R-1900M. Complete - Sharp Home Appliance Service Manual (repair manual). Page 6

R1900M

4 – 2

1. The handle lever is pulled.

2. The upper and lower latch heads are lifted up by the linked latch

lever.

3. The latch lever is lifted up by the handle lever.

4. Now both latch heads are lifted up, so they can be released from

the latch hook.

5. Now the door can be opened.

Figure D-1. Door Open Mechanism

The door sensing switch is activated by the latch head of the door and
switch lever C. When the door is opened, the contacts of the switch
open and interrupt the circuit to the coils of the primary interlock relay.
The contacts of the primary relay then open and interrupt the circuit to
the primary winding of the power transformers.

The secondary interlock switches (1), (2) are activated by the upper
latch head of the door and switch lever B. When the door is opened,
the contacts of the switch open and interrupt the circuit to the primary
winding of the power transformers (1), (2).

The monitor switches (1), (2) are mounted in the upper position of the
latch hook. The monitor switches are activated by the upper latch head
of the door and switch lever A. When the door is opened, the contacts
of the monitor switches close. Monitor switch (1) is intended to render
the oven inoperative by means of blowing the fuse F10, when the con-
tacts of the primary interlock relay and secondary interlock switch (1)
fail to open when the door is opened. Monitor switch (2) is intended to
render the oven inoperative by means of blowing the fuse F10, when
the contacts of the primary interlock relay and secondary interlock
switch (2) fail to open when the door is opened.

With the door shut, the contacts of the door sensing switch and the
secondary interlock switches (1), (2) are closed and the contacts of the
monitor switches (1), (2) are open.

1. When the door is opened, the contact of the door sensing switch

and secondary interlock switches (1), (2) are opened first, then the
contact of the monitor switches (1), (2) are closed.

2. As the door goes to a closed position, the contacts of the monitor

switches (1), (2) are opened first, then the contacts of the door
sensing switch and the secondary interlock switches (1), (2) close.

3. If the door is opened, and the contacts of the primary interlock relay

and secondary interlock switch of the same circuit fail to open, the
fuse F10 blows simultaneously with closing of the monitor switch
contacts of that circuit (1 or 2).

CAUTION: BEFORE REPLACING A BLOWN FUSE F10 TEST THE

DOOR SENSING SWITCH, PRIMARY INTERLOCK
RELAY, SECONDARY INTERLOCK SWITCHES (1), (2),
DOOR SENSING SWITCH AND MONITOR SWITCHES
(1), (2) FOR PROPER OPERATION. (REFER TO CHAP-
TER “TEST PROCEDURE”).

1. The fuse F10 blows when the contacts (COM-NO) of the primary

interlock relay (RY1) and secondary interlock switch remain closed
with the oven door open and when the monitor switch closes.

2. If the wire harness or electrical components are short-circuited, this

fuse F10 blows to prevent an electric shock or fire hazard.

3. The fuse also blows when the asymmetric rectifier, H.V. rectifier,

H.V. wire harness, H.V. capacitor, magnetron or secondary winding
of power transformer is shorted.

The temperature fuses (1), (2) located on the top of the upper and
lower waveguide, are designed to prevent damage to the magnetrons
(1), (2). If an over heated condition develops in the tube due to blower
motor failure, obstructed air ducts, dirty or blocked air intake, etc., the
circuit to the magnetrons are interrupted. Under normal operation, the
temperature fuses remains closed. However, when abnormally high
temperatures are generated within the magnetron, the temperature
fuse will open at 302

°F (150°C) causing the microwave energy to stop.

The defective temperature fuse must be replaced with new rated one.

The temperature fuse, located on the side of the exhaust duct assem-
bly, is designed to prevent damage to the oven by fire. If the food load
is overcooked, by either error in cook time or defect in the control unit,
the temperature fuse will open. Under normal operation, the oven tem-
perature fuse remains closed. However, when abnormally high tem-
peratures are generated within the oven cavity, the oven temperature
fuse will open at 248

°F (120°C), causing the oven to shut down. The

defective temperature fuse must be replaced with new rated one.

The exhaust thermistor is located on the side of the exhaust duct
assembly. The temperature in the exhaust duct is detected through the
resistance of the thermistor. If the temperature is high, the control
panel will display”EE7” and the oven will stop to avoid overheating and
catching fire. If the thermistor is open, the control panel will display
“EE8” and the oven will stop.

The thermistor is a negative temperature coefficient type. The air tem-
perature around the upper and lower magnetrons is detected through
the resistance of the thermistor. If the temperature is high, the control
panel will display “EE7” and the oven will stop to protect the magne-
trons against overheat. If the magnetron thermistor is open, the control
panel will display “EE8” and the oven will stop.

The fan motors drive blades which draw external cool air into the oven.
This cool air is directed through the air vanes surrounding the magne-
trons and cools the magnetrons. This air is channelled through the
oven cavity to remove steam and vapours given off from the heating
foods. It is then exhausted through the exhausting air vents at the
oven cavity. 

Latch hook

Latch
lever

Monitor switch (2)

Secondary
interlock switch (2)

Secondary
interlock switch (1)

Door sensing switch

Monitor switch (1)

Switch lever A

Switch lever B

Switch lever C

Latch
head

Latch
head

Handle
lever