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POWER MIG 255
TROUBLESHOOTING & REPAIR
F-25 F-25
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8. Use a commercial SCR tester or con-
struct the tester circuit shown in Figure
F.8. One 6-volt lantern battery can be
used. R1 and R2 resistor values are
±10%. Set voltmeter scale low, at
approximately 0-5 volts or 5-10 volts.
a. Test the voltage level of the battery.
Short leads (A) and (C). Close
switch SW-1. Battery voltage
should be 4.5 volts or higher. If
lower, replace the battery.
FIGURE F.8 — SCR TESTER CIRCUIT
AND SCR CONNECTIONS.
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.7.
FIGURE F.7 — LOCATION OF
LEADS X2 AND X3.
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.
ACTIVE SCR RECTIFIER ASSEMBLY TEST (continued)
NEGATIVE
CAPACITOR
BANK BUSS
BAR
LEADS X2 AND X3
CONNECTION