ESAB Heliarc 250 Instruction Manual page 15

SECTION 4
When replacing defective diodes, make sure mount-
ing surfaces are clean. Coat mounting surfaces
with Dow-Corning No. 340 silicon heat sink com-
pound, or equivalent. Replacement diode (nuts)
should be tightened only until firm, and then torqued
(recommended range is 275 inch-lbs min. to 325
inch-lbs max.).
2. Silicon Controlled Rectifier - SCR.
Disconnect the SCR wiring (but do not unclamp) to
break continuity and provide an open circuit across
the component to be tested. Using an ohmmeter
set to the R x 1 scale, check the resistance across
the SCR in both directions. A good SCR will read
high in both directions. If the reading is low or zero
in either direction, the SCR is defective.
When replacing defective SCRs, make sure the
mounting surfaces are clean. Coat the mounting
surfaces with Alcoa No. 2 electrical joint com-
pound, available in 8 oz. containers under P/N
73585002. Make certain the polarity on the re-
placement SCR is the same as on the unit being
replaced. Place the top clamp piece over the bolts
and tighten each nut hard finger tight. The clamp
piece should be parallel to the top plate. Next,
tighten each nut approximately 1/4 turn at a time
(alternately) for two complete revolutions until the
force indicator on the clamp assembly reads 1.0
kilo pounds (1000 lbs).
4-2
MAINTENANCE
15