panasonic microwave nnct878 service manual

HAD0712001C2

Microwave OvenNN-CT878SBPQ

32LUnited Kingdom

ChaosDefrost

Vegetables

Potatoes

Fish

Pizza

Chicken

Meat

Crispy Top

Bake

10

min

1

min

10

sec

1

h

Micro Power Grill 1-2-3

Convection

Combination

Turbo-Bake

Memory

Timer

Start

Stop / Cancel

lessmore

NN-CT878S

Power source 240V AC Single Phase 50Hz

Power requirements Microwave 1220W

Grill 1560W

Convection 1350W

Combi 2870W

Output (IEC60705) Microwave 1000W

Grill 1500W

Convection 1250W

Microwave frequency 2450Mhz

Timer 99 min 99 second

Oven cavity size 32L

Outside dimensions 530mm(W) x 508mm(D) X 344MM (H)

Inside dimensions 350mm(W) X 360mm(D) x 250mm(H)

Weight 21Kg

© 2007 Matsushita Electric Industrial Co., Ltd. All rights reserved.Unauthorized copying and distribution is a violation of law.

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1 Contents

1 Contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Inverter Warnings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 Feature Chart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .34 Control Panel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .45 Operation And Digital Programmer Circuit Test Procedure . . . . . . . . . . . . . . . . . . . 56 Schematic Diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87 Wiring Diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98 Description of the Operation Sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109 Cautions to be Observed when Troubleshooting . . . . . . . . . . . . . . . . . . . . . . . . . . . 1710 Part Replacement Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2011 Component Test Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2912 Measurements and Adjustments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3113 Troubleshooting Guide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3214 Description of Operating instruction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .35 15 Exploded View . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3616 Parts List . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3717 Door Assy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3918 Escutcheon Base. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4019 Packing and Accessories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4120 Trim Kit. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4321 Digital Programmer Circuit. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4422 Key Board Matrix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45



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2 Inverter WarningThe inverter board looks like a regular PCB, however, this PCBdrives the magnetron tube using very high voltages and cur-rent.It has

1. Very high voltage and high current2. An Aluminium heat sink that becomes very hot3. The capacitors on the inverter circuit will contain a high

voltage charge even when the oven is not operating. Do not

1. Do not touch the circuitry as it contains very high volt-ages. When replacing the board please take extremecare to avoid possible electric shock. High voltages mayremain in the circuit.

2. Do not touch the aluminium heat sink as it will becomevery hot. It also contains high voltages.

3. Do not attempt to repair the inverter PCB as this can bevery dangerous. Replace the high voltage inverter circuitas a complete unit. Return the old unit fully repackedin the original shipping box and completed paperwork.

4. Do not adjust or tamper with the preset volume on theinverter board. It is very dangerous to adjust this presetwithout proper test equipment.

5. Do not test the oven while the inverter grounding strip orscrews are loose. It is very dangerous to operate theinverter circuit board without a proper ground connection.

Inverter Grounding

Figure 1

Inverter PCB layout

Figure 2

3 Feature Chart

Function BPQMicrowave 6Grill 3Convection 17Combi YesSensor Reheat -Sensor Cook 5Sensor Combi 11Auto Preheat Combi -Weight Defrost 3Weight Combi 2Weight Reheat -Weight Cook -Memory Cook 3Weight Crisp -Stage Cooking 3 stageDelay / Stand YesMore / Less YesKg -> lb/oz YesClock 12hWord Prompt EnglishStep by Step LED



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4 Control Panel



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5 Operation And Digital Programmer Circuit Test Procedure



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6 Schematic Diagram



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7 Wiring Diagram

Wiring Harness



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8 Description of Operating instruction8.1. Variable power cooking controlThe HIGH VOLTAGE INVERTER POWER SUPPLY controlsthe output power by a signal from the digital Programmer circuitDPC. The power relay (RY1) turns on to supply power to theinverter circuit. The level of output power is controlled by thedrive signal level from the inverter circuit.NOTE1: The ON/OFF time ratio does not correspond withthe percentage of microwave power since approximately 2seconds are required for heating the magnetron filament.NOTE: 2 If microwave cooking is over 8 minutes on HIGHpower, the fan motor rotates for 1 minute after cooking tocool the oven and electrical components.

