Tektronix WFM601, Service Manual

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Maintenance
WFM 601 - Service Manual
6–9
c. With the negative lead of the digital multimeter connected to TP3 and
the positive lead connected to TP2, measure the circuit resistance. A
resistance of less than 20 k
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 replacements 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 P7.
3. Output Check
a. Connect the negative output from the 20 Vdc Power Supply to TP1 and
its positive output to W3 (+11 V). The circuit should draw less than 20
mA. Excessive current draw can be caused by CR16 or U2 (High Volts
power supply).
NOTE . The variable power supply used in the following checks must have a
current limit of 1 ampere or less.
b. Connect the negative output from the 20 Vdc Power Supply to TP1 and
the positive output to R69 (+40 V). The circuit should draw less than
20 mA. Excessive current draw can be caused by CR19 or Q7 (High
Volts Power Supply).
c. Connect the positive output from the 20 Vdc Power Supply to TP1and
the negative output to W2 (–11 V). The circuit should draw less than
20 mA. Excessive current draw can be caused by CR18.
d. Connect the negative output from the 5 Vdc Power Supply to TP1 and
the positive output to W1 (+5 V). The circuit should draw less than
20 mA. Excessive current draw can be caused by CR17 or Q1 and Q2
(High Volts Power Supply).
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.