Lincoln Electric SQUARE WAVE TIG 355 Service Manual page 116

F-64
FIGURE F.21 – SILICON CONTROLLED RECTIFIER TEST SETUP
To test SCRs construct the circuit outlined above.
Resistor values are plus or minus ten percent. The
voltmeter scale should be low, approximately 0-5 or
0-10 volts DC.
SILICON CONTROLLED
RECTIFIER TEST
(Heat Sink Mounted Units)
To test the SCRs, construct the circuit outlined in
Figure F.21. One 6V lantern battery can be used.
Resistor tolerances are 10%. The voltmeter scale
should be low, approximately 0-5 or 0-10 volts.
BATTERY TEST
Check the battery by shorting leads (A) and (C)
and then close switch SW-1. Replace the battery
if the voltage is less than 4.5 volts.
TROUBLESHOOTING AND REPAIR
ACTIVE SCR TEST (continued)
SQUARE WAVE TIG 355
1. Connect the SCR into the test circuit as shown:
(A) lead to anode, (C) lead to cathode, and (G)
lead to the gate. NOTE: the gate leads are
located in plug J1.
2. Close switch SW-1 (switch SW-2 should be
open). The voltmeter should read zero. If the
voltmeter reads higher than zero, the SCR is
shorted.
3. 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)
4. Open switch SW-1, disconnect the gate lead
(G), and reverse the (A) and (C) leads on the
SCR. Close switch SW-1.
should read zero. If the voltage is higher than
zero, the SCR is shorted.
F-64
The voltmeter