Sharp R-4A56, Service Manual

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R-4A56
5
FUNCTION OF IMPORTANT COMPONENTS
DOOR OPEN MECHANISM
The door can be opened by pushing the door open button on
the control panel. When the door open button is pushed, the
latch lever is moved upward, operating the latch head. The
latch head is moved upward, and released from the latch
hook. Now, the door can be opened.
MONITOR
SWITCH
UPPER
LATCH
SWITCH
STOP
SWITCH
LOWER
LATCH
SWITCH
LATCH
LEVER
PLUNGER
SWICTH
LATCH
HEAD
DOOR
OPEN
BUTTON
LATCH
HEAD
LATCH
HOOK
DOOR
Figure D-1. Door Open Mechanism
UPPER LATCH SWITCH, LOWER 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 upper and lower latch switches 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 UPPER AND LOWER LATCH
SWITCHES, MONITOR SWITCH AND MONI-
TOR RESISTOR FOR PROPER OPERATION.
(REFER TO CHAPTER “TEST PROCEDURE”.)
MONITOR RESISTOR (0.8
Ω
20W)
The monitor resistor prevents the fuse (M8A) bursting when
the fuse 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 8A 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 power transformer is cut off.
THERMAL CUT-OUT 145˚C (OVEN)
The thermal cut-out located on the top of the oven cavity is
designed to prevent damage to the oven if the foods in the
oven catch fire due to over heating produced by improper
setting of cook time or failure of control unit. Under normal
operation, the oven thermal cut-out remains closed. How-
ever, when abnormally high temperatures are reached
within the oven cavity, the oven thermal cut-out will open at
145˚C, causing the oven to shut down. The thermal cut-out
will be close its contacts again when its temperature goes
down lower than -20˚C.
FUSE (M8A)
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 blows when the asymmetric rectifier, H.V.
rectifier, H.V. wire harness, H.V. capacitor, magnetron or
secondary winding of power transformer is shorted.
3. The fuse also blows when upper and lower latch switches
remains closed with the oven door open and when the
monitor switch closes.
TEMPERATURE FUSE 150˚C (MG)
This fuse protects the magnetron against overheating. If the
temperature goes up higher than 150˚C because the fan
motor is interrupted, the air inlet duct is blocked or the
ventilation openings are obstructed, the fuse blows and puts
the oven out of operation.
The defective fuse must be replaced with new rated one.
D2 D1
ASYMMETRIC
RECTIFIER
HIGH VOLTAGE RECTIFIER
A B
C