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.
