U8, pin 3. A positive-go-
ing, 100 ms pulse should occur when J3 is shorted and unshorted. If the
pulse is not present, continue with the check.
f. Using the oscilloscope, observe the signal at U5, pin 1. It should be a
logical high when J3 is unshorted and a logical low when it is shorted.
If the signal is not present, continue with the check.
g. 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
Vdc. With J3 shorted this point should be at 5 Vdc.
6. Error Amplifier/Voltage Reference/Over Voltage Check
a. Set the variable Power Supply to 4.6 Vdc. Connect the negative lead to
TP1 and the positive output to W1 (+5 V).
b. Connect the digital multimeter common lead to TP1.