Sealed containers, such as plastic bottles, cups with screw-on lids,

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developed temper colors, it can be cleaned with alcohol as described above. A darkened radiator needs to be replaced with a new factory one; it simply pulls out of the magnetron. The old radiator, seized in its socket, should be very carefully worked loose with small pliers, and the new one should be installed by hand wearing a latex glove, to avoid soiling or scratching it.

There are three subtleties here. First, never remove the magnetron yourself. Second, don't try to extend the life of a burned-through (blown) radiator by flipping it over. In either case the oven will fall out of its proper mode and the "leakage" won't go away. Third, after any repair during which you touched the waveguide path even with a finger, be sure to check the microwave for shielding and microwave leakage, as described above.

1. Microwave technology – advantages

• High cooking speed saves time
• Food retains its natural flavor, which improves the taste of the dish
• Healthy food, prepared without the use of oils and fat, retains all its vitamins
• Energy savings, no need for lengthy heating
• Fast and easy defrosting of food
• The dish is cooked in the serving dish itself, so less cookware needs washing
• The oven is easy to use and clean

Frequencies

The frequency level is set by the number of electromagnetic wave oscillations per second.

Classification of frequency ranges:

Broadcast frequencies and microwave frequencies within the ultra-high-frequency range are established by international agreements.

To avoid any outside interference and interference, a frequency of 2450 MHz was established for microwave ovens.

Radiation range (ionizing – non-ionizing)

Fig.1 Electromagnetic wave spectrum

Microwave ovens operate in the non-ionizing range. As a result, they are not radioactive either. Radioactivity can only occur at significantly higher frequencies.

The microwave effect acts exclusively on heating water molecules, which, regardless of the oven's power, will not exceed a temperature of 100°C

2. Basic principles.

Operating principle of microwave ovens.

Microwave ovens for reheating food use electromagnetic waves, which are also known from radio and television broadcasting. A frequency of 2450 MHz means that the emitted energy completes 2,450,000,000 oscillations per second. The figure below clearly shows how microwave energy interacts with various materials, in this case – with different types of cookware.

Fig. 2 Interaction of microwave radiation with various materials

Microwave energy is reflected by metal surfaces. It is precisely this feature of microwave radiation that is used on the inner surface of the oven to improve the distribution of electromagnetic waves. However, microwave radiation will not be able to reach the dish itself if you cook it in metal cookware. This can lead to fire or, if the waves have a reverse effect on the magnetron, damage to its cathode.

For this reason, never run an empty microwave oven!!

Interpenetration

Many materials are able to let electromagnetic waves pass through without being affected by them in any way (glass, porcelain, ceramics, plastic (only if specifically intended for microwave ovens!), and paper/cardboard (dry)). These materials are suitable for use in microwave ovens: electromagnetic waves penetrate the food without losses.

Absorption

Liquids and almost any organic food absorb electromagnetic waves and convert their energy into heat during cooking. Depending on the liquid content of the food, this effect will vary.

Sealed containers, such as plastic bottles, cups with screw-on lids, and especially eggs in their shells, are not suitable for reheating in a microwave oven!

3. Cookware suitable for use in the oven

+ (suitable) – (not suitable)

Notes:

1) Except for dishware with silver, gold, platinum, or metallic decorations

2) This does not include dishware coated with a glass layer containing metal

3) Please pay attention to the maximum allowable temperature specified by the manufacturer.

Closed containers, such as bottles, closed glasses with contents, and eggs, are not intended for use in a microwave oven!!!

4. Electromagnetic energy conversion

	Any food contains positively and negatively charged liquid molecules that are in constant chaotic motion.
	If you now place this food in an electromagnetic field, its molecules will instantly align according to the +/- direction of the energy flow in that field. If you change the direction of the energy in the field, the order of the molecules in the product will change accordingly
	If this change in the field occurs quickly, the molecules will also move at a corresponding speed. As a result, friction occurs between the molecules and they generate heat. A microwave oven operates at a frequency of 2450 MHz, so the number of such changes in the electromagnetic field equals 4,900,000,000 per second. This is how the heat needed for cooking is generated.

Absorption

The thermal reaction arising from the interaction of microwave energy and water molecules weakens after penetrating into the food. This effect is measured in Watts/cm2.

