Sharp R820bk Service Manual
Have a look at the manual Sharp R820bk Service Manual online for free. It’s possible to download the document as PDF or print. UserManuals.tech offer 615 Sharp manuals and user’s guides for free. Share the user manual or guide on Facebook, Twitter or Google+.
R-820BK R-820BW In the interest of user-safety the oven should be restored to its original condition and only parts identical to those specified should be used. WARNING TO SERVICE PERSONNEL: Microwave ovens con- tain circuitry capable of producing very high voltage and current, contact with following parts may result in a severe, possibly fatal, electrical shock. (High Voltage Capacitor, High Voltage Power Transformer, Magnetron, High Voltage Recti- fier Assembly, High Voltage Harness etc..) TABLE OF CONTENTS Page PRECAUTIONS TO BE OBSERVED BEFORE AND DURING SERVICING TO AVOID POSSIBLE EXPOSURE TO EXCESSIVE MICROWAVE ENERGY ................... INSIDE FRONT COVER BEFORE SERVICING ...................................................................................................... INSIDE FRONT COVER WARNING TO SERVICE PERSONNEL ................................................................................................................ 1 MICROWAVE MEASUREMENT PROCEDURE ................................................................................................... 2 FOREWORD AND WARNING ............................................................................................................................... 3 PRODUCT SPECIFICATIONS .............................................................................................................................. 4 GENERAL INFORMATION ................................................................................................................................... 4 OPERATION .......................................................................................................................................................... 6 TROUBLESHOOTING GUIDE ............................................................................................................................ 13 TEST PROCEDURE ............................................................................................................................................ 15 TOUCH CONTROL PANEL ................................................................................................................................. 24 COMPONENT REPLACEMENT AND ADJUSTMENT PROCEDURE ................................................................ 30 PICTORIAL DIAGRAM ........................................................................................................................................ 37 POWER UNIT CIRCUIT ...................................................................................................................................... 38 CPU UNIT CIRCUIT ............................................................................................................................................ 39 PRINTED WIRING BOARD ................................................................................................................................. 40 PARTS LIST ........................................................................................................................................................ 41 PACKING AND ACCESSORIES ......................................................................................................................... 43S5812R820BPW/ R-820BK R-820BW DOUBLE GRILL CONVECTION MICROWAVE OVEN MODELS SERVICE MANUAL SHARP CORPORATION This document has been published to be used for after sales service only. The contents are subject to change without notice. CONVECTION 2GRILL COOKHELPCUSTOM HELPIinteractive COMPUPIZZAGRILLBAKEROASTCOOKDEFROSTCONVECGRILLROASTPOPCORNREHEATMINUTE PLUS KITCHEN TIMERSTOPCLEARSTARTTOUCH ONCLOCKPOWERLEVEL 100û F150û F275û F300û F325û F 350û F375û F400û F425û F450û F12345 6789O
R-820BK R-820BW PRECAUTIONS TO BE OBSERVED BEFORE AND DURING SERVICING TO AVOID POSSIBLE EXPOSURE TO EXCESSIVE MICROWAVE ENERGY (a) Do not operate or allow the oven to be operated with the door open. (b) Make the following safety checks on all ovens to be serviced before activating the magnetron or other microwave source, and make repairs as necessary: (1) interlock operation, (2) proper door closing, (3) seal and sealing surfaces (arcing, wear, and other damage), (4) damage to or loosening of hinges and latches, (5) evidence of dropping or abuse. (c) Before turning on microwave power for any service test or inspection within the microwave generating compartments, check the magnetron, wave guide or transmission line, and cavity for proper alignment, integrity, and connections. (d) Any defective or misadjusted components in the interlock, monitor, door seal, and microwave generation and transmission systems shall be repaired, replaced, or adjusted by procedures described in this manual before the oven is released to the owner. (e) A microwave leakage check to verify compliance with the Federal Performance Standard should be performed on each oven prior to releasing oven to the owner. BEFORE SERVICING Before servicing an operative unit, perform a microwave emission check as per the Microwave Measurement Procedure outlined in this service manual. If microwave emissions level is in excess of the specified limit, contact SHARP ELECTRONICS CORPORATION immediately @1-800-237-4277. If the unit operates with the door open, service person should 1) tell the user not to operate the oven and 2) contact SHARP ELECTRONICS CORPORATION and Food and Drug Administrations Center for Devices and Radiological Health immediately. Service personnel should inform SHARP ELECTRONICS CORPORATION of any certified unit found with emissions in excess of 4mW/cm 2. The owner of the unit should be instructed not to use the unit until the oven has been brought into compliance.
