Tweco Thermal Arc TA 500 - Manual (Page 44)

Service 4-10 Manual No 0-0470
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L. With the control bridge connected to the circuit, mea-
sure the resistance across the diode on the center
heat sink in both directions. It should read about 5
ohms both ways.
M. The coil of relay CSR should read about 3K ohms
measured from pin A to pin B on bottoms of relay.
N. The easiest way to check the PC Board is to replace
it with one known to be good.
O. With a soldering pencil, disconnect the ‘pigtail’
lead to the SCRs one at a time. When more than
one wire is connected to the pigtail, the two
wires should be connected to each other. Oper-
ate the unit with one and then the other SCR
disconnected and note the maximum amperage.
The output will remain unchanged when the
bad SCR is disconnected and will drop to mini-
mum when the good SCR is disconnected.
P. Transformer T1 in the Standoff Control electronics
chassis can be checked by placing the probe of a
voltmeter in pins J23-18 and J23-20 of the PC board
connector. This voltage should read close to 25 VAC.
Q. A bad varistor (VR) is difficult to detect by itself.
However, it will cause the D16 diode on the control
bridge to short and burn out. If this happens or if the
varistor looks burnt, replace both.
1. An in-rush current surge is the most probable
cause of main diode failure. The in-rush surge is
prevented by capacitor C4 and resistor R6,10 in
series with each other across the output of the
bridge rectifier. The capacity and resistor, as well
as the wires connecting them to the circuit, should
be completely checked in any case of diode prob-
lems. The capacitor is a polarized capacitor, and
it is important to be sure that the side marked + is
connected to the positive side of the circuit.
2. High frequency protection for the diodes is pro-
vided by capacitors C5, C6 and C7 which are
installed between each side of the bridge recti-
fier. These capacitors and their connections
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 25 foot pounds
(34 Nm) and that electrically conductive 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 be sure that the
air passages into and out of the unit are not ob-
structed.
4. The diode that was faulty at the time of manufac-
ture 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.
This concludes the trouble-shooting procedures.