Duty cycles for microwave cooking

8.2. Grill CookingThe digital programmer circuit controls the grill power by oper-ating the power relay RY4 in the sequence shown in the tablebelow

Duty cycles for Grill Cooking

8.3. Auto weight defrost, Autoweight Cook

When an auto control feature is selected and the start padpressed:

1. The digital programmer circuit determines the power leveland the cooking time and indicates the operating state inthe display. The table shows the corresponding cookingtimes for each category and its respective weight.

2. When the cooking time in the display window haselapsed, the oven turns off automatically via the controlsignal from the digital programmer circuit.Note: After auto cooking if the oven temperature isover the predetermined temperature the fan motorrotates to cool the oven and its components.

Defrost Menu’s

Weight Menu’s

Output Duty ON/OFFON time OFF time

High 1000W 22” 0”Defrost 440W 16” 6”Medium 600W 22” 0”Low 440W 22” 0”Simmer 440W 15” 7”Warm 440W 8” 14”

Duty ON/OFFON Time OFF Time

Grill 1 66” 0”Grill 2 54” 12”Grill 3 42” 24”

Category 1st Touch Weight Cooking TimeTurbo Defrost 500g 8m 20s

Category 1st Touch Weight Cooking Time1. Whole Chicken 1000g 23m 54s2. Chicken Pieces 500g 14m 40s3. Beef 900g 26m 59s4. Lamb 1200g 30m 06s5 Pork 1200g 35 m 00s



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8.4. Convection Cooking1. The digital programmer circuit operates the power relays

RY3,RY5 and RY6 in the sequence as shown in the figurebelow.

2. When the oven reaches a predetermined temperature thedigital programmer circuit stops supplying power to relayRY5, resulting in the convection heater turning off.

3. When the temperature drops below the predetermined

temperature, the digital programmer circuit suppliespower to power relay RY5 resulting in the convectionheater turning on.Note: After the convection process, if the oven tempera-ture is higher than the predetermined temperature, thefan motor rotates to cool the electronic components andthe oven.

Convection Cooking Duty Cycles

Figure 1



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8.5. Combination CookingCombination cooking is achieved by operating the microwaveand heater modes together during one cooking cycle. Thereare three combination modes.

1. Combination (convection and microwave)2. Combination (grill and microwave)3. Combination (grill, convection and microwave)

The digital programmer circuit operates the power relays asshown in the figures below.

When the oven temperature reaches the predetermined tem-perature, the digital programmer circuit stops supplying powerto relay (RY5) resulting in the convection heater turning off.During this time the digital programmer circuit continues tooperate relay (RY1) so that microwave activity continues at theduty cycle selected. The inverter control signal level is alsomaintained. The microwave activity continues to cycle until theentire cooking program is completed.

When the oven temperature drops below the selected tempera-ture, the digital programmer circuit operates power relay (RY5)switching on the heater elements.

In the case of grill combination the sequence applies with thedigital programmer circuit switching power relay (RY4) to con-trol, the grill elements.

With convection, grill and micro power combination. The grillelements and convection elements are operated alternativelywhilst the oven temperature is above the selected level.

Convection And Microwave Combination Duty Cycles

Grill And Microwave Combination Duty Cycles

Grill Convection And Microwave Combination Duty Cycles

Convection Heater Micropower100 - 250 C DISPLAY OUTPUT ON OFF

440 360W 66 0250 360W 48 18100 360W 21 45

Grill Heater MicropowerON OFF DISPLAY OUTPUT ON OFF

Grill 1 51 15 440 360W 66 0Grill 2 42 24 250 360W 48 18Grill 3 28 38 100 360W 21 45

Convection Heater

Grill Heater Micropower

100 - 250 C ON OFF OUTPUT ON OFFGrill 1 51 15 440 360W 66 0Grill 2 42 24 250 360W 48 18Grill 3 28 38 100 360W 21 45



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8.5.1 convection and microwave duty cycles

Figure 1



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8.5.2 grill and microwave duty cycles

Figure 2



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Figure 3

8.5.3 Grill,Convection and microwave duty cycles



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8.6. One touch Auto sensor cooking Auto sensor cook is a revolutionary way to cook by microwavewithout setting a power level or selecting a time. All that is nec-essary is to select an auto sensor program before starting tocook.