1 R-820BK R-820BW WARNING TO SERVICE PERSONNEL Microwave ovens contain circuitry capable of pro- ducing very high voltage and current, contact with following parts may result in a severe, possibly fatal, electrical shock. (Example) High Voltage Capacitor, High Voltage Power Trans- former, Magnetron, High Voltage Rectifier Assem- bly, High Voltage Harness etc.. Read the Service Manual carefully and follow all instructions. Before Servicing 1. Disconnect the power supply cord ,and then remove outer case. 2. Open the door and block it open. 3. Discharge high voltage capacitor. WARNING: RISK OF ELECTRIC SHOCK. DISCHARGE THE HIGH-VOLTAGE CAPACITOR BEFORE SERVICING. The high-voltage capacitor remains charged about 60 seconds after the oven has been switched off. Wait for 60 seconds and then short-circuit the connection of the high- voltage capacitor (that is the connecting lead of the high- voltage rectifier) against the chassis with the use of an insulated screwdriver. Whenever troubleshooting is performed the power supply must be disconnected. It may in, some cases, be necessary to connect the power supply after the outer case has been removed, in this event, 1. Disconnect the power supply cord, and then remove outer case. 2. Open the door and block it open. 3. Discharge high voltage capacitor. 4. Disconnect the leads to the primary of the power transformer. 5. Ensure that the leads remain isolated from other components and oven chassis by using insulation tape. 6. After that procedure, reconnect the power supply cord. When the testing is completed, 1. Disconnect the power supply cord, and then remove outer case. 2. Open the door and block it open. 3. Discharge high voltage capacitor. 4. Reconnect the leads to the primary of the power transformer. 5. Reinstall the outer case (cabinet). 6. Reconnect the power supply cord after the outer case is installed. 7. Run the oven and check all functions. After repairing 1. Reconnect all leads removed from components during testing. 2. Reinstall the outer case (cabinet). 3. Reconnect the power supply cord after the outer case is installed. 4. Run the oven and check all functions. Microwave ovens should not be run empty. To test for the presence of microwave energy within a cavity, place a cup of cold water on the oven turntable, close the door and set the power to HIGH and set the microwave timer for two (2) minutes. When the two minutes has elapsed (timer at zero) carefully check that the water is now hot. If the water remains cold carry out Before Servicing procedure and re- examine the connections to the component being tested. When all service work is completed and the oven is fully assembled, the microwave power output should be checked and microwave leakage test should be carried out. Dont Touch ! Danger High Voltage
2 R-820BK R-820BW MICROWAVE MEASUREMENT PROCEDURE A. Requirements: 1) Microwave leakage limit (Power density limit): The power density of microwave radiation emitted by a microwave oven should not exceed 1mW/cm 2 at any point 5cm or more from the external surface of the oven, measured prior to acquisition by a purchaser, and thereafter (through the useful life of the oven), 5 mW/cm2 at any point 5cm or more from the external surface of the oven. 2) Safety interlock switches Primary interlock relay and door sensing switch shall prevent microwave radiation emission in excess of the requirement as above mentioned, secondary interlock switch shall prevent microwave radiation emission in excess of 5 mW/cm 2 at any point 5cm or more from the external surface of the oven. B. Preparation for testing: Before beginning the actual measurement of leakage, proceed as follows: 1) Make sure that the actual instrument is operating normally as specified in its instruction booklet. Important: Survey instruments that comply with the requirement for instrumentation as prescribed by the performance standard for microwave ovens, 21 CFR 1030.10(c)(3)(i), must be used for testing. 2) Place the oven tray in the oven cavity. 3) Place the load of 275±15 ml (9.8 oz) of tap water initially at 20±5ûC (68ûF) in the center of the oven cavity. The water container shall be a low form of 600 ml (20 oz) beaker with an inside diameter of approx. 