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Maintenance
6–8
1740A/1750A/1760 – Series Service Manual
CAUTION.
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 the plug jumper P7.
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).
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Summary of Contents for 1740A Series
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