Section 4 - Maintenance & Troubleshooting; Overload Protection - Miller Gold Seal Model 420 Owner's Manual

SECTION 4 – MAINTENANCE & TROUBLESHOOTING
WARNING
ELECTRIC SHOCK can kill.
Do not touch live electrical parts.
Turn Off welding power source, and disconnect
input power before inspecting, maintaining, or
servicing.
HOT PARTS can cause severe burns.
Allow cooling period before maintaining or servicing.
Time
Before each use.
After each spool of wire.
Every 3 months.
Every 6 months.

4-1. Overload Protection

WARNING
A. Overheating
Thermostat TP1 protects the unit from damage due to overheating. If main transformer gets too hot, TP1 opens and
weld output stops, but fans keep running to cool transformer. Wait several minutes before trying to weld.
B. Motor Fuse F1
CAUTION
STATIC ELECTRICITY can damage parts on circuit boards.
Put on grounded wrist strap BEFORE handling boards or parts.
Tools Needed:
Table 4-1. Maintenance Schedule
Tighten all connections. Check gun and clean nozzle.
Blow out gun liner. Clean and check wire drive parts; replace as necessary.
Tape or replace cracked cables; clean and tighten connections. Replace unreadable labels.
Blow out or vacuum inside of unit.
3/8 in
Figure 4-1. Fuse F1 Location
MOVING PARTS can cause injury.
Keep away from moving parts.
Keep away from pinch points such as drive rolls.
Maintenance and troubleshooting to be performed only
by qualified persons.
Maintenance
READ SAFETY BLOCKS at start of
Section 4 before proceeding.
1 2
swarn8.2* 2/93
fwarn5.1* 9/91
Turn Off and unplug unit. Unlatch
door, remove handle, and remove
door/wrapper.
1 Circuit Board PC1
Located behind fan.
2 Fuse F1 (See Parts List For
Rating)
If this fuse opens, the wire drive mo-
tor does not run.
Pull fuse from fuse holder on PC1.
To reinstall, push fuse into fuse
holder.
Reinstall wrapper and handle, and
latch door.
Ref. ST-155 640 / ST-149 327
OM-174 470 Page 6