Maintenance
6-14 1705A Spectrum Monitor
CAUTION. Do not proceed until the drain of Q9 is near 0 V. Dangerousvoltage
potentials arepresent in the circuit until the capacitors discharge.
2. Rectifier/SwitcherCheck
a. Use the digital multimeter to measurethe voltage between TP2 and the
tab (drain) of Q9. Be sure the voltage is near 0 V beforeproceedi ng.
b. Unsolder and lift one end of R102.
c. Withthe negative lead of the digital multimeter connected to TP2 and
the positive lead connectedto the tab of Q9, measurethe circuit
resistance. A resistance of less than 20 kΩindicates a shorted mosfet
(Q9). Ifthe mosfet is shorted, replace it and perform the Control Circuit
Check.
d. Using the digital multimeterdiode test function, test CR21, CR22,
CR23, and CR24 for short s. Diode replacements must be fast reverse
recovery(300 ns) types to reduce conducted noise.
e. Reconnect thelifted end of R102.
3. Output Check
a. Connect the negativeoutput from the 20 Vdc Power Supply to TP1.
Connect the positive output to W4(+15 V). The circuit should draw less
than 20 mA. Excessive current draw can be caused by CR11 or U2
(High VoltsPower Supply).
b. Connect the negativeoutput from the 20 Vdc Power Supply to TP1.
Connect the positive output to W2(+40 V). The circuit should draw less
than 20 mA. Excessive current draw can be caused by CR14 or Q6
(High VoltsPower Supply).
c. Connect the positive output from the 20 Vdc Power Supply to TP1.
Connect the negativeoutput to W3 (--15 V). Thecircuit should draw
less than 20 mA. Excessive current draw can be caused by CR12 .
d. Connect the negative output from the 5 Vdc Power Supply to TP1.
Connect the positive output to W1(+5 V). The circuit should draw less
than 20 mA. Excessive current draw can be caused by CR13 or Q1 and
Q2 (High VoltsPower Supply).
4. Control Circuit Check
a. Connect the negativeoutput from the 20 Vdc Power Supply to TP2.
Connect the positive output to the cathode of CR17. Short C47 with a
clip lead. Connectthe oscilloscope probe ground to TP2.