HP 428B, Operating And Service Manual, Page: 26 / 52

Model 428B
5-59. TROUBLESHOOTING.
5-60. FRONT PANEL TROUBLESHOOTING.
5-61. The Front Panel Troubleshooting Tree was
designed for use as a means to guide the repairman to
a block of circuitry. This procedure, in combination with
the adjustment, calibration, and performance checks
should be adequate to repair most 428B's.
5-62. In any case where the front panel troubleshooting
does not lead to the problem, it will be necessary to
troubleshoot the instrument in detail.
5-63. DETAILED TROUBLESHOOTING.
5-64. Probe Check.
5-65. Resistance between pins 1-3, 1-4, 2-3, and 2-4 of
the probe cable connector should be nearly identical
and in the area of approximately 4 1/2 to 5 ohms. Where
a resistance measurement between any of the above
pairs of pins is significantly higher than between the other
pairs (by about 3 times), an open probe coil is indicated.
Any open circuit indicates a broken conductor between
the plug and the bridge. The broken conductor is probably
a broken wire in the cable. Either condition could indicate
a broken wire at the Probe Terminal Section (see wiring
diagram, Figure 6-1).
5-66. Power Supply Check.
5-67. To test the power supply:
a. Measure +272 ±6 Vdc between Test Point 5 and
ground.
b. Measure +12 ±1 Vdc between the cathode of CR10
and ground.
c. Measure -7 ±1 Vdc between the anode of CR9 and
ground.
5-68. Where the +272 V and the +12 V have both failed,
check the +12 first. Check CR10, Q1, C66, and C69.
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