Test of SCR's (1 REC, 2 REC, 5 REC)

TM 10-3930-609-35

testing, disconnect battery, discharge capacitor IC to

When replacing an SCR, apply thermal joint

prevent burning out ohmmeter, and disconnect pigtail of

compound or grease to mounting stud threads to

rectifier to be tested. With the ohmmeter set to the RX1

improve heat sinking (GE Versilube G-350-M or

scale, the resistance in the forward or conducting

equivalent is recommended). Tighten to a snug fit.

direction should be about 7 to 12 ohms. Measured on

the RX10,000 ohm range, the back resistance

2-8. Test of Card 1

(nonconducting direction) should be infinite. Allowing

The oscillator and timer sections of Card 1 may be

for meter and reading errors, replace any diode with

tested with simple test equipment after removal from the

resistance readings which are definitely outside the

panel. Remove card from panel by loosening two

specified values.

When replacing a diode, apply

screws at bottom of box and pulling box straight up to

thermal joint compound or grease to mounting stud

disengage from receptacle (fig. 2-3). Connections for

threads to improve heat sinking (GE Versilube G-350-M

the two tests may be made to card pins with insulated

or equivalent is recommended). Tighten to a snug fit.

clips.

a. Oscillator Section.

2-7. Test of SCR's (1 REC, 2 REC, 5 REC)

(1) Connect the test circuit shown in figure 2-

These are silicon controlled rectifiers. Before checking,

5. Voltmeter should be on 10 or 15 volt range to start.

disconnect battery and discharge capacitor 1C to

(2) The initial voltage reading should be less

prevent burning out ohmmeter. Before checking 1 REC

than 10 volts. Adjust CREEP SPEED trimpot until

remove Card 1 to open gate and disconnect pigtail.

voltmeter reads 0.5 volts.

Before checking 2 REC, disconnect gate lead by

(3)

Remove capacitor across meter.

opening wire 8 to terminal 5 of filter Fl (fig. 2-3), and

Voltmeter should then read less than 0.1 volts.

disconnect the pigtail.

Before checking 5 REC,

(4) Failure to obtain the specified readings

disconnect gate lead by opening wire 12 to terminal 7 of

indicates that Card 1 is defective and should be

filter Fl, and disconnect the power lead to terminal on

replaced.

top of 5 REC. The test procedure for each SCR is the

same, as follows:

a. A 3 volt battery, a 3 volt lamp, and suitable

leads are required for the test (fig. 2-4). An equivalent

continuity test may be used.

b. Connect the circuit or tester as in figure 2-4 for

the first test. The lamp should not light. If the lamp

lights the SCR is shorted and must be replaced.

c. If the SCR does not test shorted, with figure 2-4

circuit still connected, momentarily connect gate (point

2) to pigtail (point 3). The lamp should light and remain

lit when the gate con

Figure 2-5. Test circuit for oscillator section of card 1.

Timer Section.

(1) Connect the test circuit shown in figure 2-

6 with voltmeter set on 50 volt range. The battery

switch should be a momentary of spring return type.

(2) Close switch and observe change in

voltage reading. The voltage should drop to zero in

approximately one second to indicate normal timer

action. Release switch .to open battery circuit as soon

as voltage drops to zero.

Figure 2-4. SCR test circuit.

(3) If the timer operates, but the time required

nection is removed. If this does not occur the gate is

inoperative and the SCR must be replaced.

2-10