Low Voltage Circuit - Chamberlain LiftMaster Professional BG770 Installation And Maintenance Manual

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Troubleshooting
3C.
If 3a and 3b check out O.K., then manually disconnect the operator from the gate. Very carefully,
using a screwdriver with an insulated handle, press down on the open side of the contactor. Then
do the same to the close side of the contactor. Did the operator run in both directions? If it did,
the problem may be in the low voltage control circuit. It if did not, then the problem is either in the
contactor or the motor.
3D.
If the contactor is suspected to be causing the problem, first carefully check all wiring connections
at the contacts (see Figure 15). Disconnect Power! Using a V.O.M., take continuity readings
across the contacts of the contactor. Remove wires from one side of the contactor. Place one
probe on 1 and the other on 2. You should get NO continuity; now press down on the contactor;
you should get a continuity reading. Repeat this on all of the contactor's contact points.
3E.
If the problem is thought to be the motor, it is recommended that it be replaced. It is possible that
the thermal overload inside the motor has overheated. Wait approximately 15 minutes, then try
running unit.
NOTE: Some motors have the overload built into the motor itself, while other units have a
separate overload in the controller (Model BG770 uses a manual reset overload).

4. Low Voltage Circuit

4A.
The first thing to check is the circuit breaker.
4B.
The secondary voltage must be between 22 and 30 vac. This voltage can be checked at the
circuit board at terminals 3 & 6.
4C.
The contactor coils receive 24vac. To activate the motor in either the open or close direction.
There are two contactor coils (one for open and one for close).
4D.
The limit switches are S.P.D.T. (single pole, double throw). These limit switches tell the operator
to shut off at either the full open or full close position.
Doc 01-G0674
Rev D