Understanding Auto sensor cooking

As food cooks, a certain amount of steam is produced. If thefood is covered, this steam builds up and eventually escapesfrom the container. In auto sensor cooking, a carefully designedinstrument, called the humidity sensor element, senses theescape of steam. then, based upon the auto sensor programselected, the unit will automatically determine the correct powerlevel and the proper length of time it will take to cook the food.NOTE: Auto sensor cooking is successful with the foodsand recipes found in the auto sensor cooking guide.Because of the vast differences in food composition, itemsnot mentioned in the cooking guide should be prepared inthe microwave oven using the power select and time fea-tures. Please consult variable power microwave cook bookfor procedures.

Explanation of the auto sensor cooking process

1. The shaded columns in figure 1 indicate when the humid-ity sensor heater is on.

2. During the 30 second period there is no microwave activ-ity. When calculating the T2 time by using the formulabelow, make sure this 30 seconds is subtracted from theT0 time.

3. T1 time T0 time - 30 seconds4. T2 time when the steam escapes from the cooking con-

tainer placed in the oven, the humidity sensor detects itand the microprocessor calculates the balance of cookingtime. This T2 time is then shown in the display and beginscounting down.

Balance of cooking (T2 time)The balance of cooking time which is called T2 time, can becalculated by the following formula.

T2 time (in sec.) = T1 time x K factorNOTE: Remember, the T1 time starts after the start pad istapped. The coefficient K is programmed into the micro-processor memory and they are listed in the followingtables along with the P1 and P2 power.

NOTE: When “More“ or “Less” pad is selected, the K factorvaries resulting in T2 time to be increased or decreased.

Calculating the T2 timeExample 1: If the t1 time is measured to be 2 minutes and 40seconds, and the auto sensor program selected is sensorreheat. Fish (Program 4)T2 = T1 x K= 2 min. and 40 sec. X. 0.3=48 sec.

Auto sensor cooking

Figure 1

Category P1 POWER P2 POWER K FACTOR STANDARD

1. Chilled meal HIGH HIGH 1.62. Frozen meal MED HIGH 0.9



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9 Cautions to Be Observed When TroubleshootingUnlike many other appliances, the microwave oven is a highvoltage, high current device. Although it is free from danger inordinary use, extreme care should be taken during repair.

CautionServicemen should remove their watches wheneverworking close to or replacing the magnetron.

9.1. Check the groundingDo not operate on a two wire extension cord. The microwaveoven is designed to be used when grounded. It is imperative,therefore, to ensure the appliance is properly grounded beforebeginning repair work.

9.2. Inverter WarningsDANGER, HIGH VOLTAGE AND HIGH TEMPERATURE (HOT/LINE) OF THE INVERTER POWER SUPPLY (U)This high voltage inverter power supply handles very highvoltage and very high current for the magnetron tube.Though it is free from danger in ordinary use, extreme careshould be taken during repair. As you can see, it looks like aTV flyback transformer, however, the current is extremelylarge and is therefore, dangerous due to this high currentand high voltage.The aluminium heat sink is also energized with high voltage(HOT), so do not touch when the AC input terminal is con-nected. The power devices (Collector) is directly connectedto the aluminium heat sink.The aluminium heat sink may be (HOT) due to heat energy,therefore, extreme care should be taken during servicing.

WARNING FOR INVERTER POWER SUPPLY (U) GROUND-ING

Check the high voltage inverter power supply circuit ground-ing. This high voltage inverter power supply circuit boardmust have a proper chassis ground, the inverter groundingbracket must be connected to the chassis. If the inverterboard is not grounded it will expose very high voltages andcause extreme DANGER! Be sure that the inverter circuit isproperly grounded via the inverter earth bracket.

HV Inverter warning

Figure 1



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WARNING! DISCHARGE THE HIGH VOLTAGE CAPACITORS

For about 30 seconds after the oven is turned off, an elec-tric charge remains in the high voltage capacitors in theinverter power supply circuit board.When replacing or checking parts, remove the power plugfrom the outlet and short the inverter output terminal of themagnetron filament terminals to the chassis ground with aninsulated handle screwdriver to discharge. Please be sureto touch the chassis ground side first and then short to theoutput terminals.

