Philips NN-SN968B, NN-SN968BT Servise Manual

© Panasonic Home Appliances Microwave Oven
(Shanghai) Co., Ltd. 2009.

NN-SN968B
NN-SN968BT

APH (USA)

Microwave Oven

ORDER NO.PHAMOS0912075C1

E1

2

NN-SN968B / NN-SN968 BT

3

NN-SN968B / NN-SN968 BT

1 SCHEMATIC DIAGRAM 5

2 DESCRIPTION OF OPERATING SEQUENCE

6

2.1. Variable power cooking control

6

2.2. Inverter power supply circuit

6

2.3. Inverter defrost

6

2.4. Sensor cooking

6

2.5. Sensor reheat

7

2.6. Steam sensor and digital programmer circuit

7

2.7. Thermistor

7

3 CAUTIONS TO BE OBSERVED WHEN TROUBLESHOOTING

8

3.1. Check the grounding

8

3.2. Inverter warnings

8

3.3. Part replacement.

9

3.4. When the 20A fuse is blown due to the malfunction of the

monitor interlock switch:

9

3.5. Avoid inserting nails, wire etc. through any holes in the

unit during operation.

9

3.6. Verification after repair

9

3.7. Sharp edges

9

4 DISASSEMBLY AND PARTS REPLACEMENT PROCEDURE

10

4.1. Magnetron

10

4.2. Digital programmer circuit (D.P.C)

10

4.3. Low voltage transformer and/or power relays (RY1, RY2)

11

4.4. Fan motor

11

4.5. Door assembly

11

4.6. Turntable motor

12

4.7. Steam sensor

12

4.8. Inverter power supply

13

5 COMPONENT TEST PROCEDURE

14

5.1. Primary, Secondary Latch Switch interlocks & Power

Relay RY1

14

5.2. Monitor Interlock Switch

14

5.3. Magnetron

14

5.4. Inverter power supply (U)

15

5.5. Temperature thermistor

15

6 MEASUREMENTS AND ADJUSTMENTS

16

6.1. Adjustment of primary latch switch, secondary latch switch

and monitor interlock switch.

16

6.2. Measurement of microwave output

16

7 PROCEDURE FOR MEASURING MICROWAVE ENERGY

LEAKAGE

17

7.1. Equipment

17

7.2. Procedure for measuring radiation leakage

17

7.3. Record keeping and notification after measurement

17

7.4. At least once a year, have the radiation monitor checked

for calibration by its manufacture r.

18

8 TROUBLESHOOTING GUIDE

19

8.1. (Troubleshooting) Oven stops operation during cooking

19

8.2. (Troubleshooting) Other problems

20

8.3. Troubleshooting of inverter circuit (U) and magnetron

21

8.4. Trouble related to Digital Programmer Circuit

22

8.5. SIMPLE WAY OF H.V. INVERTER/MAGNETRON

TROUBLESHOOTING

23

8.6. H.V.INVERTER BOARD MAIN PARTS LIST

(F606Y8X00AP)

23

8.7. How to check the semiconductors using an OHM meter

24

9 EXPLODED VIEW AND PARTS LIST

25

9.1. EXPLODED VIEW

25

9.2. PARTS LIST

26

9.3. ESCUTCHEON BASE ASSEMBLY

27

9.4. DOOR ASSEMBLY

28

9.5. WIRING MATERIALS

29

9.6. PACKING AND ACCESSOR IES

30

10 DIGITAL PROGRAMMER CIRCUIT

31

10.1. SCHEMATIC DIAGRAM

31

10.2. PARTS LIST

33

CONTENTS

Page Page

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NN-SN968B / NN-SN968 BT

1 SCHEMATIC DIAGRAM

5

NN-SN968B / NN-SN968 BT

2.1. Variable power cooking
control

High Voltage Inverter Power Supply (U) controls output power
by the signal from Digital Programmer Circuit (DPC). Power
relay always stay on, but PWM (Pulse Width Modulation) signal
controls microwave output power.

