Lincoln Electric PRECISION TIG SVM186-A Service Manual page 67

F-27
SCR1 CATHODE/SCR2 ANODE
TEST PROCEDURE (continued)
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
TROUBLESHOOTING AND REPAIR
ACTIVE SCR TEST (continued)
FIGURE F.6 – SCR GATE LOCATIONS
SCR1 ANODE
SCR3 ANODE/SCR4 CATHODE
PRECISION TIG® 225
SCR2 CATHODE
SCR3 CATHODE
SCR4 ANODE
SCR GATE LEADS/TEST POINTS
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
Replacement.
11. Replace plug J2 into the control board.
12. Replace the case wrap-around cover.
F-27
PLUG J7
3 4
SCR3 SCR4
SCR1 SCR2
1 2
S24018-4 (INSERTION SIDE)
Removal and