SWITCH PC BOARD REPAIRS
If a test indicates that one or more FET’s are defec-
tive, both Switch PC boards must be replaced. A
defective FET is likely to damage other devices in the
power circuitry. The defect may be subtle and not
detectable by ohmmeter measurement. Unknown
defective parts may cause newly replaced parts to fail.
In addition to replacing Switch PC boards, replace C1
and C2 if the following conditions are met:
1.Switch PC boards have burned areas.
a)Switch PC Board Replacement Procedure
1) Remove the sheet of mylar insulation.
2) Carefully disconnect the leads at the top of the
Switch PC board.
3) Remove the 4-socket head cap screws with a
3/16” hex key wrench.
4) While holding the Switch PC board, remove the 2
hex head cap screws from the center of Switch
PC board.
5) Remove board.
6) Thoroughly clean the heat sink surface.
7) Apply a thin layer (ideally .002”) of Dow 340 heat
sink compound to the copper bars on the Switch
PC board and also to the 2 capacitor terminals.
Keep heat sink compound off of threads since
this will affect the torque specifications.
8) Carefully place the Switch PC board onto the
heat sink, being sure the 4 mounting holes are
lined up before making full contact with the heat
sink.
9) Carefully holding the Switch PC board, install the
4 socket headcap screws but do not tighten
them. Next, install the 2 hex headcap screws
into the capacitor but do not tighten them.
10) Torque the 4 socket head cap screws to 44” lbs.
and then torque the 2 hex head cap screws to
50-60” lbs.
11) Reconnect the leads to the Switch PC boards
and reinstall the sheet of mylar insulation.
Before applying power to the machine, check all
connections.
An incorrect connection will
result in machine damage when power is
switched on.
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12) Connect an ohmmeter set for X1000 to the hex
head screw terminals on the Switch PC board.
The meter will show the capacitors charging up.
The meter may take a minute or so to stabilize.
The value must not exceed 8000 ohms.If the
meter reads less than 6000 ohms, reverse the
meter leads and repeat the test. If OK, proceed
with power tests.
13) Fuse input supply with 5 amp fuses.
14) Disconnect P19 from J19 at the high frequency
module. Check output open circuit voltages dur-
ing first 1/2 second of output. Do not apply out-
put lead.
15) Fuse for 20 amps and test under load.
Replacement of Power Modules and
Output Diodes
Input rectifier, D9 and A1 and A2 of each Switch PC
board are power modules. When mounting power
modules, the heat sink and module material mounting
surface should be clean and free of burrs and foreign
material. For A1 and A2, apply an even coating of
joint compound to both the heat sink and module
mounting surfaces. The joint compound should be,
ideally, .002” per surface to eliminate all air pockets.
This may be verified by mounting and then removing
the module. When removed, the compound on both
surfaces will appear textured as if a vacuum had cre-
ated veinlike ridges when the parts were separated. If
the compound does not have this appearance, apply
more joint compound and recheck.
Replacement of output diodes D1 and D2 is similar.
Clean and brighten mounting surfaces with fine steel
wool. Compound should be used on surface between
diode and heat sink when mounting individual diodes.
DO NOT apply on diode stud and mounting nut
threads.
– 23 –
ELECTRIC SHOCK CAN KILL.
BEFORE PERFORMING ANY MAINTE-
NANCE THAT REQUIRES OPENING
THE CASE OF THE POWER SOURCE:
• Disconnect input power to this machine at the
Disconnect switch and discharge input capacitors
before working inside machine.
• Do not touch electrically live parts or internal wiring.
• Only qualified personnel should service this
machine.
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WARNING
CAUTION