Tweco 15XC CE Merlin PAK - Manual (Page 106)

SERVICE 4 - 34 Manual 0-2529
K. Switching Transistor
Check (continued)
The PWM compares shunt amp output on J9-5 with the
current control signal (3.3 - 10 vdc) from the remote (J7-18)
or the panel control (J10-15). A faulty shunt amp could
cause the output, normally 0 vdc with no cutting arc, to go
higher, shutting off the PWM and thus producing no DC.
If the shunt amp ribbon connector is disconnected or pin 1
is open, approximately 12 vdc is applied to J9-5, shutting
down the PWM in the same manner as a shunt amp
failure.
The switching control board sends pulses (+15 V) to the
driver board on J8-1 and J8-3. The width of these pulses
controls the on-time of Q1. The pulses are best observed
with an oscilloscope, but an AC voltmeter should read
about 6 - 7 VAC from J8-2 to both J8-1 and J8-3. If 0 VAC
is measured at both J8-1 and J8-3, with the enable (J10-9)
and shunt amp (J9-5) input low, replace the switching
control board.
If switching pulses are present at J8-1 and J8-3, check the
driver PC board output between gate and emitter of Q1.
Both gate and emitter are at -320 vdc potential.Use
extreme caution when testing driver board output.
WARNING
The metric screws provided with the transistor are to be
used for the transistor connections. Small metric screws
are to be torqued to 12 in-lbs (1.4 Nm). Larger (M6) metric
screws are to be torqued to 26 in-lbs (2.9 Nm).
The driver board output should measure about 1 VAC. If
output measures 0 to -4 vdc replace the driver board.
1. If the pilot indicator is lit and no spark is detected at the
spark gap points, check between the wire #10 and J2-4.
If 120 VAC is not present, replace the logic board.
2. If spark is visible at the points, the logic board is operat-
ing properly. Check for a faulty PCR contactor.
3. If the pilot indicator is not lit, check J3-11. If J3-11
measures +15 vdc, replace the logic board.
4. If J3-11 measures low (near 0 vdc), the switching control
board is falsely indicating main arc transfer. If the
shunt amp output (J9-5) is 0 vdc, replace the switching
control board. If shunt amp output does not measure
0 vdc, replace the shunt amp.
L. Pilot Contactor Check
(PCR)