8.5 cm (3-1/2 in.) and made of an electrically nonconductive material such as glass or plastic. The placing of this standard load in the oven is important not only to protect the oven, but also to insure that any leakage is measured accurately. 4) Set the cooking control on Full Power Cooking Mode 5) Close the door and select a cook cycle of several minutes. If the water begins to boil before the survey is completed, replace it with 275 ml of cool water. C. Leakage test: Closed-door leakage test (microwave measurement) 1) Grasp the probe of the survey instrument and hold it perpendicular to the gap between the door and the body of the oven. 2) Move the probe slowly, not faster than 1 in./sec. (2.5 cm/sec.) along the gap, watching for the maximum indication on the meter. 3) Check for leakage at the door screen, sheet metal seams and other accessible positions where the continuity of the metal has been breached (eg., around the switches, indicator, and vents). While testing for leakage around the door pull the door away from the front of the oven as far as is permitted by the closed latch assembly. 4) Measure carefully at the point of highest leakage and make sure that the highest leakage is no greater than 4mW/cm 2, and that the secondary interlock switch does turn the oven OFF before any door movement. NOTE: After servicing, record data on service invoice and microwave leakage report.
3 R-820BK R-820BW SHARP ELECTRONICS CORPORATION SHARP PLAZA, MAHWAH, NEW JERSEY 07430-2135 SERVICE MANUAL DOUBLE GRILL CONVECTION MICROWAVE OVEN R-820BK / R-820BW FOREWORD This Manual has been prepared to provide Sharp Electronics Corp. Service Personnel with Operation and Service Information for the SHARP MICROWAVE OVEN, R-820BK, R-820BW. It is recommended that service personnel carefully study the entire text of this manual so that they will be qualified to render satisfactory customer service. Check the interlock switches and the door seal carefully. Special attention should be given to avoid electrical shock and microwave radiation hazard. WARNING Never operate the oven until the following points are ensured. (A) The door is tightly closed. (B) The door brackets and hinges are not defective. (C) The door packing is not damaged. (D) The door is not deformed or warped. (E) There is no other visible damage with the oven. Servicing and repair work must be carried out only by trained service personnel. DANGER Certain initial parts are intentionally not grounded and present a risk of electrical shock only during servicing. Service personnel - Do not contact the following parts while the appliance is energized; High Voltage Capacitor, Power Transformer, Magnetron, High Voltage Rectifier Assembly, High Voltage Harness; If provided, Vent Hood, Fan assembly, Cooling Fan Motor. All the parts marked Ò*Ó on parts list are used at voltages more than 250V. Removal of the outer wrap gives access to voltage above 250V. All the parts marked ÒDÓ on parts list may cause undue microwave exposure, by themselves, or when they are damaged, loosened or removed.PRODUCT DESCRIPTION GENERAL INFORMATION OPERATION TROUBLESHOOTING GUIDE AND TEST PROCEDURE TOUCH CONTROL PANEL COMPONENT REPLACEMENT AND ADJUSTMENT PROCEDURE WIRING DIAGRAM PARTS LIST
4 R-820BK R-820BW ITEM DESCRIPTION Power Requirements 120 Volts 12.5 Amperes (Microwave) / 13.0 Amperes (Grill/ Convection) 60 Hertz / Single phase, 3 wire grounded Power Output 900 watts (IEC 705 Test Procedure) Operating frequency of 2450MHz Top Heater Power Output 1000 Watts Top & Bottom Heater Power Output 1500 Watts Case Dimensions Width 20-1/2 Height 12-1/8 Depth 19-3/4 Cooking Cavity Dimensions (0.9 Cubic Feet )Width 13-7/8 Height 7-1/2 Depth 14-1/2 Control Complement Touch Control System Timer (0 - 99 min. 99 seconds) Microwave Power for Variable Cooking Repetition Rate; P-HI ........................................... Full power throughout the cooking time P-90 ............................................................. approx. 90% of Full Power P-80 ............................................................. approx. 