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4.5
Cooling
Keep air passages in heat sinks and transformer free of dust build up and corrosive deposits.
4.5.1 Power Components
All accumulated dust, dirt, and particles of foreign matter should be removed at each periodic
inspection. A soft bristled brush should be used to clean off the power heatsinks, SCR’s diodes and
main transformer. The residue can be picked up with a vacuum cleaner.
4.5.2 Replacement of Diodes and Thyristors (SCR's)
All diodes and thyristors are accessible from the rear of the converter. The stud devices are
mounted in self-locking heatsinks so it is only necessary to loosen and remove the stud nut to
remove the device.
Each defective device must be replaced by the same device. In replacing any defective device, a
good thermal compound (Wakefield Type 120) must be used. The stud nut must be installed with
a torque wrench. See
Table 4
for values.
A. Method of Checking Diodes
Any good quality ohmmeter can be used to check if a diode is open or short circuited. The diode
to be checked must be removed from the circuit. A good diode will read a resistance in both
directions, forward (15-50 Ohms) and reverse (100k-10m Ohms). An open diode will indicate
infinite resistance in both directions. A short
circuited diode will have a zero resistance in
both directions. The resistance reading value
of a good diode may vary from one diode to
another. This variance is no indication of the
quality of the diode.
B. Method of Checking Thyristors (SCRs)
In-Circuit Check
1. Disconnect the power supply to the converter.
2. Build test fixture as illustrated in
Figure 19
.
Device
Part Number
Torque in Inch-Pounds
Diode
BN-8--Series CR-38148 (3/4-inch Stud)
275 - 325
Thyristor (SCR)
CR-38119
130 - 150
CR-38120
CR-38109
CR-38103
CR-38110
250 - 300
CR-38104
CR-38105
CR-38106
Table 4
Diode and Thyristor Torque Values
Figure 19
Checking SCR on Test Fixture
Summary of Contents for MACC
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