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POWER MIG 255
TROUBLESHOOTING & REPAIR
F-21 F-21
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FIGURE F.5 — SCR 1 TEST POINTS.
9. Test for high or infinite resistance from
the cathode to the anode of SCR 1 by
reversing the meter leads. See Figure
F.5.
a. If a high or infinite resistance is indi-
cated for both tests 6 and 7, the
SCR 1 is not “shorted”.
b. If a low resistance is indicated in
either tests 6 or 7, the SCR is faulty.
Replace the SCR assembly.
10. Repeat steps 6 and 7 to test SCR 2.
11. Reconnect leads X2 and X3 (braided
copper strap).
12. Reconnect plugs J9 and J6.
13. If this test did not identify the problem,
or to further test the SCR, go to the
Active SCR Rectifier Assembly Test.
FIGURE F.4 — LOCATION OF
LEADS X2 AND X3.
6. Disconnect leads X2 and X3 (braided
copper strap) from the negative capaci-
tor bank buss bar using a 1/2 in. open
end wrench. See Figure F.4.
7. Separate leads X2 and X3 from the
negative capacitor bank buss bar. Be
sure there is no electrical contact.
NOTE: DO NOT DISASSEMBLE THE SCR
RECTIFIER HEAT SINK ASSEMBLY.
8. Test for high or infinite resistance from
the anode to the cathode of SCR 1
using an analog ohmmeter. See Figure
F.5.
NEGATIVE
CAPACITOR
BANK BUSS
BAR
LEADS X2 AND X3
CONNECTION
STATIC SCR RECTIFIER ASSEMBLY TEST (continued)