Lincoln Electric SQUARE WAVE TIG 355 Service Manual page 110

F-58
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.
Refer to Figure F.17.
6. Place the polarity switch (S1) in a "DC" position.
7. Remove the red insulating paint from heat sink
test points. See Figure F.18. DO NOT DISAS-
SEMBLE THE HEAT SINKS.
TROUBLESHOOTING AND REPAIR
STATIC SCR TEST (continued)
FIGURE F.17 – POWER PC BOARD PLUG LOCATIONS
12
6
SQUARE WAVE TIG 355
11 10 9 8
7 10 9 8
J2
J4
5 4 3
5 4 3 2 1
1 2 3
J5
5 6 7
8. With the 1/2" wrench, remove the choke lead
from the negative plate (top).
9. With the analog ohmmeter, test the resistance
from anode to cathode of SCR1. Reverse the
meter leads and check from cathode to anode
of SCR1.
A. If a low resistance is indicated in either
direction, SCR1 is faulty.
SCR.
10. Repeat Step 9 testing SCR2, SCR3, and
SCR4.
To further check SCR functions, use an SCR
tester and proceed to the Active SCR test.
4 3
7 6
J6
2 1
2 1
4
8
SQUARE WAVE POWER
Replace the
F-58
J1