WARNINGThere is high voltage present with high current capabilitiesin the circuits of the primary and secondary windings, chokecoil and heat sink of the inverter. It is extremely dangerousto work on or near these circuits with the oven energized.DO NOT measure the voltage in the high voltage circuitincluding the filament voltage of the magnetron.

WARNINGNever touch any circuit wiring with your hand nor with aninsulated tool during operation.

Inverter Grounding

Figure 2

Discharging the high voltage capacitors

Figure 3



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9.3. When parts must be replaced,remove the power plug fromthe outlet.

9.4. When the 10A fuse is blowndue to the operation of theshort switch:

WARNINGWhen the 10A 250V fuse is blown due to the operation ofthe short switch, the primary latch switch and short switchmust be replaced. It is also important to change the powerrelay 1 (RY1) when the continuity test shows shorted con-tacts.1. This is mandatory. Refer to “adjustments and measure-

ments” for the location of these switches.2. When replacing the fuse, confirm that it has the appropri-

ate rating for these models.3. When replacing faulty switches, be sure the mounting

tabs are not bent, broken or deficient in their ability to holdthe switches.

9.5. Avoid inserting nails, wire etc.through any holes in the unitduring operation.

Never insert a wire, nail or any other metal object through thelamp holes on the cavity or any holes or gaps, because suchobjects may work as an antenna and cause microwave leak-age.

9.6. Confirm after repair1. After repair or replacement of parts, make sure that the

screws of the oven, etc. are neither loose nor missing.Microwaves might leak if screws are not properly tight-ened.

2. Make sure that all electrical connections are tight beforeinserting the plug into the wall outlet.

3. Check for microwave energy leakage. (Refer to proce-dure for measuring microwave energy leakage).

CAUTION MICROWAVE RADIATION DO NOT BECOME EXPOSED TO RADIATION FROM THEMICROWAVE GENERATOR OR OTHER PARTS CON-DUCTING MICROWAVE ENERGY

IMPORTANT NOTICEThe following components have potentials above 250Vwhile the appliance is in operation.

• Magnetron• High voltage transformer (Located on inverter (U))• High voltage diodes (Located on inverter (U))• High voltage capacitors (Located on inverter (U))

Pay special attention in these areas.When the appliance is operated with the door hinges ormagnetron fixed incorrectly, the microwave leakage canreach more than 5mW/cm3. After repair or exchange, it isvery important to check if the magnetron and the doorhinges are correctly fixed.

9.7. Sharp EdgesCaution

Please use caution when unpacking, installing or movingthe unit, as some exposed edges may be sharp to touchand cause injury if not handled with care.



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10 Parts Replacement Procedure10.1. Magnetron

1. Discharge the high voltage capacitors on the inverter cir-cuit.

2. Remove the five screws shown in figure 13. Disconnect the two high voltage leads from the magne-

tron4. Remove the four screws holding the magnetron as shown

in figure 2NOTE: After replacing the magnetron, tighten the mount-ing screws making sure that there is no gap between thewaveguide and the magnetron to prevent microwave leak-age.

CautionWhen replacing the magnetron, ensure that the antennagasket is in place.

NoteThe magnetron used for this model is unique for the inverterpower supply system. Make sure to use the magnetron aslisted in the parts list.

Removal of the magnetron

Figure 1



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10.2. Inverter power supply (U)1. Discharge the high voltage capacitors2. Unplug the H.V. Lead wires from the magnetron3. Remove the connector CN701 and CN702 from the

inverter PCB4. Remove the two screws holding the inverter base to the

chassis (See figure 3)5. Remove the one screw holding the inverter earth wire to

the oven chassis.6. Carefully remove the inverter PCB and support base from

the oven.7. Remove the air guide E by un-clipping the catch hooks8. Remove the four screws holding the PCB to the inverter

support base.Caution when replacing the inverter power supply (U)

1. Make sure that grounding plate is in place2. Securely tighten the grounding screw through the side of

the chassis (Base).

