Tektronix 1740A Series, User Manual

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Maintenance
1740A/1750A/1760 – Series Service Manual
6–9
4. Controller/Gate Drive Check
a. Connect the negative 20 Vdc Power Supply output to TP3. Connect the
positive output to the cathode of CR22. Short the emitter and base of
Q15 together. Connect the oscilloscope probe ground to TP3.
b. Table 6–4 shows the signals present when the control circuit functions
properly.
c. Use the oscilloscope to observe the signal at the cathode of CR28. 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.
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
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.
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