SERVICE TROUBLESHOOTING
4-16
Manual 0-2728
2. Measure for 115 VAC from J2-2 to J17-3 at the Logic
PC Board.
If voltage is not correct replace Logic PC Board.
3. Check for 115VAC from wire #64 to wire #65 at the
secondary gas Solenoid.
If approximately 115VAC is present, replace Solenoid.
If no voltage is present, proceed to step 4.
4. Measure for 115VAC from J17-7 to J2-6 at the Logic
Board.
If the voltage is not correct, replace the Logic Board.
J. Pilot Arc Circuit Test
NOTE
Refer to the Appendix Pages for PC Board Lay-
outs.
Check across E23 to E24 at the Pilot Output PC Board.
There should be a diode drop across E23 to E24 in one
direction and an open in the other.
If shorted remove the wires at E23 and E24 at the
Pilot Output PC Board and check again.
Replace Pilot Output PC Board if shorted from E23
to E24 (with wires removed).
K. FET/Heatsink Circuit Tests
NOTE
Refer to the Appendix Pages for PC Board Lay-
outs.
The FET/Heatsink circuit tests require various checks to
isolate the possible fault to the FET/Heatsink Assembly.
There are several internal indicators (LED's) on the Logic
PC Board, Pilot/Output PC Board and the Gate Drive
PC Board to help in troubleshooting. They are as fol-
lows:
NOTES
The upper FET Assemblies provide the power for
piloting and the main cutting arc. The lower FET
Assemblies are for main cutting arc only.
The upper Gate Drive Board controls the upper
two FET/Heatsink Assemblies and the lower Gate
Drive Board controls the bottom two FET/Heat-
sink Assemblies.
1. No DC Output
An open circuit voltage of approximately 280 to 325
vdc (depending on input power selected) is produced
when switching transistors in the FET/Heatsink As-
semblies are turned ON by a PWM Enable signal from
the Logic PC Board. A circuit on the Logic PC Board
monitors the output voltage. When the output volt-
age drops below 60 vdc, indicating a problem exists,
the Logic PC Board sends a signal which turns OFF
the PWM Enable signal to the Gate Drive PC Board.
Because this happens in less than 50 milliseconds, it
is not easy to take voltage readings to find the source
of the problem.
When the unit is at “idle” the AC OK indicator, D47,
should be ON. The Torch Switch Enable indicator,
D26, turns ON when the torch switch is pressed. At
this point the gas begins to flow. When the preflow
time is over the PWM Enable signal is given and the
DC indicator at the front panel turns ON.
If the front panel DC Indicator turns ON then OFF
immediately, the following test should be performed:
a. Disconnect J11 from the CD PC Board to dis-
able the CD signal.
b. Connect a jumper between TP1 and TP8 on
Logic PC Board. This will cause the gas to flow
continuously and the DC Indicator on the front
panel to turn ON.
WARNING
Connector J11 on the CD PC Board must be dis-
connected to prevent electrical damage to measur-
ing equipment when testing the open circuit volt-
age (OCV).
c. Press and hold the hand torch switch (Logic
PC Board Start Indicator, D26, turns ON).
d. Measure open circuit voltage between E8 (+)
to E7 (-) at the Pilot Output PC Board. If volt-
age is low, each FET/Heatsink Assembly
should be tested individually.
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