R-28STM - Sharp Home Appliance Service Manual (repair manual)

Model
R-28STM
Pages
23
Size
4.21 MB
Type
PDF
Document
Service Manual
Brand
Device
Home Appliance / Microwave Oven
File
r-28stm.pdf
Date

View Sharp R-28STM Service Manual online

CHAPTER 1. SAFTY PRECAUTIONS
CHAPTER 2. CAUTIONS
CHAPTER 3. THE HEATING PRINCIPLE OF MICRO-
CHAPTER 4. INSTALLATIONS
CHAPTER 5. SCHEMATIC DIAGRAM
CHAPTER 6. CIRCUIT DESCRIPTION
CHAPTER 7. TOOLS AND MEASURING INSTRUMENTS
CHAPTER 8. MICROWAVE LEAAGE TEST
CHAPTER 9. MEASUREMENT OF MICROWAVE 
CHAPTER 10. TROUBLE SHOOTING PROCEDURES
CHAPTER 11.  
CHAPTER 12. IMPORTANT THINGS TO DO PRIOR
CHAPTER 13. SAFTY CHEC  S AND TESTS AFTER  
CHAPTER 14. 
Parts List
MICROWAVE OVEN
SERVICE MANUAL
MODEL
CONTENTS
SHARP CORPORATION
In interests of user-safety the oven should be restored to its
original condition and only parts identical to those specified
should be used.
R-28STM
WAVE
POWER OUTPUT
COMMON BREAKDOWN AND MEANS OF 
PREPARING
CRITICAL PART SERVICING
K
SERVING
EXPLODED VIEW
(800W 240V50Hz)
R-28STM
 
 
 
 
 
 
 
 
 
 
SAFETY PRECAUTIONS
 
 
This device is to be serviced only by properly qualified service personnel. 
Consult the service manual for proper service procedures to assure continued safety operation and for precautions to 
be taken to avoid possible exposure to excessive microwave energy. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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 should be performed on each oven prior to release to the owner. 
CAUTION 
MICROWAVE RADIATION 
DO NOT BECOME EXPOSED TO RADIATION FROM THE MICROWAVE GENERATOR 
OR OTHER PARTS CONDUCTING MICROWAVE ENERGY. 
 
 
 
 
 
 
 
 
 
TABLE OF CONTENTS 
 
 
 
                                                                                                                                                (page) 
SAFETY PRECAUTIONS------------------------------------
---------------
------Inside front cover 
CAUTIONS------------------------------------------------------------------------------------------------1-1 
THE HEATING PRINCIPLE OF MICROWAVE--------------------------------------------------2-1 
INSTALLATIONS----------------------------------------------------------------------------------------3-1 
OPERATING INSTRUCTIONS----------------------------------------------------------------------4-1 
SCHEMATIC DIAGRAM--------------------------------------------------------------------------4-1 
CIRCUIT DESCRIPTION-------------------------------------------------------------------------4-2 
SERVICE INFORMATION-----------------------------------------------------------------------------5-1 
TOOLS AND MEASURING INSTRUMENTS------------------------------------------------5-1 
MICROWAVE LEAKAGE TEST----------------------------------------------------------------5-2 
MEASUREMENT OF MICROWAVE POWER OUTPUT---------------------------------5-3 
TROUBLE SHOOTING PROCEDURES------------------------------------------------------5-4 
COMMON BREAKDOWN AND MEANS OF REPAIRING-------------------------------5-8 
IMPORTANT THINGS TO DO PRIOR TO CRITICAL PART SERVICING-------------------------5-9 
SAFETY CHECKS AND TESTS AFTER SERVING--------------------------------------5-10 
EXPLODED VIEW---------------------------------------------------------------------------------------6-1 
REPLACEMENT PARTS LIST-----------------------------------------------------------------------6-1
 
 
 
 
 
 
 
 
 
 
CAUTIONS 
 
 
 
 
• DO NOT operate on a 2-wire extension cord during 
repair and use. 
• NEVER TOUCH any oven components or wiring during 
operation. 
• BEFORE TOUCHING any parts of the oven, always 
remove the power plug from the outlet.   
• For about 30 seconds after the oven stops, an electric 
charge remains in the high voltage capacitor. When 
replacing or checking, you must discharge the high 
voltage capacitor by shorting across the two terminals 
with an insulated screwdriver. 
 
