SECTION 5

TROUBLESHOOTING

 

 

 

5.1 GENERAL

DISCONNECT primary power at wall switch, or circuit breaker, before attempting inspection or work inside the power source.

If the power source is operating improperly, the follow- ing troubleshooting information may be used to locate the source of the trouble.

Check the problem against the symptoms in the follow- ing troubleshooting guide (Table 5-2.) The remedy for the problem may be quite simple. If the cause cannot be quickly located, open up the unit and perform a simple visual inspection of all the components and wiring. Check for proper terminal connections, loose or burned wiring or components, blown fuses, bulged or leaking capacitors, or any other sign of damage or discolora- tion.

5.2TESTING AND REPLACING BRIDGE ASSEM- BLY COMPONENTS

The SCRs used in the power source are devices which allow current to flow in only one direction. The SCRs are designed to provide long trouble-free operation; how- ever, should a failure occur, they may require replace- ment.

A.Testing SCRs.

1.Remove top and right side panel from the power source.

2.Locate the main rectifier assembly containing the SCRs.

3.Electrically isolate main bridge assembly by disconnecting resistor R5.

4.With the ohmmeter on RX1 scale, place the positive lead on the anode (end of SCR with screw threads) and the negative lead on the cathode (positive output terminal on the front panel). The meter should read minimum of 5 megohms.

5.Reverse leads and check each SCR. All read- ings should again show high resistance. The SCRs are bad if they show low resistance in either direction.

6.Check the gate circuit on the SCRs by installing a jumper from the gate lead to the anode of the SCR. The meter should read less than 5 ohms.

Remove the jumper from the gate. The meter reading should increase (3050 ohms).

B. Replacing the SCRs.

IMPORTANT

1.When replacing SCR’s, make sure mounting surfaces are clean. Using Alcoa No. 2 EJC Electrical Joint Compound or an equivalent, apply a thin coat to the SCR mounting surface and positively locate in place on the heatsinks. Place the clamp in position with the bolts through the holes in the heatsinks and proceed as fol- lows:

2.Tighten the bolts evenly until finger tight noting that the nuts are not rotating.

3.Tighten the bolts 3/4 turn plus an 1/8 turn using a socket wrench on the bolt heads and rotating only in 1/4 turn increments plus 1/8 turn alternat- ing between the bolts noting that the nuts are not rotating.

5.3PCB VOLTAGE TESTS

Electrical service and repair should be attempted only by a trained electrician.

When making PCB voltage measurements, refer to the schematic diagram. All voltage readings are taken with the front access panel open and the power switch “ON”.

Table 5.1 SCR Voltages (Output)

FROM

TO

READING

 

 

 

P8-5

OTB+

+10 V dc

 

 

 

P8-7

OTB+

0-10 V dc*

 

 

 

P6-6 (SCRI)

 

 

P6-5 (SCR2)

 

 

P6-4 (SCR3)

 

 

P6-3 (SCR4)

OTB+

.3 V dc with

P6-2 (SCR5)

 

contactor on

P6-1 (SCR6)

 

 

 

 

 

* Varies with setting of VCP (R1)

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LG Electronics 453cc Troubleshooting, Testing and Replacing Bridge ASSEM- BLY Components, PCB Voltage Tests, From Reading
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453cc, 553cc specifications

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