DOWNLOAD Sharp R-334 (serv.man4) Service Manual ↓ Size: 529.45 KB | Pages: 40 in PDF or view online for FREE

R-334 - 9

Closing the door activates all door interlock switches (moni-
tored latch switch and stop switch).

IMPORTANT
When the oven door is closed, the monitor switch contacts
COM - NC must be open.
When the microwave oven is plugged in a wall outlet
(220-230V 50Hz), the line voltage is supplied to the point.
Relay COM terminal + A5 in the control unit.

1. The display flashes ”88:88”.
2. To set any programmes or set the clock, you must first

touch the STOP pad.

3. “   :   ” appears in the display and the time counts up every

minute.

NOTE: When the oven door is opened, the oven lamp does

not come on at this time.

HIGH COOKING
Enter a desired cooking time with the ROTARY KNOB and
start the oven with touching START pad.

Oven lamp, Fan motor, Turntable motor

RY1

Power transformer

RY2

1. The line voltage is supplied to the primary winding of the

power transformer. The voltage is converted to about
3.3 volts A.C. output on the filament winding and high
voltage of approximately 2000 volts A.C. on the
secondary winding.

2. The filament winding voltage (3.3 volts) heats the

magnetron filament and the high voltage (2000 volts) is
sent to the voltage doubling circuit, where it is doubled
to negative voltage of approximately 4000 volts D.C..

3. The 2450 MHz microwave energy produced in the

magnetron generates a wave length of 12.24 cm. This
energy is channelled through the waveguide (transport
channel) into the oven cavity, where the food is placed
to be cooked.

4. When the cooking time is up, a single tone is heard and

the relays RY1 + RY2 go back to their home position.
The circuits to the oven lamp, power transformer, fan
motor and turntable motor are cut off.

5. When the door is opened during a cook cycle, the

switches come to the following condition.

NOTE: The ON/OFF time ratio does not exactly correspond

to the percentage of microwave power, because
approx. 3 seconds are needed for heating up the
magnetron filament.

CONDITION

DURING

DOOR OPEN

SWITCH

CONTACT

COOKING

(NO COOKING)

Monitor switch

COM-NC

Open

Closed

COM-NO

Closed

Open

Monitored latch switch

COM-NO

Closed

Open

Stop switch

COM-NO

Closed

Open

The circuits to the power transformer, fan motor and
turntable motor are cut off when the monitored   latch
switch and stop switch are made open.

6. MONITOR SWITCH CIRCUIT

The monitor switch SW2 is mechanically controlled by
oven door, and monitors the operation of the monitored
switch SW1

6-1

When the oven door is opened during or after the cycle
of cooking program, the monitored latch and stop
switches SW1 + SW3 must open their contacts first.
After that the contacts (COM - NC) of the monitor
switch SW2 should close.

6-2

When the oven door is closed, the contacts
(COM - NC) of the monitor switch SW2 must be
opened first and  the contacts (COM - NO) of the
monitored latch switch SW1  and stop switch SW3
must close after.

6-3

When the oven door is opened and the contacts of the
monitored latch switch SW1 remain closed, the
fuse F8A will blow, because the monitor switch is
closed and a short circuit is caused.

When the microwave oven is preset for variable cooking
power, the line voltage is supplied to the power transformer
intermittently within a 32-second time base through the
relay contact which is coupled with the current-limiting relay
RY2. The following levels of microwave power are given.

SETTING

32 sec. ON

24 sec. ON

18 sec. ON

12 sec. ON

6 sec. ON

8 sec. OFF

14 sec. OFF

20 sec. OFF

26 sec. OFF

HIGH

MEDIUM 
HIGH

MEDIUM

MEDIUM 
LOW

LOW

Approx. 70% = 630Watts

Approx. 50% = 450Watts

Approx. 30% = 270Watts

Approx. 10% = 90Watts

100% = 900Watts

R-334 - 10

FUSE F1 F8A 250V
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 monitored latch switch SW1

remains closed with the oven door open and when the
monitor switch SW2 closes.

3. 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.

