Tweco Thermal Arc TA 500 - Manual (Page 43)

Manual No 0-0470 4-9 Service
4.9 Test Procedures
The following tests are suggested for specific problems
listed in the preceding trouble shooting chart. The let-
ter designation corresponds to those listed in the “Rem-
edy” section of the chart.
WARNING
Several of these tests involve voltage measure-
ments that must be made with power on. In or-
der to make these measurements, the Supply
Console cover interlock switch must be propped
closed or bypassed. Use extreme care when mak-
ing these tests and be sure to return the inter-
lock switch to proper operation after work is com-
pleted.
Test requiring voltage measurements are marked with
the warning symbol:
All other tests are to be made with the primary power
to the system turned off.
A. Pressure switches are located in the Supply Con-
sole on the PLASMA GAS line just inside of the
solenoid valve on the inlet fitting and on the
line running from the power manifold to the
NEG connection on the bulkhead board. The
switches are normally open. The pressure at
which the switches will close may be adjusted
by turning the knurled wheel just under the
plastic part of the switch. Turn clockwise to in-
crease the pressure at which switch is actuated.
B. The two time delay relays, TD1 and TD2, are
interchangeable and can normally be checked
by swapping them. They are the “delay on op-
erate” type with a 2 second delay.
C. The relay can be tested by removing it from the
relay socket and with the power on, measure
the voltage between pins A and B on the socket.
The voltage should read close to the voltage
specified for the relay. If it is, then the relay is
bad. If not, then the trouble lies in the circuitry.
D. To check the main contactor remove the left side
panel of the machine. The contactor should
close when 115 volt AC is applied to lines 1 and
80 (when the START button is pressed). At this
time, 220 volt 3 phase power should be present
at the connections from the reactor. (220 volt 3 phase
power should always be present at lines 105, 106 and
107 when the primary power is turned on).
E. To check the toroid coil, measure the resistance from
J400-5 to J400-6. It should be 22 ohms.
F. The Arc Starter transformer (T3) has two dif-
ferent voltage outputs to check to verify its sta-
tus. Using a voltmeter, place the probes be-
tween J400-19 and 20. This voltage should measure
280-330 VAC. Measure the voltage between J400-15
and 17. This should read 10-15VAC. Both voltages
must check out or the transformer is faulty.
G. A “quick check” can be made on the main diodes
without removing them from the circuit as follows:
Using an ohmmeter with the RX1 or RX10 scale,
measure the resistance of each diode in both direc-
tions. The readings should differ by at least a factor
of 10. If they do not differ (both high or both low)
the diode has failed and must be replaced.
H. The 3-phase AC input to the main bridge rectifier is
220 volts. This can be measured at the top of the
main bridge rectifier where the diode pigtails are joined
with the reactor leads (see Fig. 5-8) at any time that
the DC power light is on.
I. The best way to check the P.C. board is to replace it
with one known to be good. The current control PC
board in the PAK 10, 22, 44, and 350 can be used.
J. Locate the control bridge (see Fig. 5-8). Disconnect
the multi-conductor plug next to this component.
Using an ohmmeter with the RX10 scale, all diodes
should be checked for high resistance in one direc-
tion and low resistance in the other direction. The
SCRs should read a high resistance in both direc-
tions. If diode D16 is shorted, see ‘R’ below.
K. Remove the current regulating printed circuit card
(see illustration below) from the power supply. With
the CURRENT CONTROL knob set at MIN measure
the resistance between the 3rd (pin 20) and 7th (pin
16) pins, counting from the top (see below). This
should read 0 ohms and increase smoothly to 10,000
ohms as the CONTROL knob is rotated to MAX.
Now measure the resistance between the 3rd (pin 19)
and 4th (pin 20) pins. This should read 10,000 ohms.