Tweco PAK 10 - Manual (Page 35)

Manual No. 0-0515 23 Service
2. High frequency protection for the diodes is pro-
vided by capacitors C-2, C-3, and C-4 which are
installed between each side of the bridge recti-
fier and ground, and across the output of the
bridge rectifier. These capacitors and their con-
nections should be checked.
3. Overheating of the diode can occur if air flow
over the heat sink is inadequate or if the diode
is not properly fastened to the heat sink. Check
to see that the diodes are torqued to 125 inch
pounds (15 Nm) and that electrically conduc-
tive heat sink compound (this is a white grease)
is present between the diode and heat sink. Also
check for normal operation of the fan and to be
sure that the air passages into and out of the
unit are not obstructed.
4. The diode that was faulty at the time of manu-
facture is difficult to diagnose. This usually fails
during the first few hours of operation. Before
deciding that this was the case, be sure to check
out the other three possibilities.
F. To check the high frequency capacitor, it is neces-
sary to try to start the torch. The spark between the
spark gap points should be bright blue. If the spark
appears to be weak or nonexistent, disconnect the
wire between the spark gap and the capacitor and
try to start the torch again. If the spark is stronger
with this wire disconnected, the high frequency ca-
pacitor is faulty and must be replaced.
G. To check the toroid coil, measure the AC voltage
supplied to relay CSR (measured between pins J1-
14 to J2-15). This voltage is measured with the torch
switch closed (2 seconds after closing the switch)
and should read approximately 40 VAC. If it reads
115 VAC the toroid coil winding(s) is shorted and
the toroid must be replaced.
H. The high frequency transformer has too much volt-
age (6000 volts AC) to check under power. The re-
sistance of the primary coil should be 5 ohms and
the resistance of the secondary coil should be about
20 K ohms.
J. The open circuit voltage can be checked between
the two heat sinks of the main bridge rectifier. This
should be 205 volts DC.
K. The pilot arc resistor is located horizontally at the
back of the unit. It should read 3.5 ohms.
L. To check for DC voltage at the torch leads it is nec-
essary to disconnect the high frequency first to avoid
damage to the voltmeter. The 115 volt primary to
the high frequency transformer is connected through
two push-on connectors at the high frequency trans-
former (near the front panel of the unit). These are
lines 1 and 18, and the high frequency can be dis-
connected by removing one of these wires.
Once the high frequency is disconnected, the torch
switch should be closed. 205 volts DC will imme-
diately be present between the negative lead and
ground. After 2 seconds relay PCR should close and
205 volts can then be measured between the nega-
tive lead and the positive lead. If no voltage is
present check the PCR coil and contacts.
M. The 3 phase AC input to the main bridge rectifier is
150 volts. This can be measured at the top of the
main bridge rectifier where the diode pigtails join
together (lines 30, 31, and 32) at any time that the
DC power light is on.
N. Locate the control bridge near the top of the unit (2
SCR’s and 3 diodes on three aluminum heat sinks,
Item No. 2 & 3, Fig. 5-3). Disconnect the multi-con-
ductor plug next to this component. Using an ohm-
meter with the RX10 scale, all three diodes should
be checked for high resistance in one direction and
low resistance in the other direction. If diode D8
(middle heatsink) is shorted see T below. The SCR’s
should read a high resistance in both directions.
(This is a “quick check” - diodes and SCR’s may be
checked as described in test F.)
O. Remove the current regulating printed circuit card
(Item No. 7, Fig. 5-2) from the machine and mea-
sure the resistance between the 3rd and 7th pins on
the receptacle (counting from the bottom). This
should read 0 ohms with the dial set at “MIN” and
increase smoothly to 5,000 ohms as the dial is ro-
tated to “MAX”.
P. With the control bridge (see N above) connected to
the circuit, measure the resistance across the diode
on the center heat sink in both directions. It should
read about 5 ohms both ways.