3. Securely connect the 3 lead wire connectors4. Make sure that the heat sink has enough space (gap)

from the oven. Take care not to touch any lead wire to thealuminium heat sink because it is hot.

Screws to remove the magnetron

Figure 2

Removal of the inverter PCB

Figure 3



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10.3. Digital Programmer Circuit(DPC) and membrane keyboard.

NOTE: Ground any static electric built up on your body beforehandling the DPC.

1. Disconnect all connectors from the DPC.2. Remove the four screws holding the escutcheon base

and slide the escutcheon base upward slightly. removal iseasier with the door open.

3. Release the flat cable.4. Remove the five screws holding the DPC HU assembly5. Remove the door lever6. Remove the eight screws holding the DPC AU assembly

To remove escutcheon pad1. Remove the escutcheon bracket from the escutcheon

base by freeing the 4 catch hooks on the escutcheonbase.

2. Peel away the display window from the inside of the base.3. Remove the membrane assembly by pushing from the

inside of the base and then peeling away from the outsidesurface.

NOTE: 1. The escutcheon key board is attached to the escutcheon

base with double faced adhesive tape. Therefore, apply-ing hot air such as using a hair dryer is recommended forsmoother removal

2. When installing a new escutcheon key board, make surethat the surface of the escutcheon base is cleared suffi-ciently so that problems such as, shorted contacts anduneven surfaces can be avoided.

10.4. Low voltage transformer and/or power relays

NoteBe sure to ground your body to discharge any staticbefore handling the DPC.1. Using a solder wick or a de soldering tool and a 30W sol-

dering iron, carefully remove all solder from the terminalpins of the low voltage transformer and/or power relays.

2. With all of the terminal pins cleaned and separated fromthe DPC contacts, remove the defective transformer/power relays and install the new components makingsure that the terminal pins are inserted completely. Care-fully re solder all terminal contacts carefully.

NoteDo not use a soldering iron or de soldering tool of morethan 30 watts on DPC contacts

Disconnecting the PCB lock connector

Figure 4 Removal of DPC AU and DPC HU

Figure 5



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10.5. Fan Motor1. Disconnect the two lead wires from the fan motor termi-

nals2. Remove the power cord by removing the two wires from

the noise filter and the ground connection from the ovenchassis.

3. Remove the noise filter4. Remove the two screws holding the orifice assembly5. Remove the two screws holding the fan motor assembly6. Detach the orifice assembly and the fan motor assembly

from the oven assembly.7. Remove the fan blade from the fan motor by pulling out-

ward.

10.6. Door disassembly1. Remove door C from door E by carefully pulling outward

starting from the upper right hand corner using a flatblade screwdriver.

2. Remove four screws holding the door E to the door Aassembly

3. Replace the door A and screen B as a complete unit.4. Remove the door key and spring from the door E

After replacement of the defective component parts of thedoor, reassemble it and follow the instructions below for

proper installation and adjustment so as to prevent exces-sive microwave leakage.

1. When mounting the door to the oven, be sure to adjustthe door parallel to the bottom line of the oven face plateby moving the upper hinge in the direction necessary forproper alignment.

2. Adjust so that the door has no play between the innerdoor surface and the oven front surface. if the doorassembly is not mounted properly, microwave power mayleak from the clearance between the door and oven.

3. Perform the microwave leakage test.

10.7. Turntable Motor1. Remove the motor cover by breaking off at the 8 spots

indicated by the arrows.2. Disconnect the two lead wires connected to the turntable

motor3. Remove the turntable motor by removing the two screws

Removing the fan motor

Figure 6

Disassembly of the door

Figure 7



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Note: After breaking off the motor cover, make sure that cut-off portions are properly trimmed off or bent inside so that no sharp edges are exposed.Note: To secure the motor cover use a 4 x 6 screw.

10.8. Quartz Heater1. Remove the sensor cable from the DPC connector CN7.2. Remove the exhaust guide A by removing the two screws

shown in figure 10.3. Disconnect the cables to the grill heater4. Remove the grill cover by removing the 5 screws in figure

11 5. Remove the two screws holding the grill heater

(Figure12).6. Loosen the 2 screws holding the ceramic grill holders,

turn the holders to release the grill element.7. Remove the heater bracket D by using a flat blade screw

driver to lever back the tabs.8. The grill element can now be removed from inside the

oven.

