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PRECISION TIG 185
TROUBLESHOOTING & REPAIR
F-27 F-27
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6. Connect SCR into the test circuit as
shown (A) lead to anode (C) lead to
cathode and (G) lead to the gate.
7. Close switch SW-1 (switch SW-2
should open), voltmeter should read
zero. If the voltmeter reads higher
than zero the SCR is shorted.
8. With switch SW-1 closed, close
switch SW-2 for two seconds and
release. The voltmeter should read 3
to 6 volts before and after switch SW-
2 is released. If the voltmeter does not
read, or reads only while SW-2 is
depressed, the SCR or battery is
defective (repeat Battery Test
Procedure above).
9. Open switch SW-1, disconnect the
gate lead (G) and reverse the (A) and
(C) leads on the SCR. Close switch
SW-1. The voltmeter should read
zero. If the voltage is higher than
zero, the SCR is shorted.
10. Replace SCR Bridge Assembly if
any SCRs do not pass the test in
Step #3. See SCR Bridge
Assembly Removal and
Replacement.
11. Replace plug J2 into the control
board.
12. Replace the case wrap-around
cover.
ACTIVE SCR TEST (continued)
SCR1 ANODE
SCR1 CATHODE/SCR2 ANODE
SCR2 CATHODE
SCR3 CATHODE
SCR4 ANODE
SCR3 ANODE/SCR4 CATHODE
FIGURE F.6 SCR GATE LOCATIONS
3
4
2
1
SCR1
SCR4SCR3
SCR2
S24018-4 (INSERTION SIDE)
PLUG J2
SCR GATE LEADS/TEST POINTS