NOTE:

The ON/OFF time ratio does not correspond with the
percentage of microw ave power since approximately 2
seconds are required for heating of magnetron
filament.

Variable Power Cooking

POWER SETTING OUTPUT

POWER(%)

APPROX.

MANUAL MICROWAV E

DUTY

ON(SEC) OFF(SEC)

HIGH P10 100% 22 0

P9 90% 22 0

P8 80% 22 0
MEDIUM-HIGH P7 70% 22 0
MEDIUM P6 60% 22 0

P5 50% 22 0

P4 40% 22 0
MEDIUM-LOW P3 30% 22 0

P2 20% 15 7

P1 10% 8 14
DEFROST P3 30% 22 0

2.2. Inverter power supply circuit

The Inverter Power Supply circuit powered from the line
voltage, 120V 60Hz AC input supplies 4,000V DC to the
magnetron tube, and functions in place of the H.V. transformer,
the H.V. capacitor and H.V. diode.

1. The AC input voltage 120V 60Hz is rectified to DC voltage
immediately.

2. DC voltage will be supplied to the switching devices called
IGBT. These devices are switched ON-OFF by the 20 to 40
kHz PWM (pulse width modulation) signal from the
microcomputer in the DPC.

3. This drives the High voltage transformer to increase voltage
up to 2,000V AC.

4. Then the half-wave doubler voltage rectifier circuit,
consisting of the H.V. diodes and capacitors, generates the
necessary 4,000V DC needed for the magnetron.

5. Output power of the magnetron tube is always monitored by
the signal output from the current transformer built into the
inverter circuit.

6. This signal is fed back to the microcomputer in the DPC to
determine operating conditions and output necessary to
control PWM signal to the Inverter Power Supply for control
of the output power.

2.3. Inverter defrost

When the Auto Control feature is selected and the Start button
is tapped:

1. The digital programer circuit determines the power level and
cooking time to complete cooking and indicates the
operating state in the display window. Table shows the
corresponding cooking times for respective serving by
categories.

Inverter Turbo Defrost

SELECTED WEIGHT COOKING TIME

1.0 LB 4 min.45 sec.

2. When cooking time in the display window has elapsed, the
oven turns off automatically by a control signal from the
digital programmer circuit.

2.4. Sensor cooking

Auto sensor cooking without setting a power level or selecting
a time. All that is necessary is to select an Auto Sensor
Program before starting to cook.

Understanding Auto Sensor Cooking

As the food cooks, a certain amount of steam is produced.
If the food is covered, this steam builds up and eventually
escapes from the container. In Auto Sensor Cooking, a
carefully designed instrument, called the steam sensor
element, senses this escape of steam. Then, based upon
the Auto Sensor Program selected, the unit will
automatically determine the correct power level and the
proper length of time it will take to cook the food.

NOTE:

Auto Sensor Cooking is successful with the foods and
recipes found in the Auto Sensor Cooking Guide.
Because of the vast differences in food composition,
items not mentioned in the Cooking Guide should be
prepared in the microwave oven using power select
and time features. Please consult Variable Power
Microwave Cookbook for procedures.

2 DESCRIPTION OF OPERATING SEQUENCE

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NN-SN968B / NN-SN968 BT

Explanation of the Auto Sensor Cooking process

1. During the first 10 second period there is no microwave
activity. When calculating the T2 time by using the
formula below make sure this 10 seconds is subtracted
from the T1 time. In other words, T1 time starts at the
end of the 10 second period.

2. T1 time The total amount of time it takes the microwave
oven to switch to T2 time after the 10second period.

3. T2 time When the steam escapes from the cooking
container placed in the oven, the steam sensor detects
it and the microprocessor calculates the balance of
cooking time. This T2 time is then shown in the display
and begins counting down.