Removing the exhaust guide and grill cover

Figure 10

Removing the Turntable motor cover

Figure 8

Two screws to remove the turntable motor Figure 9



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Removing the screws holding the grill heater

Figure 11

Removing the grill element

Figure 12



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10.9. Convection Element And Circulation Fan Motor1. Remove one screw to remove the exhaust guide D. Figure 13. 2. Remove the rear heater cover by removing the three screws. Figure 133. Remove the temperature sensor by removing the two screws. Figure 144. Remove the power cord.5. Remove the wire terminals from the circulation fan motor and the convection heater element.6. Remove the four screws from the oven base. Figure 15 7. Carefully place the oven onto its base and remove the backplate.8. Remove the nut holding the circulation fan. Figure 169. Remove the circulation fan by removing the three screws. Figure 16

10. Remove the convection element by removing the two screws. Figure 16

Removing the rear heater cover

Figure 13



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Removing the oven back plate

Figure 14

Removing the base plate

Figure 15



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Removing the circulation motor

Figure 16



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11 Component Test ProcedureCaution

1. High voltage is present at the high voltage terminal of theinverter unit, including the aluminium heat sink.

2. It is not necessary or advisable to attempt to measure thishigh voltage.

3. Before touching any oven components, or wiring, alwaysunplug the oven from its power source and discharge thehigh voltage capacitors.

11.1. Primary Latch Switch, Second-ary Latch Switch and powerrelay B interlocks.

1. Unplug the lead connectors to power relay B and verifythe continuity of the power relay B 1-2 terminals.

2. Unplug the lead connectors to the primary latch switchand secondary latch switch.

3. Test the continuity of the switches with the door open andclosed with an ohm meter on the lowest scale.Normal continuity readings should be as followed.

11.2. Short Switch and Monitor Cir-cuit

1. Unplug the lead wires from the HV inverter primary termi-nals.

2. Connect the test probes of the ohm meter to these leads3. Test the continuity of the short switch with the door open

and the door closed using the lowest ohm scale.

11.3. MagnetronContinuity checks can only indicate an open filament or ashorted magnetron. To diagnose an open filament or shortedmagnetron.

1. Isolate the magnetron from the circuit by disconnectingthe HV leads

2. A continuity check across the magnetron filament termi-nals should indicate one ohm or less

3. A continuity check between each filament terminal andthe magnetron case should read open.

11.4. Push Button KeyboardCheck the continuity between the switch terminals, by tappingan appropriate pad on the keyboard. The keypad matrix isshown on Page 50.

Door Open Door ClosedPrimary Latch Switch infinite Ω(Open) 0Ω (Close)Secondary Latch switch

infinite Ω(Open) 0Ω (Close)

Power relay B infinite Ω(Open) infinite Ω (Close)

Door Open Door ClosedMonitor switch 0Ω infinite Ω



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11.5. Inverter Power SupplyCaution

DO NOT try to repair this inverter power supply. Replace as a whole H.V. Inverter Unit.

Inverter power supply diagram

Figure 3

11.6. Inverter Power Supply Unit Warning

Do not attempt to make any measurements in the high volt-age circuitry of the inverter or magnetron.

See troubleshooting of the inverter circuit and magnetron onpage 27 to determine if the inverter power supply is still func-tioning.

11.7. Temperature SensorA temperature sensor is mounted on the rear of the oven. Theresistance reading across the thermistor should read 300K ohmwithin a temperature range of 10 to 30 degrees centigrade. Thiswould be the temperature range within a kitchen environment.If the resistance measured is outside this range the thermistoris defective and should be replaced.NOTE: When measuring the resistance of the thermistor dis-connect the connector from the digital programmer circuit.NOTE: If the microwave oven has been operated allow to coolto room temperature before attempting to measure the ther-mistor resistance.

11.8. Humidity sensor and digitalprogrammer circuit.

1. Check the resistance across the sensor heater terminals.Normal cold resistance should read approximately 4.5ohms.

2. Check the voltage across the sensor heater terminals. Itshould read approximately 2.5VDC

3. In order to determine if the auto cooking by humidity sen-

sor is working, perform the following test.

