Marconi LORAIN RHM800E50 Manual page 46

3.02 Open the equipment access doors or covers and
remove any inner protective grilles or covers to gain
access to the diode or thyristor assembly. Proceed as
follows:
If a thyristor is being replaced, disconnect the
cathode (K) and gate (G) leads from their
terminal board. These leads are usually red and
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white wires, and are connected to a terminal
board mounted on the heatsink near the
thyristor.
Loosen and remove the two mounting clamp
bolts. The clamp, heatsink, and semiconductor
may now be removed from the equipment
NOTE THE
SEMICONDUCTOR BEFORE REMOVING IT
FROM ITS HEATSINK.
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One mounting surface is the anode connection,
whereas the other mounting surface is the
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cathode connection. The polarity of the device is
usually indicated by a symbol stamped on the
ceramic portion of the device. Refer to the
Identification Outlines (Figure 1) if any doubt
exists concerning the polarity of the device. It
may be necessary to remove circuit wiring or
loosen a busbar connection from the heatsink
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before it can be removed from the equipment.
Remove the faulty semiconductor from its
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heatsink. Using a clean rag or paper towel dean
both portions of the heatsink assembly to
remove any thermal compound residue.
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If a thyristor is being replaced, carefully inspect
the gate lead connection at the pin entering the
ceramic body of the thyristor. tt is of utmost
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importance that this connection is tight. lf this
connection point is suspected of being loose,
carefully solder the push-on-connector to the
pin to insure good electrical contact
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Apply a thin coatinq of thermal joint compound
to the mounting surfaces of the replacement
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remicnductor.
POLARITY OF THE FAULTY
WARNING
joint compound on the mamting se-feces of the
replacement davice. The compowtd possesses
device
the semiconductor
Install the replacement device on the heatsink,
observing correct polarity and insuring it is
correctly centered on the pins imbadded in the
and secure them finger-tight only.
Using a force gauge (LP. Part No. 6774-007-W)
placed against the damp springs as shown in
Figure 2, tighten the mounting clamp bolts
alternately, l/4 turn at a time. The bolts should
be tightened equally until the force gauge
indicates rated force is applied as shown in
Figure 2 (B).
excessive force is applied by overtightening the
Zf
mounting clomp bolts, QS
gauge indication in Figure 2 fC), completely
loosen the two bolts and repeat the tightening
process. Never attempt to adjust spring tension
by backing off the bolt.9 since the force gauge
may ptovide a indication in this case.
false
When the mounting clamp bolts are correctly
tightened, remove the force gauge. Reconnect
any wiring removed from the heatsinks, insure
that all hardware used to make electrical
connections is tightened securely.
If replacing a thyristor. reconnect gate and
cathode leads to their respective terminals.
Refer to the equipment Wiring Diagram
(T-drawing) for details.
WARNING
device may re.uIt when power is appZied.
Section 2966
from functioning
shown by the force
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