Procedure for Replacing P.C. Board

D-8	TROUBLESHOOTING	D-8

TROUBLESHOOTING

PROCEDURES

(The P.C. Board is located behind the front control panel. Remove the nameplate screws to loosen the control panel.)

When the P.C. Board is to be replaced, follow this pro- cedure:

Visually inspect P.C. Board in question. Are any of the components damaged? Is a conductor on the back side of the board damaged?

1.If there is no damage to the P.C. Board, insert a new one and see if this remedies the problem. If the problem is remedied, re-insert the old P.C. Board and see if the problem still exists with the old P.C. Board.

a.If the problem does not exist with the old board, check the harness plug and P.C. Board plug for corrosion, contamination, or oversize.

b.Check leads in the harness for loose connec- tions.

2.If there is damage to the P.C. Board, refer to the Troubleshooting Guide.

OUTPUT VOLTAGE

The open circuit voltage of the machine should be 67 to 71 volts and should not vary when the rheostat is varied. If any other condition exists, refer to the Troubleshooting Guide.

OVERLOAD PROTECTION

E500’s have built-in protective thermostats. If the recti- fier or transformer reaches the maximum safe operat- ing temperature because of frequent overload or high room temperature plus overload, the line contactor drops out stopping the welder. The thermostats auto- matically reset and the line contactor pulls in when the temperature reaches a safe operating level.

The power rectifier bridge is also protected against short term, high current overloads generally caused by poor operating techniques. For example, if an arc

gouging carbon or the electrode is allowed to touch – or almost touch – the work for a couple of seconds or more, the overload protection P.C. Board automatical- ly reduces the output to minimum and keeps it there until the overload is removed or the machine is turned off.

CHECKING SNUBBER CIRCUIT

In case of an SCR malfunction or failure, the snubber assembly should be checked. Turn the machine off and disconnect one lead of the snubber assembly. (Either 221, 222, or 223 depending on the SCR in question. See wiring diagram.) The sides of the machine have to be removed to do this. (See parts list for the exact location.)

1.Visually inspect the snubber assembly for overheat- ed components.

2.Using a V.O.M meter on the X10 scale connect the positive lead to the lead removed. Touch the nega- tive lead to the shunt. The indicating needle on the meter will move quickly to the right (low resistance value) and then slowly return to the left (high resis- tance value). This indicates that the capacitor in the snubber circuit is taking a charge.

3.If the needle stays to the right, the capacitor is shorted and the assembly is defective.

4.If the needle does not move, the capacitor or resis- tor on the snubber assembly is open and the assembly is defective.

CHECKING CURRENT CONTROL RHEOSTAT ON MACHINE

Turn the machine off.

Remove the control panel screws and open the front cover.

Turn the current control switch to remote.

Disconnect the harness plug from the control board.

Put current range switch to “B” range.

With an ohmmeter on X1K, connect it to lead 210 and 211 on SW #2.

Rotate the current control rheostat. The resistance reading should be from around zero to 10K ohms.

E500