1. Place a water load (150cc) in the oven.2. Press the auto reheat panel.3. Press the start pad4. The humidity sensor detects steam about 2 to 4 minutes

after the start pad has been pressed.5. The auto cooking (T1) automatically switches to cooking

time. (T2)6. The remaining cooking time T2 appears in the display

window. If 48s -1min 36 secs appears in the display, thehumidity sensor function is normal.



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12 Measurements and AdjustmentsWarning

• For continued protection against radiation hazard, replaceonly with identical parts.

• When the 10 amp fuse is blown due to the operation of theshort switch, you must replace the primary latch switch andshort switch. Then follow the installation procedures below.

• Interlock switch replacement - In replacing faulty switches,be sure mounting tabs are not bent, broken or otherwisedeficient in their ability to hold the switches.

• Refer to the schematic and wiring diagram to ensure properconnection

12.1. Installation of primary latchswitch, secondary latch switchand short switch.

1. When mounting the primary latch switch, secondary latchswitch and short switch to the door hook assembly. Fol-low the instructions in figure 1.NOTE: No specific adjustment during the installation ofeach switch into the door hook is necessary.

2. When mounting the door hook assembly to the ovenassembly, adjust the door hook assembly by moving it inthe direction of the arrow in figure 1. Ensuring the doordoes not have any play in it. Check for play by pulling thedoor assembly. Make sure that the latch keys movesmoothly after adjustment is completed. Completelytighten the screws holding the door hook assembly to theoven assembly.

3. Reconnect the short switch, primary switch and second-ary latch switches and check the continuity of the monitorcircuit and latch switches by following the component testprocedures on page 29.

12.2. Measurement of microwaveoutput

The output power of the magnetron can be determined by per-forming the IEC standard test. However, due to the complexityof the IEC test procedures, it is recommended to test the mag-netron using the simple method outlined below.Necessary equipment:

• 1 litre beaker• Glass thermometer• Wrist watch or stop watch

NOTE: Check the line voltage under load. Low voltage willlower the magnetron output. Take the temperature readingsand heating time as accurate as possible.

1. Fill the beaker with exactly one litre of tap water. Stir thewater using the thermometer and record the beakers tem-perature (Recorded as T1)

2. Place the beaker on the center of the glass cook plate.3. Stir the water again and read the temperature of the bea-

ker (Recorded as T2)4. The normal temperature rise at the high power position

for each model is shown in the table. (Figure 2)

Adjustment of latch switch assembly

Figure 1

TABLE (1L - 1min test)

Figure 2

RATED OUTPUT TEMPERATURE RISE1000W 8°C



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13 Troubleshooting Guide



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Fuses are not blown

Fuses are blown

Checking the semiconductors using a resistance meter



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6A5BPQ

Other problems Inverter circuit troubleshooting

Alternate way to troubleshoot oven with AC Ampere meter used

H95, H97, H98, H99 appears in display window a short time after start key is pressed and no microwave oscillation with AC Ampere

meter used for troubleshooting.

Alternative method for toubleshooting, the inverter circuit



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14 Description of Operating instruction14.1. Auto weight defrost, Auto

weight CookWhen an auto control feature is selected and the start padpressed:

1. The digital programmer circuit determines the power leveland the cooking time and indicates the operating state inthe display. The table shows the corresponding cookingtimes for each category and its respective weight.

2. When the cooking time in the display window haselapsed, the oven turns off automatically via the controlsignal from the digital programmer circuit.Note: After auto cooking if the oven temperature isover the predetermined temperature the fan motorrotates to cool the oven and its components.

Auto Weight Defrost

Category 1st Touch Weight Cooking TimeBread 100g 45sMeat Items 200g 3m 40sMeat Joints 400g 6m 20s

Category 1st Touch Weight Cooking TimeWhole Chicken 1000g 18m 20sChicken Portions 300g 10m 00sBeef / Lamb 500g 14m 40sCasserole 800g 56m 40sChilled Crispy Top 300g 10m 00sFrozen Crispy Top 300g 14m 00sPastry 600g 15m 00sCake 400g 11m 40s



panasonic microwave nnct878 service manual

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