Sharp R-771 OE, Service Manual

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CHAPTER II
OPERATION
COOKING SEQUENCE
To start cooking:
1.
2.
3.
4.
Place the food to be cooked in the oven and close the
door firmly.
Set the cooking control selector for a desired position,
Set the timer for a desired cooking time. The oven
lamp will come on and the fan motor will rotate im-
mediately.
Push the cook button. The cook lamp will glow to
indicate the oven is in the cook cycle, and the turn-
table motor, timer motor and vari-motor will operate.
5.
Note: If the oven door is opened during the cook
cycle, the oven will go out of the cook cycle (no
microwave energy) and the timer will stop. This is
done so the exact cooking time remains on the timer.
To restore cooking operation, the door must be closed
and the cook button must be pushed again.
At the end of the cook cycle (timer returned to “O”),
cook lamp will go out and the timer bell will ring. The
oven will stop supplying the microwave energy.
DESCRIPTION OF OPERATING SEQUENCE
The following is a complete description of component
during the three stages of oven operation.
l-6. The negative 4000 volts D.C. is applied to the
cathode of the magnetron tube. This causes it to
I
OFF CONDITION
With the tinier dial at “0” position and the door closed, no
components in the oven will operate; (Figure 1). But
the oven lamp and fan motor will turn on with the door
opened.
IDLE CONDITION
Closing the door activates both door interlock switches;
upper latch switch and lower latch switch.
Selecting a
cooking time closes the timer contacts leading to the
oven lamp, fan motor and lower latch switch, therefore
the oven lamp and fan motor turn on; (Figure 2).
COOKING CONDITION
1. HIGH CONDITION
When the cook switch is pushed and the cooking
control selector set to “HIGH” position, the follow-
ing operation occurs; (Figure 3).
l-l. Cook lamp, timer motor, turntable motor and
vari-motor are energized.
1-2. The 240 volts AC. is supplied to a primary wind-
ing of the power transformer through the
switch.
1-3. The power transformer converts 3.4 volts A.C.
output on the filament winding and approximately
2,000 volts on the high voltage winding.
14. The 3.4 volts A.C. output from filament winding
heats the magnetron
l-5. The 2,000 volts output from the secondary
winding is sent to a voltage doubler circuit con-
sisting of a high voltage capacitor and a rectifier.
The 2,000 volts A.C. is-converted to approximate-
ly 4,000 negative D.C. voltage (peak to peak) by
the voltage doubler circuit and sent to the magne-
tron assembly.
oscillate and produce a 2450 MHz cooking fre-
.
1-7. The RF energy produced by the magnetron tube
is channeled through a waveguide into the cavity
and then into the oven cavity where the
food is placed to be cooked.
l-8. Upon completion of the selected cooking time,
the timer bell rings once. And the timer switch is
deactivated. The oven reverts to the off condition.
In the case upper latch switch fails with its contacts
closed when the door is opened, the monitor switch
will form a short circuit through the fuse and the upper
latch switch.
2. MED. LOW (DEFROST), MED., MED. HIGH and
LOW CONDITION
When the cook switch is pushed and the cooking
control selector is set to “MED. LOW”, “MED.“,
“MED. HIGH”, or “LOW”, the following operation
occurs.
2-l. Above l-1 operation occurs.
2-2. The vari-motor rotates at two (2) revolutions per
minute. -In case the cooking control selector is set
to “MED. LOW”, the 240 volts A.C. is supplied to
the power transformer intermittently through the
vari-switch which is operated, at approx. 9.8
seconds ON and 20.2 seconds OFF repetition rate
by means of a cam linked to the vari-motor shaft.
The repetition rate for others are as follows;
LOW: approx. 5 seconds ON, 25 seconds
OFF
MED. : approx. 16.5 seconds ON, 13.5
seconds OFF
MED. HIGH:
approx. 23.2 seconds ON, 6.8
seconds OFF
2-3. The power transformer converts while the 240
volts is supplied to the primary winding.
2-4. Then above 1-3, 1-4, l-5, 1-6, l-7 and l-8 opera-
tions occur.
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