Model 428B
5-69. Where the + 272 V is at fault, try replacing V9, V10,
or V11. If the tubes are not defective, check the associated5-80. Synchronous Detector.
circuitry, especially CR11 (V11 in some instruments).5-81. With TP1 and TP2 still grounded and on the 100
5-70. Where the -7 V has failed, measure -17 Vdc at mA range, perform the test indicated. Beyond a tedious
junction of R102 and C65. If less than -7 volts is measured ohmmeter test of the circuitry between T3 and pin 7 of
V6, little can be done to check the detector circuit. It
at this point, check F3 and C65. If the junction of R102 and should be noted, however, that R46 should be able to
C65 measures more than - 7 Vdc, check CR9 and Q2. provide the + 0.5 Vdc which is required at pin 7 of V6
during normal operation.
5-71. Oscillator - Buffer Amp. Check.
5-72. Short Test Point 1 and Test Point 2 to ground. 5-82. DC Amplifier Check.
5-83. Remove the ground wire from Test Point 1 and
5-73. Compare the waveform at pin 2 of Jl to the waveform Test Point 2 (installed for previous checks). Disconnect
shown at this point of the schematic diagram. If these the probe from the 428B front panel. Set the 428B to the
waveforms are not comparable, check the waveform at 300 mA range. Zero the 428B with the front panel ZERO
Test Point 7 and follow the Troubleshooting Tree. If the control. Measure and note the voltages at pin 7 and pin
waveform at pin 2 of J1 is acceptable, continue to the next 2 of V6. Turn the ZERO control to deliver a ? 1 Vdc
paragraph.
5-74. Set up equipment as described in Paragraph 5-51. swing around the voltage noted for pin 2. The difference
Compare the oscilloscope display to Figure 5-7. between the voltages at pins 7 and 2 of V6 should not
exceed 0.05 Vdc at any point during this swing.
5-75. If the display is unacceptable, clip the probe around 5-84. If the DC Amplifier tracks properly as tested in
100 mAdc and downrange to the 1 mA range. Find a 2.5 V Paragraph 5-83, proceed with the performance checks.
pk-pk sine wave at the junction of R1 and L5. If this If the DC Amplifier has failed, continue to perform the
waveform is unacceptable, adjust or repair the input tuning tests in the Troubleshooting Tree.
circuit (L5, C1, C2, and R1).
5-76. If the waveform described in Paragraph 5-75 is 5-85. CALIBRATION.
5-86. Zero the 428B on, the 1 mA range using the front
alright, use the same test setup to find a 2.5 V pk-pk sine panel ZERO control. Switch to the 100 mA range. Clip
wave at pin 7 of VI. If this waveform is unacceptable, check the probe around an accurate 100 mA current (as in
VI and the range switch. Paragraph 5-14). Adjust Meter Cal, R69, for a meter
5-77. If the waveform described in Paragraph 5-76 is reading of 100 mA. Connect a 1000 ohm ? 1 % resistor
alright, use the same test setup to look for a 1 V pk-pk sine in parallel with an accurate (0.25%) voltmeter to the
wave at pin 7 of V2. Follow this branch of the
Troubleshooting Tree to its conclusion. output jack of the 428B. Turn the front panel Output
Level control full counterclockwise past the detent.
5-78. Detector Gate Amp. Check. Adjust the Output Cal control, R63, for a reading of
5-79. No test equipment need be set up for these checks exactly 0.73 V on the voltmeter.
except an oscilloscope and a wire to ground TP1 and TP2. 5-87. Perform the entire In-Cabinet Performance Tests
(Paragraphs 5-5 through 5-24) to insure that the 428B
meets all of its specifications.
21 |
ACEC
HEWLETT-PACKARD