80% of Full Power P-70 ............................................................. approx. 70% of Full Power P-60 ............................................................. approx. 60% of Full Power P-50 ............................................................. approx. 50% of Full Power P-40 ............................................................. approx. 40% of Full Power P-30 ............................................................. approx. 30% of Full Power P-20 ............................................................. approx. 20% of Full Power P-10 ............................................................. approx. 10% of Full Power P-0 ............................................. No power throughout the cooking time Grill Cooking (GRILL) ........ Top and Bottom Grill Mode / Top Grill Mode only Convection Cooking (CONVEC) .......................... 100 - 450ûF Temp. control Roast Cooking (ROAST) .......................... 325ûF with 30% microwave power CUSTOM HELP pad, COMPU COOK/ BAKE/ PIZZA/ GRILL/ ROAST pads COMPU DEFROST pad, CONVEC pad, GRILL pad, ROAST pad POPCORN pad, REHEAT pad, MINUTE PLUS pad Number and temperature selection pads, KITCHEN TIMER pad, CLOCK pad STOP/CLEAR pad, POWER LEVEL pad, START / TOUCH ON pad Oven Cavity Light Yes Safety Standard UL Listed FCC Authorized DHHS Rules, CFR, Title 21, Chapter 1, Subchapter J SPECIFICATION GENERAL INFORMATION GROUNDING INSTRUCTIONS This oven is equipped with a three prong grounding plug. It must be plugged into a wall receptacle that is properly installed and grounded in accordance with the National Electrical Code and local codes and ordinances. In the event of an electrical short circuit, grounding reduces the risk of electric shock by providing an escape wire for the electric current. WARNING: Improper use of the grounding plug can result in a risk of electric shock. Electrical Requirements The electrical requirements are a 115 -120 volt 60 Hz, AC only, 15 or 20 amp. fused electrical supply. It is recommended that a separate circuit serving only this appliance be provided. When installing this appliance, observe all applicable codes and ordinances.
5 R-820BK R-820BW A short power-supply cord is provided to reduce risks of becoming entangled in or tripping over a longer cord. Where a two-pronged wall-receptacle is encountered, it is the personal responsibility and obligation of the customer to contact a qualified electrician and have it replaced with a properly grounded three-pronged wall receptacle or have a grounding adapter properly grounded and polarized. If the extension cord must be used, it should be a 3-wire, 15 amp. or higher rated cord. Do not drape over a countertop or table where it can be pulled on by children or tripped over accidentally. CAUTION: DO NOT UNDER ANY CIRCUMSTANCES CUT OR RE- MOVE THE ROUND GROUNDING PRONG FROM THIS PLUG. 3-Pronged PlugGrounded Receptacle Box Grounding Pin 3-Pronged Receptacle OVEN DIAGRAM 1. Oven light 2. Top heaters (Grill heaters) 3. Oven door with see through window 4. Door hinges 5. Safety door latches 6. Door seals and sealing surfaces 7. Bottom heater 8. Oven cavity 9. Turntable motor shaft 10.Ventilation openings 11.Waveguide cover 12.One touch door opening button 13.Auto-Touch control panel 14.Time display 15.Power supply cord 16.Outer cabinet 17.Menu label NOTE: 1. Ensure that the bottom heater is in the lowest position as shown in the figure, as it is possible to move it up and down to help with cleaning. 2. Place the turntable support over the turntable motor shaft on the floor of the cavity. 3. Then place the turntable onto the turntable support. TOUCH CONTROL PANELTurntable Turntable support Rack Turntable motor shaft Bottom heaterCOOKHELP CUSTOM HELP Iinteractive COMPU PIZZAGRILL BAKEROAST COOKDEFROST CONVECGRILL ROAST POPCORNREHEATMINUTE PLUS KITCHEN TIMER STOPCLEARSTARTTOUCH ON CLOCKPOWERLEVEL 100û F 150û F 275û F 300û F 325û F 350û F 375û F 400û F 425û F 450û F12345 6789O 1 4 u 3 00 0 0 5 5 2 6w7 9 8qert y
