Sharp R-7H55, Service Manual

Page: 9 / 44

R-7H55
7
FUNCTION OF IMPORTANT COMPONENTS
DOOR OPEN MECHANISM
The door is opened by
pushing the open button on
the control panel, refer to
the Figure D-1.
When the open button is
pushed, the open button
pushes up the switch lever,
and then the switch lever
pushes up the latch head.
The latch heads are moved
upward and released from
latch hook. Now the door
will open.
Figure D-1. Door Open Mechanism
1ST. LATCH SWITCH, 2ND. LATCH SWITCH AND
STOP SWITCH
1. When the oven door is closed, the contacts COM-NO)
must be closed.
2. When the oven door is opened, the contacts .(COM-NO)
must be opened.
MONITOR SWITCH
1. When the oven door is closed, the contacts (COM-NC)
must be opened.
2. When the oven door is opened, the contacts (COM-NC)
must be closed.
3. If the oven door is opened and the contacts (COM-NO)
of the 1st. latch switch, 2nd. latch switch and relay RY2
fail to open, the fuse M8A blows simultaneously with
closing the contacts (COM-NC) of the monitor switch.
CAUTION: BEFORE REPLACING A BLOWN FUSE M8A
TEST THE 1ST. LATCH SWITCH, 2ND. LATCH
SWITCH MONITOR SWITCH, RELAYS (RY1
AND RY-2) AND MONITOR RESISTOR FOR
PROPER OPERATION.
MONITOR RESISTOR R 0.8
Ω
20W
The monitor resistor prevents the fuse M8A bursting when
the fuse M8A blows due to the operation of the monitor
switch.
ASYMMETRIC RECTIFIER
The asymmetric rectifier is a solid state device that prevents
current flow in both directions. And it prevents the tempera-
ture rise of the power transformer by blowing the fuse M8A
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 asymmetric
rectifier or high voltage rectifier are shorted when the each
peak reverse voltage goes beyond the each rated peak
reverse voltage. (The process of blowing the fuse M8A.)
1. The high voltage rectifier is shorted by any causes when
microwave 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 voltage
winding of the power transformer.
5. The large electric currents beyond M8A flow through the
primary winding of the power transformer.
6. The fuse M8A blows by the large electric currents.
7. The power supply to the all electrical parts are cut off.
FUSE M8A
1. The fuse M8A blows when the contacts (COM-NO) of the
1st. latch switch 2nd. latch switch and relay RY2 remain
closed with the oven door open and when the monitor
switch closes.
2. The fuse M8A also blows when asymmetric rectifier,
H.V. rectifier, H.V. wire harness, H.V. capacitor, magn-
etron or secondary winding of power transformer is
shorted.
3. If the wire harness or electrical components are short-
circuited, this fuse M8A blows to prevent an electric
shock or fire hazard.
THERMAL CUT-OUT 145˚C (MG)
This thermal cut-out protects the magnetron against over-
heating. If the temperature goes up higher than 145˚C
because the fan motor is interrupted, the ventilation open-
ings are blocked, the thermal cut-out will open and the line
voltage to the power transformer will be cut off and the
operation of the magnetron will be stopped.
The defective thermal cut-out must be replaced with new
one.
THERMAL CUT-OUT 150˚C (CONVECTION)
The thermal cut-out protect the convection motor against
over heating. If its temperature of the convection motor rises
above 150˚C because the convection fan motor is inter-
rupted, the ventilation openings are obstructed or the other
abnormal matter occurs, the thermal cut-out opens and
switch of the oven. When thermal cut-out cools itself down
to 130˚C, the contacts of the thermal cut-out will close again.
THERMAL CUT-OUT 95˚C (FAN MOTOR)
The thermal cut-out protect the fan motor against overheat-
ing. If its temperature goes up higher than 95˚C because the
fan motor is locked or the ventilation operating are blocked,
the contacts of the thermal cut-out will open and switch off
the oven. When the oven cools itself down to 75˚C, the
contacts of the thermal cut-out will close again.
LATCH
HEADS
LATCH HOOK
2ND. LATCH
SWITCH
MONITOR
SWITCH
SWITCH
LEVER
STOP SWITCH
1ST. LATCH
SWITCH
D2 D1
ASYMMETRIC
RECTIFIER
HIGH VOLTAGE RECTIFIER
A B
C