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TROUBLESHOOTING & REPAIR
F-48 F-48
COMMANDER 400
ACTIVE SCR TEST (continued)
FIGURE F.21 - SCR TESTER CIRCUIT AND SCR CONNECTIONS
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.
R1 = 4 ohms/10 watts
R2 = 3 ohms/10 watts
6 VOLT
LANTERN
BATTERY
SCR
UNDER
TEST
A
V
R2
R1
SW2
C
G
SW1
+
ANODE
CATHODE
REMOVE
INSULATING
PAINT
10. Construct the circuit shown in Figure F.21.
One 6-volt lantern battery can be used.
Set voltmeter scale low, at approximately
0-5 volts or 0-10 volts.
11. Test the voltage level of the battery. Short
leads (A) and (C). Close switch SW-1.
Battery voltage should be 4.5 volts or high-
er. If lower, replace the battery.
12. Connect the tester to the SCR A as shown
in Figure F.21.
a. Connect tester lead (A) to the anode.
b. Connect tester lead (C) to the cathode.
c. Connect tester lead (G) to the gate.
13. Close switch SW-1.
NOTE: Switch SW-2 should be open.
14. Read meter for zero voltage.
a. If the voltage reading is higher than
zero, the SCR is shorted.
15. Close or keep closed switch SW-1.
16. Close switch SW-2 for 2 seconds. Release
and read meter.
a. If the voltage is 3-6 volts while the
switch is closed and after the switch is
open, the SCR is functioning.
b. If the voltage is 3-6 volts only when the
switch is closed or if there is no voltage
when the switch is closed, the SCR is
defective.
NOTE: Be sure the battery is functioning prop-
erly. A low battery can affect the results of the
test. Repeat Battery Test Procedure in Step 11
if needed.
17. Open switch SW-1.
18. Reconnect the tester leads. See Figure
F.21.
a. Connect tester lead (A) to the cathode.
b. Connect tester lead (C) to the anode.
c. Disconnect test lead (G) from the gate.
19. Close switch SW-1.
20. Read meter for zero voltage.
a. If the voltage is zero, the SCR is func-
tioning.
b. If the voltage is higher than zero, the
SCR is shorted.
21. Perform the Active Test Procedure outlined
in Steps 12-20 for SCRs B and C.
22. Replace all SCR assemblies that do not
pass the above tests.
23. Replace all molex plugs onto the control
board and snubber board. Reconnect lead
#204B resistor R1.
24. Replace the snubber board cover and con-
trol board cover.
25. Replace all case covers. (See the Case
Cover Removal and Replacement pro-
cedure.)