Lincoln Electric SQUARE WAVE TIG 355 Service Manual page 114

F-62
G2081-[ ]
PROCEDURE
1. Remove the input power to the machine.
2. With the 5/16" nutdriver, remove the machine
case top and sides.
3. Perform the Input Power Factor Capacitor
Voltage Check and Discharge Procedure.
4. Remove the output welding cables from the
machine.
5. Remove plug J1 from the power PC board. See
Figure F.19.
6. Place the polarity switch (S1) in a "DC" position.
TROUBLESHOOTING AND REPAIR
ACTIVE SCR TEST (continued)
FIGURE F.19 – POWER BOARD PLUG LOCATIONS
SQUARE WAVE TIG 355
7. Remove the red insulating paint from heat sink
test points. See Figure F.20. DO NOT DIS-
ASSEMBLE THE HEAT SINKS.
8. With the 1/2" wrench, remove the choke lead
from the negative plate (top).
9. Perform the test procedure as outlined in
Figure F.21. Repeat the test for all four
SCRs.
10. Replace any SCR or SCR assembly that does
not pass the test in Step 9.
F-62
SQUARE WAVE POWER