Depending on the level of the food's electrical non-conductivity, the energy level decreases with increasing penetration depth.

On the left is a graphic showing the loss of microwave energy power when interacting with food. Thermal throughput decreases from the surface toward the interior of the product. Therefore, it is recommended to reduce the power if, for example, you are cooking meat cut into large pieces, and to increase the cooking time to allow the thermal energy to penetrate deeper into the product. For the same reason, various liquids – for example, soups – should be stirred. This will help distribute the thermal energy and avoid overheating the surface. In an extreme case, there is a risk that such overheating of the liquid could cause damage to the upper edge of the dishware used, if it is a plastic container.

5. Path of electromagnetic waves

Wave propagation channel

The electromagnetic waves emitted by the magnetron are directed into the interior cavity of the oven through a special metal waveguide.

Wave stirrer

The wave stirrer (diffuser) is made of reflective metal and is in continuous motion. Since its position is constantly changing, the reflection and propagation of the waves is also unstable. The wave stirrer can take different shapes, but in most cases it is a propeller.

A motor drives it. In older oven models, the effect of moving air from the fan's operation was also partly used to drive the wave stirrer.

Fig. 3 Oven with wave stirrer:

Fig. 4 Oven without a wave stirrer; a special energy feed inlet and a rotating platform ensure uniform distribution of the electromagnetic waves.

Different ways of feeding and distributing electromagnetic waves.

Fig. 5 Using a rotating platform ensures uniform distribution of the waves for cooking food.

6. Microwave oven power

The magnetron cannot adjust its power smoothly. During operation it is always engaged at 100% of its capacity. To control the power, the magnetron must operate in pulses; in other words, the desired power is achieved by periodically switching the magnetron on/off for varying intervals of time. This same operating principle is found in various electric ovens.

Fig. 6 Principle of magnetron power control

Fig. 7 Repeated short-duration mode of magnetron operation

Duration of the standard time cycle is 29/30 sec (example: 29 sec)

If the required power level is 48(50)%, this means that the magnetron remains on for 14(1) seconds and off for 15.

Time cycles may differ slightly across various electronic timers, but the operating principle always remains unchanged!

Fig. 8 Energy distribution in a microwave oven.

Pay attention to this important note!

Never turn on the oven if there is no food inside capable of absorbing the energy. If microwave energy is fed into an empty oven, the electromagnetic waves will find an outlet for that energy, but in an unfavorable location.

The result will be sparks and scorched areas. In the worst case, the anode antenna in the magnetron may be damaged or destroyed!

7. Control panel

Mechanical timer 1. Power 2. Timer 3. Door opening

Electronic control 1. Display 2. Power 3. Timer 4-8. Start button and functions 9. Door opening

Shown here is the mechanical timer, replaced with an electronic one with a display. In the appliance's operating manuals you will find the specific features and operating programs of the oven.

The basic functions of microwave ovens are, in general, the same.

8. Metal objects in the oven: useful tips

	When cooking liquid foods, particularly when reheating or in tall narrow containers, a so-called delayed boiling effect may occur. In this case the liquid is in a boiling state, but the usual air bubbles do not rise from the bottom. A slight jolt, for example when you remove the container from the oven, can cause the contents to suddenly boil over as a result of a chemical reaction, and the user may be scalded by liquid escaping over the edge. To avoid this, it is recommended to place, for example, a teaspoon in the container while it is boiling. This way, the delayed boiling effect can be avoided and the liquid will boil "normally".
	Foods with a food casing, such as sausages and tomatoes, tend to burst during cooking due to internal formation and accumulation of steam. To avoid this unpleasant effect, the food should be pricked slightly with a fork beforehand. As a result, the steam will escape and internal pressure will not build up, and in most cases the skin will remain intact. However, if you use a high power level - which means an accelerated cooking mode - there is a chance that bursting of the product's skin cannot be avoided, even if pricked with a fork.
	Ready meals in aluminum packaging can also be reheated in a microwave oven. In this case you should keep in mind that the microwave energy will only act on the upper layers of the food, and as a result the time needed to prepare the food will increase. Moreover, a distance of 2 cm should be maintained between the plate and the wall of the microwave oven to avoid arcing.
	Wrap thin parts of the food, such as the tips of poultry legs, in foil; this will shield the product from direct contact with the electromagnetic waves and protect it from drying out.
	Browning dishware is coated with a special absorbing material. Thanks to this absorption, the dishware heats up and additionally functions as a frying pan. With the grill function enabled, you can brown the product, for example when preparing pizza or vegetable pie, on top and bottom simultaneously. As a rule, browning dishware needs 2-3 minutes to heat up to maximum temperature before food is placed in it.
Closed containers, such as bottles, threaded glass jars, as well as eggs (in the shell), are not intended for use in a microwave oven!!! Use available lids to retain moisture, but do not overdo it! When reheating a product, remember that its interior may reach a temperature of - at minimum - 70°C. However, never use liquid or mercury thermometers in the oven!!!