 
 
• Remove your watches whenever working close to or 
replacing the Magnetron. 
• DO NOT touch any parts of the control panel circuit. A 
resulting static electric discharge may damage this 
P.C.B.  
• NEVER operate the oven with no load. 
• NEVER injure the door seal and front plate of the oven 
cavity. 
• NEVER put iron tools on the magnetron. 
• NEVER put anything into the latch hole and the 
interlock switches area.` 
 
 
 
 
 
 
 
Unlike other appliances, the microwave oven 
is 
high-voltage and high-current equipment. 
Though it is free from danger in ordinary use,
extreme care should be taken during repair. 
MICROWAVE RADIATION 
Personnel should not be exposed to the 
microwave energy which may radiate from the 
magnetron or other microwave generating 
device if it is improperly used or connection. 
All input and output microwave connections, 
waveguide, flange and gasket must be secure 
never operate the device without a microwave 
energy absorbing load attached. 
Never look into an open waveguide or antenna 
while the device is energized. 
• Proper operation of the microwave oven 
requires that 
the magnetron be assembled to the waveguide 
and 
cavity. Never operate the magnetron unless it is
properly installed.   
• Be sure that the magnetron gasket is 
properly 
installed around the dome of the tube 
whenever 
installing the magnetron. 
 
THE OVEN IS TO BE SERVICED 
ONLY 
BY PROPERLY QUALIFIED SERVICE
PERSONNEL.
 
1-1 
 
 
 
THE HEATING PRINCIPLE OF MICROWAVE 
Microwave is one kind of radio wave whose wave length is very short, frequency is very high. Therefore, it is called ultrahigh 
frequency electromagnetic wave. Microwave can heat food mainly result in the mutual affect of the food in the microwave field and 
microwave field itself. 
Under the affect of microwave field, the thermal effect mechanism produced from the mutual affect of the microwave and the food 
includes two aspects: One is Dielectric loss of polar molecule, the other is conductive loss of ion. 
Usually, food is constitute of organism (plant and animal).The organism is formed by all kinds of polar water molecule, polar 
protein molecule, and all sorts of salt ion. The center of gravity of the positive and negative charge in the molecule is not coincide. 
In normal condition, the molecule is in irregular order due to its thermal action, thus the food do not appear polarity (FIG.1-
1a).Under the action of outer electric field, the positive end of the polar molecule trend to the negative electric field, the negative 
end of polar molecule trend to the positive electric field, and somewhat arrange in order through the direction of the electric 
field(FIG.1-1c).This phenomenon usually be called “TORQUE POLARITY”. When the outer electric field apply for the opposite 
polarity, the polar molecule then arrange an opposite direction order accordingly (FIG.1-1b).If the direction of the outer electric field 
changed repeatedly, the polar molecule would repeatedly sway accordingly. During the swaying, it understands that the polar 
molecule would produce heat due to somewhat similar friction among them.   
 
 
 
 
 
 
 
 
When the electric field is applied for ultrahigh frequent microwave field from the outside, its direction would change tens billion 
times per second, so do the molecule. This kind of molecule swaying producing similar frictional heat from the interference and 
block of the action strength among the molecule, and changed to microscopic microwave heating. Microwave heating not only 
concerned the nature of the matter itself, but also closely connected with the electric field strength and frequency. When the 
frequency is low, the molecule swaying rate and the acute degree of the mutual friction among the molecule is low, and would 
produce much heat. When the frequency is too high, as the swing of the polar molecule is with rotating inertia, it made the swing 
do not in line with the changing rhythm of the electric field because of the friction drag, thus, actually lowed the polar molecule 
swaying speed. The friction dragging degree is concerning about the magneto electric wave frequency, polar molecule shape, and 
the matter’s sticky degree. To different matter’s molecule, there are different special frequency zone. Those ado
 
