TROUBLESHOOTING & REPAIR
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SQUARE WAVE TIG 275
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ACTIVE SCR TEST
(continued)
FIGURE F.8 – SCR GATE LOCATIONS
SCR4 CATHODE
SCR1 / SCR2 CATHODE
SCR3 CATHODE
SCR2 ANODE
SCR1 ANODE
SCR3 / SCR4 ANODE
DIODE D1
6. Connect SCR into the test circuit as shown in
Figure F.7: (A) lead to anode (C) lead to cath-
ode and (G) lead to the gate.
7. Close switch SW-1 (Switch SW-2 should be
open). The 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 described in
Step 5.)
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 the SCR Bridge Assembly if any
SCRs do not pass the test in Step #3. See
SCR Bridge Assembly Removal and
Replacement.
11. Replace plug J1 on the control board.
12. Replace the case sides and top.
TROUBLESHOOTING & REPAIR
F-32
F-32
SQUARE WAVE TIG 275
