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
(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):
(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
(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:
960 Hz + 5v p-p
15.36 KHz + 5v p-p
60 Hz + 5v p-p
120 Hz + 5v p-p