absorb microwave 
energy from these zone are most capable to turn microwave energy to heat energy. 
Apart from the above said action, there is another action which is electric ion under the action of microwave field, act fiercely 
accompanied with the acceleration of electric field. The positive ion transfer to the negative polarity of the field while the negative 
ion do opposite. Accompanying with the changing electric field, the electric ion changing accordingly. During the transferring, heat 
produced with the crash among the ion. This kind of action take the main effect to those microwave heating of high salt molecule. 
No matter it is the polar molecule swaying or the ion transferring, they both are turning the microwave energy which the heating 
matter got from the microwave field to hear energy. From the analysis of theory, we can draw such a conclusion that the power 
which a unit of volume matter absorbed from the microwave field as the following formula: 
Pa=K·E· F·Er·tgδ 
Pa Stands for the power the heated matter absorbed from the microwave field.
 
K Stands for a constant 
E Stand for microwave field strength 
F Stands for the microwave frequency. 
Er Stands for relative dielectric constant of the heated matter.
 
tgδStands for loss angle tangent of the heated matter 
2-1
 
 
 
INSTALLATIONS 
 
INSTALLING 
1. Empty the microwave oven and clean inside it with 
a soft, damp cloth. Check for damage such as 
misaligned door, damage around the door or dents 
inside the cavity or on the exterior. 
 
2. Put the oven on a counter, table, or shelf that is 
strong enough to hold the oven and the food and 
utensils you put in it. (The control panel side of the 
oven is the heavy side. Use care when handling.) 
 
3. Do not block the vent and the air intake openings. 
Blocking vent or air intake openings can cause 
damage to the oven and poor cooking results. 
Make sure the microwave oven legs are in place to 
ensure proper air flow. 
 
4. The oven should not be installed in any area where 
heat and steam are generated, because they may 
damage the electronic or mechanical parts of the 
unit. 
Do not install the oven next to a conventional 
surface unit or above a conventional wall oven. 
 
5. Use microwave oven in an ambient temperature 
less than 104°F(40°C). 
 
6. Place the microwave oven on a sturdy and flat 
surface at least 10 cm(4 inches) from the wall. 
 
7. Place the microwave oven as far away as possible 
from TV, RADIO, COMPUTER, etc., to prevent 
interference. 
 
 
BEFORE YOU BEGIN, READ THE FOLLOWING INSTRUCTIONS COMPLETELY 
EARTHING INSTRUCTIONS 
This microwave oven is designed to be used in a fully 
earthed condition. 
It is imperative, therefore, to make sure it is properly 
earthed before servicing 
WARNING-   
THIS  APPLIANCE 
MUST BE EARTHED 
IMPORTANT
 
 
The wires in this mains lead are colored in 
accordance with the following code: 
Green-and-yellow:              Earth 
Blue:                                      Neutral 
Brown:                                  Live
 
As the colors of the wires in the mains lead of this 
appliance may not correspond with the colored 
markings identifying the terminals in your plug, 
proceed as follows. 
 
The wire which is colored 
green-and-yellow must be 
connected to the terminal in the plug which is marked 
with the letter 
or by the earth symbol (
 ) or 
colored 
green or green-and-yellow
 
The wire which is colored 
blue must be connected to 
the terminal in the plug which is marked with the letter 
or colored black
 
The wire which is colored 
brown must be connected 
to the terminal in the plug which is marked with the 
letter 
or colored red
3-1 
SCHEMATIC DIAGRAM
TCO.
 