Balance of cooking time (T2 time)

The balance of cooking time which is called T2 time,
can be calculated by the following formula.
T2 time (in sec.) = T1 time X K factor

NOTE:

Remember, the T1 time starts after the 10 second
period. The coefficient K is programmed into the
microprocessor memory and they are listed in the
following tables along with the P1 and P2 powers.

NOTE:

When "More" or "Less" button is selected, the K factor
varies resulting in T2 time to be increased or decreased.

Example of calculating the T2 time

Example 1: If the T1 time is measured to be 2 minutes and
40 seconds after the 10 second period, and the Auto
program selected is Oatmeal:

T2 = T1 × K

= 2 min. and 40 sec. × 0.1

= 160sec. × 0.1

= 16 sec.

Category P1

Power

P2

Power

K Factor

Standard

Omelet MEDIUM MEDIUM 0.6

2.5. Sensor reheat

Auto Sensor Reheat is a quick and easy way to reheat
refrigerated and room temperature foods.

Simply press the reheat button. There is no need to select
power level and cooking time.

NOTE:

The Auto Sensor Reheat process is same as Auto Sensor
Cooking process.

Category P1

Power

P2

Power

K Factor

Standard

Sensor Reheat HIGH MEDIUM 0.2

2.6. Steam sensor and digital
programmer circuit

In order to determine if the steam sensor function of the digital
programmer circuit is working, do the following test.

1. Place a water load (150 cc) in the oven.

2. Tap Sensor Reheat button.

3. Tap Start button.

4. Steam Sensor detects steam about 1.5 to 4 minutes after

the Start button is tapped.

5. T1 time cooking automatically switches to remaining time

for cooking (T2).

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

window. If the following cooking time appears, Steam
Sensor function is normal.

T1 TIME T2 TIME (Remainingcooking time)

1 Min. 30 Sec. ~ 4 Min. 27 Sec. ~ 1Min.12 Sec.

2.7. Thermistor

The thermistor which fixed on magnetron detects magnetron
temperature and make power down when the temperature
becomes abnormal high. A normal thermistor´s resistan ce
values for temperature ranges between 75-85 degree F is from
37kΩ to 57kΩ.

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NN-SN968B / NN-SN968 BT

Unlike many other appliances, the microwave oven is a high
voltage, high current device. Though it is free from danger in
ordinary use, extreme care should be taken during repair.

CAUTION

Servicemen should remove their watches and rings
whenever working close to or replacing the magnetron.

3.1. Check the grounding

Do not operate on a two wire extension cord. The microwave
oven is designed to be grounded when used. It is imperative,
therefore, to ensure the appliance is properly grounded before
beginning repair work.

3.2. Inverter warnings

WARNING HIGH VOLTAGE AND HIGHTEMPERATURE

(HOT/LIVE) OF THE INVERTERPOWER SUPPLY (U)

The High Voltage Inverter Power Supply generates very
high voltage and current for the magnetron tube. Though it
is free from danger in ordinary use, extreme care should be
taken during repair.

The aluminum heat sink is also energized with high voltage
(HOT), do not touch when the AC input terminals are
energized. The power device Collector is directly connected
to the aluminum heat sink.

The aluminum heat sink may be HOT due to heat energy,
therefore, extreme care should be taken during servicing.

H.V. Inverter warning

WARNING FOR INVERTER POWER SUPPLY (U)

GROUNDING

Check the High Voltage Inverter Power Supply circuit
grounding. The high voltage inverter power supply circuit
board must have a proper chassis ground. The inverter
grounding plate must be connected to the chassis. If the
inverter board is not grounded it will expose the user to very
high voltages and cause extreme DANGER! Be sure that
the inverter circuit is properly grounded via the inverter
grounding plate.

Grounding of the inverter circuit board

WARNING DISCHARGE THE HIGH VOLATGE

CAPACITORS

For about 30 seconds after the oven is turned off, an
electric charge remains in the high voltage capacitors of the
Inverter Power Supply circuit board.

