Note: Before disassembling the existing
rectifier, note which heat sink the outer
metal ring of the power SCR is mounted
towards. Also, note the positioning of
the gate lead of the SCR. Failure to
reinstall the new SCR in the same orien-
tation as the original may result in sub-
sequent damage to the new SCR and
other components of the welder. See
Figure 20.

Fig. F.20 -SCR

The unclamping and clamping proce-
dure outlined below is critical for the
prevention of internal SCR damage.
Failure to follow this procedure may
result in subsequent damage of the
SCR. Handle all SCRs with care.

__________________________

Alternately loosen nuts 1/2 turn
each until heat sinks are loose.
Remove nuts and leaf spring. IT IS
RECOMMENDED THAT

NEW

HARDWARE, LEAF SPRING AND
HOUSING BE USED FOR
REASSEMBLY.

Remove the old SCR.

Clean the area on the heat sink
around the SCR mounting surface.
Using a putty knife, or similar tool.
DO NOT SCRATCH THE SCR
MOUNTING SURFACE.

Polish each heat sink's mounting
surface using NO. 000 fine steel
wool. Wipe surface clean with a lint
free cloth or paper towel.

Inspect the mounting surfaces of
each new SCR.

Remove all burrs and wipe
clean. Do not use steel wool or
any abrasive cleanser on the
SCR mounting surfaces.

Apply a thin (0.001" to 0.003") layer
of PENETROX A-13 (L.E. CO.
#E2529) or PENETROX A, heat sink
compound, to each heat sink's SCR
mounting surface.

Care must be used to prevent
foreign material contamination
of the SCR to heat sink junction.

Place the new SCR between the
heat sinks. Be sure that the outer
metal ring of the SCR is facing
towards the same heat sink as the
old SCR's metal ring. Be sure that
the roll pin of the heat sink engages
the "hole" in the SCR. The SCR con-
tact surfaces must sit flat against
both heat sink surfaces.

SCR REMOVAL AND REPLACEMENT (CONTINUED)

OUTER METAL

RING

ANODE

GATE

CATHODE

CAUTION

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