 
4-2 
 
 
CIRCUIT DESCRIPTION
 
 
 
 
 
 
 
GENERAL DETAILS
• The low voltage transformer supplies the necessary
voltage to the micom controller when power cord is
plugged in.
• When the door is closed, the primary switch is ON, the
secondary switch is ON, and the monitor switch opens
(contact COM and NO).
WHEN SELECTING COOKING POWER
LEVEL AND TIME
• The micom controller memorizes the function you set.
• The time you set appears in the display window.
• Each indicator light turns on to indicate that the stage
has been set.
WHEN TOUCHING THE START PAD
• The coil of the relay is energized by the micom
controller.
• Power input is supplied to the high voltage transformer
through the fuse to the primary switch and relay 2.
• Turntable rotates.
• The fan motor rotates and cools the magnetron by
blowing the air (coming from the intake on the
baseplate)
• The air is also directed into the oven to exhaust the
vapor in the oven through the upper plate.
• Cooking time starts counting down.
• 3.2 volts AC is generated from the filament winding of
the high voltage transformer. This 3.2 volts is applied
to the magnetron to heat the magnetron filament
through two noise preventing choke coils.
• A high voltage of approximately 2100 volts AC is
generated in the secondary of the high voltage
transformer which is increased by the action of the high
voltage diode and charging of the high voltage
capacitor.
• The negative 4,000 Volts DC is applied to the filament
of the magnetron.
WHEN THE OVEN IS SET AT ANY LEVEL
EXCEPT MAXIMUM.
• The micom controller controls the ON-OFF time of relay
2 by the applied signal to vary the average output 
power of microwave oven as POWER LEVEL.
WHEN THE DOOR IS OPENED DURING
COOKING
• Both the primary switch and relay 2 are cut off primary
winding voltage of the high voltage transformer.
• ON-OFF of relay 2 is coupled electrically with opening
and closing of the secondary switch.
• When the door is opened, the secondary switch is
opened and when the door is closed, the secondary
switch is closed.
• The cooking time stops counting down.
• Relay stops functioning.
• As the door is opened, if the contact of primary switch
and relay 2 and/or secondary switch fails to open, the
fuse opens due to the large current surge caused by
the monitor switch activation, which in turn stops
magnetron oscillation.
L
FUSE
TRANS-
FORMER
RELAY 2
MICOM CONTROLLER
SECONDARY
SWITCH
SWITCH
N
MONITOR
SWITCH
PRIMARY
MONITOR
SWITCH
L
FUSE
RELAY 2
MICOM CONTROLLER
SECONDARY
SWITCH
PRIMARY
SWITCH
N
TRANS-
FORMER
 
 
 
SERVICE INFORMATION 
TOOLS AND MEASURING INSTRUMENTS
 
NECESSARY TOOLS 
Tools normally used for TV servicing are sufficient. 
Standard tools are listed below. 
• Diagonal pliers 
• Long nose pliers 
• Phillips screwdriver 
• Flat blade screwdriver 
• Wrench (size 5mm) 
• Nutdriver (size 5mm) 
• Adjustable wrench 
• Soldering iron 
• Solder 
• Vinyl insulation tape 
• Polishing cloth 
 
 
MICROWAVE LEAKAGE TEST
 
CAUTIONS 
• Be sure to check microwave leakage prior to 
servicing the oven if the oven is operative prior to 
servicing. 
• The service personnel should inform the 
manufacture importer, or assembler of any certified 
oven unit found to have a microwave emission 
level in excess of 5 mW/cm
2
 
and should repair any 
unit found to have excessive emission levels at no cost 
to the owner and should ascertain the cause of the 
excessive leakage. The service personnel should 
instruct the owner not to use the unit until the oven has 
been brought into compliance. 
• If the oven operates with the door open, the service 
personnel should: 
- Tell the user not to operate the oven. 
- Contact the manufacturer. 
• The service personnel should check all surface and 
vent openings for microwave leakage. 
• Check for microwave leakage after every servicing. The 
power density of the microwave radiation leakage 
emitted by the microwave oven should not exceed 
5 mW/cm
2
. Always start measuring of an unknown field 
to assure safety for operating personnel from radiation 
leakage. 
 