When replacing or checking parts, remove the power plug
from the outlet and short the inverter output terminal of the
magnetron filament terminals to the chassis ground with an
insulated handle screwdriver to discharge. Please be sure
to contact the chassis ground side first and then short to the
output terminal.

3 CAUTIONS TO BE OBSERVED WHEN

TROUBLESHOOTING

8

NN-SN968B / NN-SN968 BT

Discharging the high voltage capacitors

WARNING

There is high voltage present with high current capabilities
in the circuits of the primary and secondary windings, choke
coil and heat sink of the inverter. It is extremely dangerous
to work on or near these circuits with the oven energized.
DO NOT measure the voltage in the high voltage circuit
including the filament voltage of the magnetron.

WARNING

Never touch any circuit wiring with your hand or with an
insulated tool during operation.

3.3. Part replacement.

When troubleshooting any part or component is to be replaced,
always ensure that the power cord is unplugged from the wall
outlet.

3.4. When the 20A fuse is blown
due to the malfunction of the
monitor interlock switch:

WARNING

When the 20A 120V fuse is blown due to the malfunction of
the monitor interlock switch, replace all of the components
(primary latch switch, monitor interlock switch and power
relay RY1).

1. This is mandatory. Refer to “measurements and
adjustments” for the location of these switches.

2. When replacing the fuse, confirm that it has the
appropriate rating for these models.

3. When replacing faulty switches, be sure the mounting
tabs are not bent, broken or deficient in their ability to
hold the switches.

3.5. Avoid inserting nails, wire etc.
through any holes in the unit
during operation.

Never insert a wire, nail or any other metal object through the
lamp holes on the cavity or any holes or gaps, because such
objects may work as an antenna and cause microwave
leakage.

3.6. Verification after repair

1. After repair or replacement of parts, make sure that the
screws of the oven, etc. are neither loosen or missing.
Microwave energy might leak if screws are not properly
tightened.

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

3. Check for microwave energy leakage. (Refer to procedure
for measuring microwave energy leakage).

CAUTION OF MICROWAVE RADIATION LEAKAGE

USE CAUTION NOT TO BECOME EXPOSED TO
RADIATION FROM THE MICROWAVE MAGNETRON OR
OTHER PARTS CONDUCTING MICROWAVE ENERGY.

IMPORTANT NOTICE

1. The following components have potentials above 2000V

while the appliance is operated.

• 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 to these areas.

2. When the appliance is operated with the door hinges or

magnetron installed incorrectly, the microwave leakage
can exceed more than 5mW/cm

2

. After repair or
exchange, it is very important to check if the magnetron
and the door hinges are correctly installed.

3.7. Sharp edges

CAUTION

Please use caution when disassembling or reassembling
internal parts. Some exposed edges may be sharp to the
touch and can cause injury if not handled with care.

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NN-SN968B / NN-SN968 BT

4.1. Magnetron

1. Discharge high voltage charge.

2. Remove 1 screw holding air guide A on the oven cavity.

3. Remove 1 screw holding air guide B.

4. Remove 1 screw holding themistor.

5. Disconnect 2 high voltage lead wires from magnetron
filament terminals.

6. Remove 4 screws holding the magnetron.

NOTE:

After replacement of the magnetron, tighten mounting
screws properly in an x pattern, making sure there is no
gap between the waveguide and the magnetron to
prevent microwave leakage.

CAUTION

When replacing the magnetron, be sure the antenna gasket is in
place.

4.2. Digital programmer circuit
(D.P.C)

CAUTION:

Be sure to ground any static electric charge built up in
your body before handling the DPC.

1. Disconnect all connectors from D.P.C.

2. Remove 1 grounding screw.

3. Remove 1 screw holding escutcheon base and slide the

escutcheon base upward slightly.

4. Remove all screws holding D.P.C. board on escutcheon

base.

5. Separate D.P.C board from tabs on the escutcheon base

and remove D.P.C board.

4 DISASSEMBLY AND PARTS REPLACEMENT

PROCEDURE

10

NN-SN968B / NN-SN968 BT