Failure Isolation and Repair

TM 11-6130-377-14

Symptom

Failure Isolation and Repair

a. A1CB1 trips at turn on.

a. Place an oscilloscope on the collector (case of A2Q11

(A2Q22)). Ground the oscilloscope to C2 negative. Set

the sweep to 50 ms/cm and the sensitivity to 20v/cm.

(Use an X10 probe if necessary.)

(1) With the variable dc source set at 110 vdc, turn

on A1CB1 and check oscilloscope trace. Trace should start

at zero and go to 110 vdc. If oscilloscope trace does not

move upward, A2Q11 (A2Q22) or A2CR5 (A2CR13) is

shorted, or the drive to A2Q11 (A2Q22) is faulty. Turn

OFF A1CB1.

(2) To check drive circuitry to (A2Q22), disconnect

lead to A1CB1-C and separate red connector A1P3 from

A1J3. Turn on A1CB1 and measure voltage at the follow-

ing points (1973 model only):

(A2Q19)

Collector

(A2Q17)

Collector

+5 vdc

(A1Q14)

Collector

(A2Q12)

Collector

+4 vdc

(A2Q8)

Collector

(A2Q6)

Collector

+4 vdc

(3) If the voltages in (2) above are not as indicated,

replace the faulty component.

b. Trip sensor circuits do not operate at the proper voltage

b. Disconnect A1P3 from A1J3. If the low or high voltage

(refer to para 6-23 and 6-24) (1973 model only).

sensor does not operate at the proper voltage, check the

board voltage on A2CR2 cathode (should be 10 to 12 vdc)

and the voltage on A2U1-2 which should be 4.5 to 5.5 vdc.

Check the voltage on A2U1-2 and varying the 100 vdc

input from 104 to 141 vdc. The voltage on A2U1-2 should

stay within 4.5 to 5.5 vdc. If the voltage varies with the

changing input, replace A2U1 and recheck for proper

operation. If the voltage on A2U1-2 stays within 4.5 to

5.5 vdc limits, replace A2U2 if the low sensor is faulty

and A2U7 if the sensor is faulty.

c. The inverter fails to start (operate) after 5 seconds,

c. Disconnect A1P3 from A1J3. Place an oscilloscope on

there is no drive to the SCR Gates.

A1P3-1 and -3 (in turn) with common on A1P3-2. With

A1P3 disconnected from the SCR's, the voltage on A1P3-1

and -3 should be a 60-Hz square wave at 5 volts (peak to

peak).

(1) If this voltage is not present, connect the oscillo-

scope reference to A2C17 (A2C14) negative and measure

the voltage on the collector of A2Q4 and A2Q5 (A2Q21).

This voltage should be a 20v p-p square wave operating

at 60 Hz. If this voltage is present but there is no voltage

on A1P3-1 and -3 check the voltage on A2J1-9 and -3

with reference to A2J1-12. This voltage should be 5v p-p,

60 Hz. If this voltage is present, check the cabling and

connectors between A2J1 and A1P3 and replace the faulty

connector or cable as necessary.

(2) If there is no voltage on A2Q4 (A2Q20) and A2Q5

(A2Q21) collector, measure the voltage on the base of

A2Q4 (A2Q20) and A2Q5 (A2Q21). The base voltage

should be 0.6v p-p at 60 Hz. If this is present, replace

A2Q4 (A2Q20) and A2Q5 (A2Q21).

(3) If this voltage is not present, check the voltage on

A2U4-4 (A2U9-1 and -2) or A2J2-2 and -11. This wave-

shape should be 5v p-p at 60 Hz, square wave. If the

voltage is present, replace A2U4 (A2U9). If it is not

present, check the voltages at the following points:

A2U2

960 Hz + 5v p-p

A2U1

15.36 KHz + 5v p-p

A2U3-11

60 Hz + 5v p-p

(A2U6-8)

120 Hz + 5v p-p

Change 1

6-11