6 R-820BK R-820BW OPERATION DESCRIPTION OF OPERATING SEQUENCE The following is a description of component functions during oven operation. OFF CONDITION Closing the door activates door sensing switch and second- ary interlock switch. (In this condition, the monitor switch contacts are opened.) When oven is plugged in, 120 volts A.C. is supplied to the control unit. (Figure O-1). 1. The display will show , , , , , , , , and . To set any program or set the clock, you must first touch the STOP/CLEAR pad. The display will clear, and : will appear. MICROWAVE COOKING CONDITION Program desired cooking time by touching the NUMBER pads. Program the power level by touching the POWER LEVEL pad and then a Number pad. When the START pad is touched, the following operations occur: 1. The contacts of relays are closed and components connected to the relays are turned on as follows. (For details, refer to Figure O-2) RELAY CONNECTED COMPONENTS RY-2 Power transformer RY-5 Fan motor RY-6 Oven lamp/ Turntable motor 2. 120 volts A.C. is supplied to the primary winding of the power transformer and is converted to about 3.1 volts A.C. output on the filament winding, and approximately 2260 volts A.C. on the high voltage winding. 3. The filament winding voltage heats the magnetron filament and the H.V. winding voltage is sent to a voltage doubler circuit. 4. The microwave energy produced by the magnetron is channelled through the waveguide into the cavity feed- box, and then into the cavity where the food is placed to be cooked. 5. Upon completion of the cooking time, the power transformer, oven lamp, etc. are turned off, and the generation of microwave energy is stopped. The oven will revert to the OFF condition. 6. When the door is opened during a cook cycle, monitor switch, door sensing switch, secondary interlock switch, relay (RY5), relay (RY6) and primary interlock relay(RY2) are activated with the following results. The circuits to the turntable motor, the cooling fan motor, and the high voltage components are de-energized, the oven lamp remains on, and the digital read-out displays the time still remaining in the cook cycle when the door was opened. 7. The monitor switch electrically monitors the operation ofthe secondary interlock switch and primary interlock relay and is mechanically associated with the door so that it will function in the following sequence. (1) When the door opens from the closed position, the relay (RY5) , primary interlock relay(RY2) and secondary interlock switch open their contacts. And contacts of the relay(RY6) remains closed. Then the monitor switch contacts close. (2) When the door is closed from the open position, the monitor switch contacts open first. Then the contacts of the secondary interlock switch and door sensing switch close. And contacts of the relay(RY6) open. If the secondary interlock switch and primary interlock relay (RY2) fail with the contacts closed when the door is opened, the closing of the monitor switch contacts will form a short circuit through the monitor fuse, secondary interlock switch, relay(RY6) and primary interlock relay(RY2), causing the monitor fuse to blow. POWER LEVEL P-0 TO P-90 COOKING When Variable Cooking Power is programmed, the 120 volts A.C. is supplied to the power transformer intermittently through the contacts of relay(RY-2) which is operated by the control unit within a 32 second time base. Microwave power operation is as follows: VARI-MODE ON TIME OFF TIME Power 10(P-HI) 32 sec. 0 sec. (100% power) Power 9(P-90) 30 sec. 2 sec. (approx. 90% power) Power 8(P-80) 26 sec. 6 sec. (approx. 80% power) Power 7(P-70) 24 sec. 8 sec. (approx. 70% power) Power 6(P-60) 22 sec. 10 sec. (approx. 60% power) Power 5(P-50) 18 sec. 14 sec. (approx. 50% power) Power 4(P-40) 16 sec. 16 sec. (approx. 40% power) Power 3(P-30) 12 sec. 20 sec. (approx. 30% power) Power 2(P-20) 8 sec. 24 sec. (approx. 20% power) Power 1(P-10) 6 sec. 26 sec. (approx. 10% power) Power 0(P-0) 0 sec. 32 sec. (0% power) Note: The ON/OFF time ratio does not correspond with the percentage of microwave power, because approx. 2 seconds are needed for heating of the magnetron filament. GRILL COOKING CONDITIONS The oven has two grill cooking conditions. They are the TOP HEATER mode and TOP AND BOTTOM HEATER mode.