9. Electrical circuit diagram of a microwave oven with timer control function

Fig. 9 Simplified electrical circuit diagram of a microwave oven.

10. Mechanical safety system

Fig. 10 Microwave energy leakage protection

The task of this system is to suppress RF energy leakage between the oven body and the door.

• Blocking mesh filter, whose size and structure depend on the length of the microwave oven.
• Door seal made of magnetic material to thereby absorb any possible residual radiation.

Electromagnetic safety system

«Interlock – microswitch system»

-Primary

-Secondary

-Monitor

When the door is opened, all three microswitches are actuated by at least 2 mechanical systems. Thus, the primary and secondary switches interrupt the power supply.

If for some reason one of these switches fails to open, the monitor switch will short the internal circuit of the high-voltage transformer. Blowing of the fuse is acceptable in this situation.

Safety first!

Fig.11 Electromagnetic door interlock

11. Safety system interlocks

The safety interlock system may have different modifications in different oven models; one of the possible variants is shown below.

But the basic functions are always identical!

Fig. 12 Door safety system interlocks

12. Primary circuits (low voltage)

Fig. 13 Primary (low-voltage) electrical circuits

• As a rule, the cooking-time control panel is integrated into the timer itself or provided as a separate component.
• The timer, fan and turntable motor are activated only after the oven is switched on.
• For the operating principle of the primary, secondary and monitor switches, see the section «Mechanical safety system».
• Other components, such as, for example, the safety thermostat that detects magnetron overheating, may be fitted depending on the model (or a thermostat for the oven cavity, grill or ventilation duct)
• The noise suppression filter blocks high-voltage interference, and its installation is required by law.

13. Secondary circuits (high voltage)

Below is an explanation of the operation of the secondary and/or high-voltage circuit.

Fig. 14 Secondary (high-voltage) electrical circuits

14. High-voltage transformer

	A high-voltage transformer usually has three windings: – primary winding – high-voltage secondary winding – low-voltage secondary winding
	The primary winding is powered from the 220 V mains; the following voltages are induced on the secondary windings: 3.1 Volts - low voltage, needed for heating the magnetron cathode filament. 2750 Volts - high voltage, supplied to the voltage multiplier.
	This is what the high-voltage transformer looks like when mounted in the housing. The bonded rubber parts are needed to reduce vibration and housing hum

15. Voltage multiplier

	The voltage multiplier consists of a diode and a capacitor. All measurements, especially those involving parts under high voltage, must be performed only as resistance measurements, and the oven must be disconnected from the mains!!!
	The 2750 Volt alternating voltage in the secondary circuit of the high-voltage transformer is converted into a direct voltage of 4000 Volts. This energy serves to power the magnetron.
	After the oven is switched on, the first wave of positively charged energy is directed into the capacitor at a voltage exceeding 2000 Volts.
	The change in polarity does not affect the operation of the diode, but the capacitor loses its charge. The voltage of the positive half-wave is added to the total energy of the capacitor. In this way, the required amount of direct voltage, approximately equal to 4000 Volts, is achieved.