NECESSARY MEASURING 
INSTRUMENTS 
• TESTER(VOLTS-DC, AC., Ohmmeter) 
• Microwave survey meter 
- Holaday HI-1710(A) 
- Narda 8100 
8200 
• Inch scale 
• 600 cc non conductive material beaker (glass or 
plastic), inside diameter: approx. 8.5 cm(3
1/2 
in.) 
• Cylindrical and made of borosilicate glass vessel. 
max. thickness: 3 mm 
outside diameter: approx. 190mm 
height: approx. 90mm 
• Glass thermometer: 100°C or 212°F (1 deg scale) 
MEASURING MICROWAVE ENERGY 
LEAKAGE 
• Pour 275±15 cc of 20±5°C(68±9°F) water in a 
beaker 
which is graduated to 600 cc, and place the beaker 
on the center of the turntable. 
• Set the energy leakage monitor to 2,450 MHz and 
use it following the manufacturer's recommended 
test procedure to assure correct result. 
• When measuring the leakage, always use the 2-
inch (5 cm) spacer supplied with the probe. 
• Operate the oven at its maximum output. 
• Measure the microwave radiation using and 
electromagnetic radiation monitor by holding the 
probe perpendicular to the surface being measured 
Move probe along shaded area 
Probe scanning speed: Less than 2.5 cm/sec 
 
5-1 
 
 
 
 
MEASUREMENT WITH OUTER CASE 
REMOVED 
• When you replace the magnetron, measure for 
microwave energy leakage before the outer case is 
installed and after all necessary components are 
replaced or adjusted. 
Special care should be taken in measuring the 
following parts. 
- Around the magnetron 
- The 
waveguide 
 
 
 
 
 
 
 
 
MEASUREMENT WITH A FULLY 
ASSEMBLED OVEN 
• After all components, including the outer case, are fully 
assembled, measure for microwave energy leakage 
around the door viewing window, the exhaust opening, 
and air inlet openings. 
• Microwave energy leakage must not exceed the values 
prescribed below. 
NOTE: Leakage with the outer case removed less than 
3 mW/cm
2
. Leakage for a fully assembled 
oven (Before the latch switch (primary) is 
interrupted) with the door in a slightly opened 
position-less than 1 mW/cm
2
 
 
 
 
 
 
 
 
 
 
 
 
 
 
NOTES WHEN MEASURING
 
• Do not exceed meter full scale deflection. 
• The test probe must be removed no faster than 
1 inch/sec (2.5 cm/sec)
 
along the shaded area, 
otherwise a false reading may result. 
• The test probe must be held with the grip portion of 
the 
handle. 
A false reading may result if the operator's hand is 
between the handle and the probe. 
• When testing near a corner of the door, keep the 
probe 
perpendicular to the surface making sure the probe 
horizontally along the oven surface, this may possibly 
cause probe damage. 
RECORD KEEPING AND NOTIFICATION 
AFTER MEASUREMENT 
• After adjustment and repair of any microwave energy 
interruption or microwave energy blocking device, 
record the measured values for future reference. Also 
enter the information on the service invoice. 
• The microwave energy leakage should not be more 
than 1.0 mW/cm
2
. after determining that all parts are in 
good condition, functioning properly and genuine 
replacement parts which are listed in this manual have 
been used. 
• At least once a year, have the electromagnetic energy 
leakage monitor checked for calibration by its 
WARNING : AVOID CONTACTING ANY
HIGH VOLTAGE PARTS 
(Magnetron, H.V. Transformer, 
H.V. Capacitor, H.V. Cable Ass’y, 
H.V. Fuse)
 
5-2 

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Here you can view online and download Sharp R-28STM Service Manual in PDF. R-28STM service manual will guide through the process and help you recover, restore, fix, disassemble and repair Sharp R-28STM Microwave Oven. Information contained in service manuals typically includes schematics / circuit diagrams, wiring diagrams, block diagrams, printed wiring boards, exploded views, parts list, disassembly / assembly, pcb.