7 R-820BK R-820BW TOP AND BOTTOM HEATER MODE In this mode, the food is cooked by both the top heaters and bottom heater. Press the GRILL pad and number 1 pad and then enter the cooking time by pressing the number pads. When the START pad is pressed, the following operations occur (Figure O-3a): 1. The relays(RY3, RY4, RY5 and RY6) are energized. 2. The numbers of the digital read-out start the count down to zero. 3. Then the top heaters, bottom heater, turntable motor, oven lamp and fan motor are energized. 4. Now, the food is grilled by the top heaters and the bottom heater. 5. Upon completion of the selected cooking time, audible signal sounds and the contacts of relays(RY3, RY4 and RY6) are opened, then the top heating elements, bottom heating element, turntable motor and oven lamp are de- energized. But the relay(RY5) stays closed and the fan motor operates for 5 minutes. But if the cooking time is less than 2 minutes the relay(RY5) will not stay closed. NOTE: If the total grill cooking time is beyond 35 minutes, the relay(RY1) is energized and the convection motor is energized. TOP HEATER MODE In this mode, the food is cooked by the top heaters. Press the GRILL pad and number 2 pad and then enter the cooking time by pressing the number pads. When the START pad is pressed, the following operations occur (Figure O-3b): 1. The relays(RY3, RY5 and RY6) are energized. 2. The numbers of the digital read-out start the count down to zero. 3. Then the top heaters, turntable motor, oven lamp and fan motor are energized. 4. Now, the food is grilled by the top heaters. 5. Upon completion of the selected cooking time, audible signal sounds and the contacts of relays(RY3 and RY6) are opened, then the top heaters, turntable motor and oven lamp are de-energized. But the relay(RY5 ) stays closed and the fan motor operates for 5 minutes. But if the cooking time is less than 2 minutes the relay(RY5) will not stay close. NOTE: If the total grill cooking time is beyond 35 minutes, the relay(RY1) is energized and the convection motor is energized. CONVECTION COOKING CONDITION WITH PREHEAT Touch the CONVEC pad and select the convection with preheat mode by touching the number 1 pad. 1. Program desired convection temperature by touching the Temperature pad. Enter the cooking time by touching the number pads. When the START pad is touched, the following operations occur: PREHEATING CONDITION 2. The coil of shut-off relays (RY5 and RY6) are energized, the oven lamp, cooling fan motor and turntable motor are turned on. 3. The coil of relays(RY3 and RY4) are energized by the control unit, and the top heaters and the bottom heaterwork with 100% power. The relay(RY1) and the convection motor are not energized at this preheating condition. NOTE: If the cavity temperature is lower than 100ûF(38C) at 4 minutes and 15 seconds after the preheat is started, the control unit judges that the thermistor is opened. And the oven is stopped. 4. When the oven temperature reaches the selected preheat temperature, the following operations occur: 4-1 The relays(RY3 and RY4) are de-energized by the control unit temperature circuit and thermistor, opening the circuit to the heating element. 4-2. The oven will continue to function for 30 minutes, turning the top heaters and the bottom heater on and off, as needed to maintain the selected preheat temperature. The oven will shutdown completely after 30 minutes CONVECTION TIME COOKING CONDITION 5. When the preheat temperature is reached, a beep signal will sound indicating that the holding temperature has been reached in the oven cavity. Open the door and place the food to be cooked in the oven. When the START pad is touched, the following operations occur: 6. The numbers on the digital read-out start to count down to zero. 7. The relays(RY1, RY5 and RY6) are energized and the oven lamp, turntable motor, cooling fan motor and convection motor are energized. 8. The relays(RY3 and RY4) are energized (if the cavity temperature is lower than the selected temperature) and the main supply voltage is applied to the heating element to return to the selected cooking temperature. The top heaters and the bottom heater work in accordance with the following table while the heaters are energized. Selected Temp- Top Heaters Bottom Heaterserature (ûF) Power (%) Power (%) 100 20 10(after 3min. 0%) 150 20 10 275 40 40 300 40 50 325 50 50 350 50 60 375 60 60 400 60 70 425 70 70 450 70 80 9. Upon completion of the cooking time, the audible signal will sound, and oven lamp, turntable motor, cooling fan motor and convection motor are de-energized. At the end of the convection cycle, if the cavity air temperature is above 250ûF(120ûC), the circuit to RY5 will be maintained (by the thermistor circuit) to continue operation of the cooling fan motor until the temperature drops below 220ûF(104ûC), at which time the relay will be de- energized, turning off the fan motor. Relay RY1 will however, open as soon as the convection cycle has ended, turning off the convection fan motor.
