Tektronix 1740A Series, User Manual

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Maintenance
6–10
1740A/1750A/1760 – Series Service Manual
g. Using the oscilloscope, observe the signal at the collector of Q13. With
J3 un shorted 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.
c. Using the digital multimeter measure the voltage at U3, pin 6. It should
be 0 V.
d. Using the digital multimeter measure the voltage at U3, pin 1. 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 U3, pin 6. It should
be 5 V.
g. Slowly increase the voltage of the variable Power Supply. Before 6 V is
reached the over voltage protection circuit should fire and pull the
voltage below 1 V.
7. Shut Down Logic Check
a. Connect the negative output of the 20 Vdc Power Supply to TP3.
Connect the positive output to the cathode of CR22.
b. Connect the digital multimeter common lead to TP3.
c. Measure the voltages according to Table 6–5 to see if the shut down
logic circuit is functioning properly.
Table 6–5: Shut Down Logic Levels
Circuit Location Approximate Voltage
U7, pin 1 0 V
U7, pin 4 2 Vdc
U7, pin 6 3 Vdc
U7, pin 7 4.15 Vdc
U7, pin 9 0.3 Vdc
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