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F-26

TROUBLESHOOTING & REPAIR

F-26

ACTIVE SCR RECTIFIER ASSEMBLY TEST (continued)

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9.Connect the tester to the SCR 1 as shown in Figure F.8.

a.Connect tester lead (A) to the anode.

b.Connect tester lead (C) to the cathode.

c.Connect tester lead (G) to the gate.

10.Close switch SW-1.

NOTE: Switch SW-2 should be open.

11.Read meter for zero voltage.

a.If the voltage reading is higher than zero, the SCR is shorted.

12.Close or keep closed switch SW-1.

13.Close switch SW-2 for 2 seconds and release and read meter.

a.If the voltage is 3 to 6 volts while the switch is closed and after the switch is open, the SCR is functioning.

b.If the voltage is 3 to 6 volts only when the switch is closed or there is no voltage when the switch is closed, the SCR is defective.

NOTE: Be sure battery is functioning prop- erly. A low battery can affect the results of the test. Repeat Battery Test Procedure in Step 6 if needed.

15.Reconnect the tester leads. See Figure F.8.

a.Connect tester lead (A) to the cathode.

b.Connect tester lead (C) to the anode.

c.Disconnect test lead (G) from the gate.

16.Close switch SW-1.

17.Read meter for zero voltage.

a.If the voltage is zero, the SCR is functioning.

b.If the voltage is higher than zero, the SCR is shorted.

18.Perform the Active Test Procedure out- lined in Steps 6-15 for SCR 2.

19.Replace all SCR assemblies that do not pass the above tests.

20.Reconnect plug J6 onto the control PC board and J9 to the snubber PC board.

21.Reconnect leads X2 and X3 to the neg- ative capacitor bank bus bar.

22.Replace the tool tray and case sides.

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14. Open switch SW-1.

POWER MIG 255

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Lincoln Electric SVM144-B service manual Open switch SW-1