8 R-820BK R-820BW ROAST COOKING CONDITION Touch the ROAST pad once. And then program desired cooking time by touching the number pads. The ROAST pad is preprogrammed for 325ûF with 30% microwave power. NOTE: The microwave power can not be changed. To change the temperature, touch the ROAST pad twice. When the display says SELECT TEMP, touch desired temperature pad. When the START pad is pressed, the following operations occur: 1. The numbers on the digital read-out start to count down. 2. The shut-off relays(RY1, RY5 and RY6) are energized, turning on the oven lamp, turntable motor, cooling fan motor and convection motor. 3. The microwave relay(RY2) is energized for 16 seconds within a 48 second time base. 4. The heater relays(RY3 and RY4) are energized intermittently within a 48 second time base. The power output is set in accordance with the following table. Selected Temp- Top Heaters Bottom Heaterserature (ûF) Power (%) Power (%) 100 20 40 150 30 50 275 50 60 300 60 70 325 70 70 350 70 70 375 70 70 400 70 70 425 70 70 450 70 70 5. The microwave relay(RY2) and the heater relays(RY3 and RY4) are not energized simultaneously. ON/OFF TIME RATIO In grill cooking, convection cooking or roast cooking, the top heaters, bottom heater or magnetron operate whithin a 48 second time base. The following table is the ON / OFF time ratio at each power output of the top heaters, bottom heater or magnetron. POWER OUTPUT ON TIME OFF TIME 100% 48 sec. 0 sec. 90% 44 sec. 4 sec. 80% 40 sec. 8 sec. 70% 36 sec. 12 sec. 60% 32 sec. 16 sec. 50% 26 sec. 22 sec. 40% 22 sec. 26 sec. 30% 16 sec. 32 sec. 20% 12 sec. 36 sec. 10% 8 sec. 40 sec. FIRE SENSING FEATURE (MICROWAVE MODE) This model incorporates a sensing feature which will stop the ovens operation if there is a fire in the oven cavity during microwave cooking. This is accomplished by the LSI repeatedly measuring the voltage across the temperature measurement circuit (ther- mistor) during its 32-seconds time base comparing the obtained voltage measurements. If the most recent voltage measured is 700mV greater than the previous voltage measured, the LSI judges it as a fire in the oven cavity and switches off the relays to the power transformer, fan motor, oven lamp and turntable motor. The LSI also stops counting down. Please refer to the following section for a more detailed description. Operation Please refer to the timing diagrams below. 1. The thermistor operates within a 32-seconds time base and it is energized for three (3) seconds and off for 29 seconds. Two (2) seconds after the thermistor is energized, the voltage across the temperature measurement circuit is sampled by the LSI and twenty one (21) seconds after the thermistor is cut off the LSI turns on the cooling fan for six (6) seconds. 2. The above procedure is repeated. If the difference between the first voltage measured (in step 1) and the voltage measured when the procedure is repeated (step 2) is greater than 700mV the LSI makes the judgment that there is a fire in the oven cavity and will switch off the relays to the power transformer, fan motor, oven lamp and turntable motor. The LSI also stops counting down. 3. Once the fire sensor feature has shut the unit down, the programmed cooking cycle may be resumed by pressing the START pad or the unit may be reset by pressing the CLEAR pad. THERMISTOR SENSING VOLTAGEON OFF ON OFF 0 2 32 64 (sec.) 3 sec. Sensing the voltage across temperature measurement circuit.