Tektronix WFM601, Service Manual

Page: 143 / 304

Maintenance
WFM 601 - Service Manual
6–11
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
d. Using the digital multimeter, measure the voltage at U6, pin 3. It should
be approximately 5 Vdc. Short the base to the emitter of Q15. Pin 3
should now measure 0 V.