16. Protective diode (fuse diode)

Negative half-cycle UABmax = - 800 Volts D1: forward direction D2: reverse voltage less than Vr2 Positive half-cycle UABmax = +4600 Volts D2: forward direction D1: reverse voltage less than Vr1 Operating mode not disturbed	Negative half-cycle UABmax = - 3000 Volts D1: forward direction D2: reverse voltage more than Vr2 (1200 V) Breakdown of diode D2 leads to a short circuit The current in diode D1 exceeds the norm and leads to a short circuit Secondary winding in short-circuit mode. The magnitude of the electric current in the secondary coil exceeds the norm and causes the main fuse to blow. Transformer protected

17. Magnetron

The magnetron is the heart of the microwave oven. Using a transformer, a rectifier and a magnetic field, it converts a 230 Volt 50 Hz electric current into electromagnetic energy at a frequency of 2.450 MHz

The magnetron consists of the following parts: The main body with the anode (positive) and cathode (negative), heatsink fins, a permanent magnet, connections for the filament and high voltage, as well as an antenna for transmitting electromagnetic energy. The inner cylinder is both the filament element and the cathode at the same time. The outer cylinder is the anode block with resonating cavities. Due to the significant voltage difference between the anode and cathode, the electrons move from the inner cylinder to the outer one.
For the magnetron to operate, high voltage alternating current (4000 Volts), low voltage direct current for cathode heating, and a magnetic field are required. The cathode is heated in order to cause electron emission, and the electrons then travel toward the anode	.
A permanent magnet is needed to achieve current oscillation. Due to its magnetic field, the electrons continuously rotate around the cathode instead of moving directly from cathode to anode. As a result, the electrons pass through the resonating cavities and cause oscillation of the electromagnetic field.
	These resonating cavities have a special structure. They have a capacitive and even an inductive effect within the high-frequency range. They also set the frequency of the electromagnetic energy produced. These electric current oscillations are then directed by the antenna directly into the oven cavity.

18. Practical measurements

Before, as well as during, the taking of measurements, safety precautions must be observed to avoid exposure to electromagnetic energy.

• Never operate the oven with the door open!
• Before switching on an oven intended for repair, perform a safety check.

Check the door interlock mode (seal and surfaces, hinges and latch bolts, mechanical intervention outside the oven).

When performing measurements or repair work:

Pull the plug out of the outlet!!

!Be sure to discharge the high-voltage capacitor beforehand.

This must be done with a suitable jumper with insulated ends.

When working with any microwave oven, it is important to always check for RF energy leakage (radiation leakage measurement) and to perform a check per VDE 0701 standard after any repair work (even simply opening the oven housing is considered repair work).

Repair of a microwave oven must be carried out by a specialist who has the appropriate qualifications, possesses the relevant information, and has the necessary equipment, measuring tools and technical documentation.

19. Measuring the emitted power level (simplified method)

1 liter of water (17-24°C) 62 seconds of operation at full power		Temperature difference (°C)	Electromagnetic energy output (in Watts)
	Important aspects: Use containers made of thin material. Thick container walls absorb particles of water heated to the maximum temperature, which leads to a false measurement result. If possible, set the initial water temperature to 20°C When taking measurements, observe timing accuracy Use an accurate thermometer, preferably an electronic one Pour the water into the container immediately before the experiment	7 8 9 10 11 12 13 14 15 16 17	490 560 630 700 770 840 910 980 1050 1120 1190

Measuring the power consumption

The most rational method for measuring power consumption is timing with a stopwatch (left) and using a clamp meter (right). If the power of the electric current suddenly increases, the parts responsible for high voltage are activated and electromagnetic energy is emitted.

The correct quantitative ratio of the appliance's on/off states should be determined from the technical documentation.
Simplest rule: 50% on – 50% off 50% power 30% on – 70% off 30% power Time cycle used: 30 seconds

20. Measuring microwave energy leakage

In most countries, the limit on radiation leakage is set by law and is no more than 5 mW/cm2 at a distance of 5 cm.

In modern microwave ovens, the doors are designed so that they transmit no more than 1 mW/cm2

A similar result can also be achieved by adjustment.

Even though the human body is capable of «withstanding» radiation of 100 mW/cm2, this is not a reason for careless leakage measurements!!!

Measuring radiation leakage

Pour 275 ml of water into a measuring cup and place it in the center of the oven.

Close the door Set the timer to 3 minutes Set the power to 100% Turn on the oven

To use measuring instruments, consult their operating manual.

