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F-34F-34
PRECISION TIG 275
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TROUBLESHOOTING & REPAIR
STATIC SCR TEST (continued)
TEST PROCEDURE
1. Remove main supply power to the PRECI-
SION TIG 275 and remove the case top and
left side.
2. Locate and remove plug J4 from the control
board. See Figure F.5.
3. Using an analog ohmmeter, test the resis-
tance from anode to cathode of SCR1.
Reverse the meter leads and check from
cathode to anode of SCR1. See Figure F.5. If
a low resistance is indicated in either direc-
tion, SCR1 is faulty. Replace the SCR Bridge
Assembly. See SCR Bridge Assembly
Removal and Replacement.
4. Repeat Step #3 testing SCR2, SCR3 and
SCR4.
5. To further check the SCRʼs functions, use an
SCR tester and proceed to the Active SCR
Test.
NOTE: Also check diode D1.
6. Replace plug J4 on the control board.
7. Replace the case top and left side.
Plug
J4
G3909
Figure F.5 - Plug J4 Location
DIODE D1
SCR4 CATHODE
SCR3 / SCR4 ANODE
SCR3 CATHODE
SCR2 ANODE
SCR1 / SCR2 CATHODE
SCR1 ANODE
LIFT BAIL STABILIZER
Figure F.6 - SCR TEST POINTS