Lincoln Electric SVM 122-A service manual SCR Test

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F-28 F-28

TROUBLESHOOTING & REPAIR

STATIC SCR/DIODE RECTIFIER BRIDGE TEST (continued)

FIGURE F.11 – HEAT SINK AND SCR TEST POINTS

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8.Remove the red insulating paint from the heat sink test points. See Figure F.11.

NOTE: Do not disassemble the heat sink.

9.Measure the resistance from the anode to the cathode of SCR 1 using an analog volt/ohmmeter (multimeter) set at R x 1000 scale. See Figure F.11.

a.Reverse the meter leads and measure the resistance from the cathode to the anode of SCR 1. See Figure F.11.

b.If a low resistance is measured in either meter polarity, SCR1 is faulty. Replace SCR 1.

10.Test the resistance of SCR 2 and SCR 3 using the same procedure described in Step 9.

11.Measure the resistance of diode D1 from anode (+probe) to cathode (-probe) using an analog ohmmeter set at R x 1000 scale. The resistance should be low. See Figure F.11.

a.Reverse the meter leads and measure the resistance from cathode (+probe) to anode (-probe) of diode D1. The resistance should be high. See Figure F.11.

b.If a low resistance is measured in both meter polarities, diode D1 is shorted. Replace diode D1.

c.If a high resistance is measured in both meter polarities, diode D1 is open. Replace diode D1.

12.Test diodes D2, D3 and D4 for proper oper- ation using the same procedure described in Step 11.

13.Reconnect all leads and molex plugs.

14.If this test did not identify the problem or to further test the SCRs, go to the Active

SCR Test.

IDEALARC DC-400

LINCOLN ®

ELECTRIC