Inspect the oven for radiation leakage by moving the test probe clockwise across the top, starting from the door, at no faster than 25 mm/sec, back to the starting position.

The measurement should be taken at a distance of 5 cm. In most cases, the probe has parts made of plastic or other materials that help maintain this distance. Also check the entire front of the door, openings, and corners where energy leakage occurs.

21. Checking the door interlock safety system. Measuring the fuse diode. Checking the high-voltage diode

Fig. 15 Microwave oven door interlock

In the table below you can review the values that should be measured and recorded in tabular form, in this case > O Ohm < or >infinity<

Primary/secondary switch

Measurements of the switches are taken after disconnecting the wires and opening/closing the door.

Monitor switch:

It is recommended to check its operability after working with the main switches. Its wires must be disconnected.

In any case, the monitor switch must close if the door is open.

Measuring the fuse diode (fuse diode)

	Diagram of high-voltage connections between the capacitor and the diodes. Attention: Incorrect connections will cause instant damage and may destroy the components!
Unplug the power cord from the outlet before taking any measurements!!! Short the capacitor with any insulated jumper to discharge it. Check the fuse diode in both directions at its contacts. An undamaged fuse diode should not show resistance in both directions

Checking the high-voltage diode

Disconnect the oven from the mains! Make sure the capacitor is discharged Disconnect the diode to take measurements Take resistance measurements in both directions using an ohmmeter on a high measurement range (MOhm) In one direction the diode should show no resistance at all; in the other direction you need to take a measurement and obtain a value within a few hundred kOhm. The instrument used for the measurements must support a voltage above 9 Volts, which is the threshold at which the diode should open.

22. Checking the magnetron, high-voltage capacitor, high-voltage transformer

Measuring the magnetron Disconnect the oven from the mains! Make sure the capacitor is discharged! Disconnect the magnetron Measure the resistance between the contacts The cathode (filament) resistance should be approximately equal to 0.5 Ohm There should be no resistance to the case

Checking the high-voltage capacitor

Low resistance and rapidly increasing… …OK Low resistance persists… …short circuit Very high resistance persists… …normal

If replacing the capacitor, please use an original component.

Correct operation largely depends on precise power and capacitance.

Measuring the high-voltage transformer

23. Fault-finding algorithms

Fault-finding algorithm for microwave ovens with a timer control function. Mains outlet is connected, timer is running, door is closed

The interior light does not come on, and the oven does not work either

The oven does not work, but the interior light comes on

The oven runs for a few minutes and then shuts off

Fault-finding algorithm for microwave ovens. The oven does not heat, the fan runs

Fault-finding algorithm for microwave ovens. The oven does not work, no response to any action. The fuse blows

24. Useful tips for consumers

Finally, useful tips for consumers:

	Don't skimp on time when reheating food. Some pieces require more time. The dish should not be served straight from the oven. If you leave it covered for a few minutes, the heat will be able to distribute evenly throughout the food. Large pieces of meat, for example, are considerably warmer on the outside than on the inside. However, round, cylindrical pieces of food 20-50 mm in diameter tend to heat up more on the inside than on the outside.
	Before cooking the food, remove any packaging and foil. This is a basic rule. However, if you need to cook a dish covered, try to keep a distance between the packaging and the oven walls to avoid sparking. Such dishes cook very quickly, and there is active steam release. When the dish is wrapped (loosely only!) nothing will splatter the oven. Heat spreads better inside packaged dishes. Packaged dishes do not dry out as quickly.
	When a dish needs to be browned to a crust, packaging should not be used. Using the grill function also implies the absence of any packaging. Liquid dishes should be stirred from time to time. Add a little water when cooking hard vegetables (e.g. carrots). There is no need to add water when cooking soft vegetables (for example, tomatoes). If possible, defrost unpackaged meat in glass or porcelain containers. Ideally on moisture-absorbing dishware, to get rid of the liquid that forms after defrosting the product. It is useful to turn the defrosting product over, if possible, when half of the total defrosting time has passed.
	If possible, avoid defrosting the product using the microwave oven alone, since this can lead to violations of certain standards (e.g. dried-out edges) Do not defrost the product in its packaging. It does not matter what material the packaging is made of: cardboard, paper, plastic or metal – it's all the same. This is a basic rule.
	Fruits and vegetables also should not be fully defrosted in the microwave oven if they will not be used immediately. By the end of the defrosting process, room temperature should be maintained. This will have a good effect on the taste of the product. If necessary, fruits and vegetables can be used in cooking even in a frozen state. Naturally, the power and cooking time must be set accordingly.
	Semi-finished products in plastic or metal containers, wrapped in metal foil (which, of course, must be pierced!) can be heated in a microwave oven according to the manufacturer's instructions on the packaging. The picture on the left shows the requirement: «place the product on a plate», meaning it needs to be unpacked! However, information from the product manufacturer cannot always serve as an «excuse».

