Tektronix WFM 601A, Service Manual

Page: 148 / 368

Maintenance
6–28
WFM 601A, WFM 601E, & WFM 601M Service Manual
Table 6–4: Low volts supply voltages
Test point Voltage range
TP4 (+5 V) +5.0 V
TP5 (–5.2 V) –5.2 V
W1 (+11 V) +10.5 to +12.0 V
W2 (–11 V) –10.5 to –12.0 V
R69 (+40 V) +39.0 to +41.0 V
NOTE . No AC power is necessary to perform the Low Volts Power Supply
troubleshooting.
b. Disconnect AC power from the instrument. Disconnect the instrument
from the Power Supply by removing the jumper from J7.
c. Use the digital multimeter to measure the voltage between TP2 and the
tab (drain) of Q14. Check that the voltage is near 0 V.
WARNING. Do not proceed until the drain of Q14 is near 0 V. Dangerous voltage
potentials are present in the circuit until the capacitors discharge.
2. Rectifier/Switcher/Snubber Check
a. Use the digital multimeter to measure the voltage between TP2 and TP3.
Be sure the voltage is near 0 V before proceeding.
b. Remove jumper P7.
c. With the negative lead of the digital multimeter connected to TP3 (line
common) and the positive lead connected to TP2, measure the circuit
resistance. A resistance of less than 20 k
W
indicates a shorted MOSFET
(Q14). If the MOSFET is shorted, replace it and perform the Control
Circuit Check.
d. Using the digital multimeter diode test function, test CR32, CR33, CR34,
and CR35 for shorts. Diode replacement parts must be fast reverse
recovery (300 ns) types to reduce conducted noise.
e. Using the digital multimeter diode test function, test the snubber diodes
CR23 and CR25 for shorts.
f. Replace jumper P7.