°

The thermal cut-out protects the magnetron against
overheat. If this temperature goes up higher than 125

°

C

because the fan motor is interrupted, the ventilation openings
are blocked, the thermal cut-out TC1 will open and line
voltages to the power transformer T will be cut off and the
operation of the magnetron MG will be stopped. The defective
thermal cut-out TC1 must be replaced with new one.

°

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
125

°

C, causing the oven to shut down. The defective

thermal cut-out TC2 must be replaced with new one.

TURNTABLE MOTOR
The turntable motor drives the turntable roller assembly to
rotate the turntable.

FAN MOTOR
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 va-
pours given off from the heating foods. It is then exhausted
through the exhausting air vents at the oven cavity.

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

DOOR OPEN MECHANISM
The door can be opened by pulling the door open.

MONITORED LATCH SWITCH SW1
STOP SWITCH SW3
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 SW2
1. When the oven door is closed, the contacts (COM - NC)

must be opened and the contacts (COM - NO) must be
closed.

2. When the oven door is opened,  the contacts (COM -

NC) must be closed and the contacts (COM - NO) must
be open.

3. If the oven door is opened and the contacts (COM - NO)

of the monitored latch switch SW1 fail to open, the fuse
F1 F8A blows simultaneously with closing the contacts
(COM - NC) of the monitor switch SW2.

CAUTION: BEFORE REPLACING A BLOWN FUSE

F1 F8A TEST THE MONITORED LATCH
SWITCH SW1, MONITOR SWITCH SW2
FOR PROPER OPERATION. (REFER TO
CHAPTER ”TEST PROCEDURE”.)

Latch Hooks

Stop Switch
SW3

Monitor Switch
SW2

Monitored Latch
Switch SW1

R-334 - 11

ASYMMETRIC RECTIFIER
The asymmetric rectifier is a solid state device that prevents
current flow in both directions. It prevents the temperature
rise of the power transformer by blowing the special fuse F1
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 F1)
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 F1 blows by the large electric currents.
7. The power supply to the power transformer is cut off.

R-334 - 12

PROBLEM

Home fuse blows when power cord is plugged
into wall outlet.

Fuse F8A blows when the door is opened.
F8A fuse blows when power supply cord is
plugged into wall outlet.
“88:88” does not appear in display when power
cord is plugged into wall outlet.
Display does not operate properly when STOP
pad is touched.
Oven lamp does not light when door is opened.
(Display appears.)

Oven does not start when START pad is touched.
(Display appears.)

Oven lamp does not light. (Display appears.)

Fan motor does not operate. (Display appears.)
Turntable motor assembly does not operate.
(Display appears.)
Oven or any electrical parts do not stop when
cooking time is 0 or STOP pad is touched.
Oven seems to be operating but little or no heat is
produced in oven load. (Microwave power level is
set at ”HIGH”)
Oven does not operate properly during the variable
cooking condition except ”HIGH” cooking condition.
(Oven stops when STOP pad is touched.)
Oven goes into cook cycle but shuts down before
end of cooking cycle.

Oven stops as soon as the START pad is touched.

POSSIBLE CAUSE

AND

DEFECTIVE PARTS

TEST PROCEDURE

CONDITION

OFF
CONDITION

COOKING
CONDITION

A

B

C

D

E

E

E

F

G

H

K

M

A
G
N

E
T
R

O
N

M

O
N

I

T

O
R

E

D

L

A

T
C

H

S

W

I

T
C

H

H
V

.

C
A

P

A
C

I

T

O
R

H
V

.

R
E
C
T

I

F

I

E
R

A

S
S
E

M

B
L
Y

P

O

W

E
R

T
R

A
N

S
F

O
R

M

E
R

F

A
N

M

O

T

O
R

N
O

I

S
E

F

I

L
T
E
R

F

U

S
E

F
8

A

T

H

E
R

M

A

L

C
U
T

|

O
U

T

1
2
5

C

S
T

O

P

S

W

I

T
C

H

M

O
N

I

T

O
R

S

W

I

T
C

H