R-757M (serv.man6). Operation sequence and function of important components - Sharp Home Appliance Service Manual (repair manual). Page 3

R-757M - 8

CAUTION: BEFORE REPLACING A BLOWN FUSE  F8A,

TEST THE MONITORED LATCH SWITCH
AND  MONITOR SWITCH FOR PROPER
OPERATION.  (REFER TO CHAPTER “TEST
PROCEDURE”).

If the wire harness or electrical components are short-
circuited, this fuse blows to prevent an electric shock of
fire hazard.

This fuse also blows when monitored latch switch remains
closed with the oven door open and when the contact
(COM-NC) of monitor switch closes.
1. If the wire harness or electrical components are short-

circuited, this fuse blows to prevent an electric shock
or fire hazard.

2. The fuse also blows when the asymmetric rectifier,

H.V. rectifier,.H.V. wire harness, H.V. capacitor,
magnetron or secondary winding of high voltage trans-
former is shorted.

This thermal cut-out protects the magnetron against over-
heat. If the temperature goes up higher than 125˚C be-
cause the fan motor is interrupted or the ventilation open-
ings are blocked, the thermal cut-out will open and line
voltage to the high voltage transformer will cut off and
operation of the magnetron will be stopped. The defec-
tive thermal cut-out must be replaced with a new one.

The thermal cut-out located on the top of the oven cavity
is designed to prevent damaged to the oven if the food in
the oven catch fire due to over heating produced by
improper sitting of cooking time or  failure of control unit.
Under normal operation, the oven thermal cut-out remains
closed. However, when abnormally high temperature is
reached within the oven cavity, the oven thermal cut-out
will open at 145˚C, causing the oven to shut down. When
the thermal cut-out cools itself down to the operating
temperature of 115˚C, the contacts of the thermal cut-out
will close again.

The turntable motor rotates the turntable.

The fan motor drives a blade which draws external cool air.
This cool air is directed through the air vanes surrounding
the magnetron and cools the magnetron. This air is chan-
nelled through the oven cavity to remove steam and
vapours given off from heating food. It is then exhausted
through the exhausting air vents of the oven cavity.

The grill heating element is provided to brown the food and
is located on the top of the oven cavity.

The noise filter prevents radio frequency interference that
might flow back in to the power circuit.

The asymmetric rectifier is solid state device that prevents
current flow in both directions. And it prevents the tem-
perature rise of the high voltage transformer by blowing
the fuse F8A when the high voltage rectifier is shorted.

The rated peak reverse voltage of D1 of the asymmetric
rectifier is 6 KV The rated peak reverse voltage of D2 of the
asymmetric rectifier is 1.7 KV. D1 and D2 of the asymmet-
ric rectifier or high voltage rectifier are shorted when the
each peak reverse voltage goes beyond the each rated
peak reverse voltage. (The process of the blowing the fuse
F8A.)
1. The high voltage rectifier is shorted by some fault when

microwave cooking or dual cooking.

2. The peak reverse voltage of D2 of the rectifier goes

beyond the rated peak reverse voltage 1.7 KV in the
voltage doubler circuit.

3. D2 of the rectifier is shorted.
4. The large electric currents flow through the high volt-

age winding of the high voltage transformer.

5. The large electric currents beyond 8A flow through the

primary winding of the high voltage transformer.

6. The fuse F8A blows by the large electric currents.
7. The power supplying to the high voltage transformer is

cut off.