Service Manual Part 5
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The checks listed below have been included to provide assistance in locating faults. It is
sometimes convenient to disable a complete section of the power supply in order to iso-
late a fault. If individual sections are isolated the rest of the unit should operate nor-
mally (refer to Figure 8.1.2, “Typical Waveforms” for details).
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Faulty diodes and transistors can generally be found by a simple ohmmeter check, as
follows (an AVO model 8 or equivalent meter should be used for taking the measure-
ments, using only the medium or low resistance ranges):
Set the ohmmeter to the ohms x 1 range.
Measure the forward and reverse resistance of each junction. The resistance in one
direction should be low (generally 30 to 100Ω), and the resistance in the other
direction should be high. In a faulty transistor or diode, the junctions will usually
be either short or open circuited.
Other components in the circuit may affect these readings unless the junctions
are isolated.
The collector current drawn by multijunction transistors is a further guide to their oper-
ating performance.
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The following table shows voltages under normal operating conditions, and those fol-
lowing thermal shutdown:
Normal Operation
(V)
Thermal Shutdown
Operating (V)
output voltage 13.8 0
Q3 base-emitter voltage 0.2 approx. 0.7 approx.
Q4 base-emitter voltage 0.7 approx. 0 approx.
IC1 pin 4 voltage 0 5
IC1 pin 14
(voltage reference)
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