
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 associated
circuitry, especially CR11 (V11 in some instruments).
5-70. Where the -7 V has failed, measure -17 Vdc at
junction of R102 and C65. If less than -7 volts is measured
at this point, check F3 and C65. If the junction of R102 and
C65 measures more than - 7 Vdc, check CR9 and Q2.
5-71. Oscillator - Buffer Amp. Check.
5-72. Short Test Point 1 and Test Point 2 to ground.
5-73. Compare the waveform at pin 2 of Jl to the waveform
shown at this point of the schematic diagram. If these
waveforms are not comparable, check the waveform at
Test Point 7 and follow the Troubleshooting Tree. If the
waveform at pin 2 of J1 is acceptable, continue to the next
paragraph.
5-74. Set up equipment as described in Paragraph 5-51.
Compare the oscilloscope display to Figure 5-7.
5-75. If the display is unacceptable, clip the probe around
100 mAdc and downrange to the 1 mA range. Find a 2.5 V
pk-pk sine wave at the junction of R1 and L5. If this
waveform is unacceptable, adjust or repair the input tuning
circuit (L5, C1, C2, and R1).
5-76. If the waveform described in Paragraph 5-75 is
alright, use the same test setup to find a 2.5 V pk-pk sine
wave at pin 7 of VI. If this waveform is unacceptable, check
VI and the range switch.
5-77. If the waveform described in Paragraph 5-76 is
alright, use the same test setup to look for a 1 V pk-pk sine
wave at pin 7 of V2. Follow this branch of the
Troubleshooting Tree to its conclusion.
5-78. Detector Gate Amp. Check.
5-79. No test equipment need be set up for these checks
except an oscilloscope and a wire to ground TP1 and TP2.
5-80. Synchronous Detector.
5-81. With TP1 and TP2 still grounded and on the 100
mA range, perform the test indicated. Beyond a tedious
ohmmeter test of the circuitry between T3 and pin 7 of
V6, little can be done to check the detector circuit. It
should be noted, however, that R46 should be able to
provide the + 0.5 Vdc which is required at pin 7 of V6
during normal operation.
5-82. DC Amplifier Check.
5-83. Remove the ground wire from Test Point 1 and
Test Point 2 (installed for previous checks). Disconnect
the probe from the 428B front panel. Set the 428B to the
300 mA range. Zero the 428B with the front panel ZERO
control. Measure and note the voltages at pin 7 and pin
2 of V6. Turn the ZERO control to deliver a ± 1 Vdc
swing around the voltage noted for pin 2. The difference
between the voltages at pins 7 and 2 of V6 should not
exceed 0.05 Vdc at any point during this swing.
5-84. If the DC Amplifier tracks properly as tested in
Paragraph 5-83, proceed with the performance checks.
If the DC Amplifier has failed, continue to perform the
tests in the Troubleshooting Tree.
5-85. CALIBRATION.
5-86. Zero the 428B on, the 1 mA range using the front
panel ZERO control. Switch to the 100 mA range. Clip
the probe around an accurate 100 mA current (as in
Paragraph 5-14). Adjust Meter Cal, R69, for a meter
reading of 100 mA. Connect a 1000 ohm ± 1 % resistor
in parallel with an accurate (0.25%) voltmeter to the
output jack of the 428B. Turn the front panel Output
Level control full counterclockwise past the detent.
Adjust the Output Cal control, R63, for a reading of
exactly 0.73 V on the voltmeter.
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
Содержание 428B
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Страница 27: ...Model 428B Figure 5 9 Detailed Troubleshooting Tree 20 ...
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Страница 35: ...Model 428B Figure 6 1 Parts Breakdown current probe 28 ...
Страница 37: ...Model 428B Figure 7 1 Block Diagram 30 ...
Страница 39: ...Model 428B Figure 7 5 Power Supply 32 ...
Страница 40: ...Model 428B Figure 7 6 Block Diagram 33 ...
Страница 44: ...Model 428B Figure A 1 428B Side Views 37 ...
Страница 45: ...Model 428B Figure A 2 Backdating Schematics for 428B 38 ...
Страница 46: ...Model 428B Figure A 3 Backdating Schematics for 428B 39 ...
Страница 52: ...Model 428B 00428 90003 PRINTED IN U S A 45 ...