25. Notes on setting the correct power level

The following overview shows which power levels are suitable for certain processes.

Common faults and troubleshooting methods

Before considering in detail how to fix the faults listed below, we consider it necessary to warn that before diagnostics and repair it is necessary to physically disconnect the appliance from the mains power supply, that is, pull the plug out of the outlet.

No response to the power button.

In this case, diagnostics and repair should follow the algorithm below:

1. Check for the presence of voltage in the mains power supply. If it is absent, resolve the power supply issue; otherwise, proceed to the next step.
2. Check the PSU of the control module. Start with the fuse. If it is blown, replace it. Then turn on the appliance and try to heat, for example, a glass of water. If everything works, the repair is complete. If the fuse blows again, the problem is in the control module, and it should be repaired or replaced.

To repair the control module yourself, you need certain skills in electronics; without them, it is not recommended to attempt to repair the control module on your own.

Example of fuse location on the control module

The microwave oven does not turn off after the cycle ends.

In most cases, this problem indicates a fault in the door position microswitch. To fix the problem, find, check, and if necessary, replace the switch.

If the microswitches are normal, then the problem may be related to the relay that supplies voltage to the power transformer in the magnetron's power circuit. «Test» the relay contacts with a multimeter; if they are «stuck», replace the electric switch with a new one.

If no problems are found with the relay, then the fault is related to the control unit; replace or repair it.

Weak heating.

Most often, this fault is related to a voltage drop in the household electrical network. If it drops below 205.0-210.0 V, there is a sharp decrease in the intensity of the microwave flow. This problem is typical for private houses in rural areas, where overvoltage in the power grid regularly occurs, and as a result, voltage drops.

If the multimeter shows an acceptable level of household mains voltage, then the power circuit of the magnetron should be checked, as we will describe how to do this in the next section.

When diagnostics of the magnetron circuit did not yield results, everything points to problems with the control module.

No heating.

This fault clearly indicates a fault in the magnetron's power circuit. Diagnostics are performed as follows:

1. Check with a multimeter for the presence of voltage on the primary winding of T1 (see fig. 1). If it is absent, the problem should be sought in the control module.
2. The presence of voltage indicates that it is necessary to check the high-voltage circuit fuse, transformer T1, the fuse, capacitor C1, diode VD1, and the magnetron itself. Checking the listed components is performed with the power disconnected!
3. «Test» the fuse with a multimeter, switching it to diode test or resistance measurement mode. If the device shows an open circuit, replace the fuse.
4. Check T1, for an open circuit and short circuit of the primary and secondary windings.
5. Test capacitor C1; how to check a capacitor with a multimeter was described on our website.
6. «Test» VD1. If anyone is unfamiliar with the technology for testing diodes, it can be found in a previously published article.
7. Check the magnetron. There are certain peculiarities to testing it, so let's look at this process in more detail:

• First of all, it is necessary to «test» the filament; if the multimeter shows a resistance close to zero (see a in fig. 6), then it is normal, continue testing. If the device shows an open circuit, check the contact of the filter coils (marked with yellow arrows on b, fig. 6). If there are problems with the contact of the coil mounting, the device can be restored; otherwise, everything indicates the need

продолжение следует...

Продолжение:

Часть 1 Microwave Oven Repair: Algorithm and Example of Fault Finding
Часть 2 Sealed containers, such as plastic bottles, cups with screw-on lids,
Часть 3 Algorithm for troubleshooting a microwave oven - Microwave Oven Repair: