How to repair microwave oven

How to repair microwave oven

Modern microwave ovens make it much easier to heat and often cook food. They are used by all members of the family, including children. But, if not properly maintained, these household helpers often break down.

In this article I show how an ordinary home electrician with average skills can repair a microwave with his own hands: I help him in detail with my tips and illustrative pictures. Given the many years of experience as a workman and the experience of a high-voltage laboratory relayman, I immediately draw attention to the dangers inherent in the design of the microwave oven. I give them half of the material, but you can go straight to the main part. If this information will be difficult for you, I advise you to contact Brooklyn oven repair.

What are the dangers of the microwave: what you need to clearly imagine in its device before you start repairing or even buying 

The main law of repair of unfamiliar electrical devices can be formulated briefly: do no harm! Therefore, it is important to understand the principle of their operation, the risks of damage to the equipment and personal injury.

Threat #1. Ultra-high radiation is the main danger when repairing and operating microwave equipment

In any household microwave oven, food is heated by irradiating it with electromagnetic waves. They are emitted by the magnetron antenna. Then they are concentrated by the waveguide and directed through its window directly to the heated object.

In any household microwave oven, food is heated by irradiating it with electromagnetic waves. They are emitted by the magnetron antenna. Then they are concentrated by the waveguide and directed through its window directly to the heated object.

The design of the device, as you can see, uses the energy of microwave radiation, affecting all objects that are in its path. These beams are capable of:

  • Heat food;
  • Boil water;
  • Heat solids;
  • Induce large currents in metals;
  • Disable semiconductor elements;
  • Create burns on the skin;
  • Burn out an eyeball;
  • Do useful work or make a variety of mischief.

Manufacturers take all these risks into account. They created a solid, double metal cage construction based on the Faraday cage principle.

It is designed to block microwave radiation from escaping outside the cooking chamber in any direction, whether it is the back, bottom, top, side wall, or open door.

I will not focus on the fact that microwave radiation in different models is capable of phoning, breaking through all sorts of defects and cracks. That’s another topic.

Just in case, consider that the industry does not produce microwave radiation detectors for nothing, which are used to test operating furnaces in working condition.

If there is a small gap between the door and the cabinet, electromagnetic waves from the magnetron will penetrate through it.

Therefore, the safety of the construction is ensured by a tight fit of the door to the side walls and a rigid control of this condition by triggering limit switches.

In the open state, the two contacts SWA and SWB exclude power supply to the high-voltage transformer with a magnetron forming high-frequency pulses, and the third SWC – prepares a circuit for creating a short circuit in case of accidental sticking of SWA.

It will be eliminated by burning the built-in 220 volt fuse and backed up by the work of the circuit breaker in the apartment switchboard.

Such double protection by micro switches and manufacturing of body in the form of Faraday cage was introduced by the manufacturers against accidental exposure of users to hazardous electromagnetic waves. We are not allowed to violate it.

If, when repairing with the lid removed, you artificially short-circuit the door’s microns to check the current flow through them, the dangerous microwave radiation will spread throughout the room. A hapless handyman can easily get a burned eye or lose his eyesight altogether. When other people, such as curious young children, are around, there will be much more trouble.

Since few people have a microwave radiation detector at home, it is acceptable to test the work of the Faraday cage with any cell phone. It is placed inside the microwave, close the door and dial a number from a second phone. The call signal should be clearly blocked by the shielding grid.

Never forget the danger posed by high-frequency electromagnetic radiation.

Threats #2. Types of high-voltage voltage – where and what risks the repairman is exposed to

The name “magnetron” is the name given to the ordinary electron tube, which is based on a diode circuit. It transmits electric current in one direction using the ancient principle of thermionic emission.

All power to the magnetron comes from the output windings of a transformer connected to the domestic mains by a control circuit. On its secondary coils are transformed:

  • 4 volts to power the heated cathode circuit;
  • 2000 volts supplied to the voltage multiplier, assembled on capacitor C1 and diode VD1.

The heater circuits are made with thick wire designed for a constant load of 10 amperes, and the high-voltage coil and its wires have high dielectric properties.

When checking the operation of the magnetron, the repairman can be affected by the current both when the circuit is energized and when it is de-energized.

Check the output circuits of the transformer under voltage

The high voltage sine wave of two kilovolts can be traced at the output coil pin ends or between ground and capacitor terminals.

The 4 kilovolt voltage is present at the anode and cathode of the magnetron or between the ground potentials and the anode of the high voltage diode VD1.

The danger in this situation is a common set of household electrical measuring instruments designed for use with voltages of only up to one kilovolt. Pay attention to the insulation class and the ability of your instruments to operate under such high voltage potentials.

Here it is necessary to exercise the utmost caution.

What risks lurk in checking the transformer with the power off

Pay attention to the high voltage capacitor, which is able to store the charge received during operation. It must be discharged in advance before working with the transformer and its circuits.

In some designs this possibility is provided by the circuit design (parallel connection of a current-limiting resistor, which can even be built inside a common case with the capacitor), while in others it is absent.

The discharge of the charged capacitance of a capacitor should be performed not only with a high dielectric protection instrument, but also through a current-limiting resistor. Creating even a short-circuit mode for a short time with a jumper of low resistance has a negative effect on the state of the terminals.

The risks of a high-voltage discharge are high and must be considered during repairs.

Threat #3. 230 volts of the domestic mains is a “familiar” danger

I am not going to emphasize anything here. I have a lot of information on this subject on my blog. I will emphasize only one important point, for the sake of which I put the word familiar in quotation marks.

When working with electricity, you should never relax and should always be clear about the dangers of your actions. The use of monotonous, monotonous work and loss of vigilance can be harmful to the body.

From this rule follows another point to be considered not only for repairs.

Threat #4. How you can get an electric shock from a microwave oven in your apartment by accident – hidden dangers that you need to prevent in advance

It applies to almost all owners of complex electronic equipment, not just microwave ovens powered by the old TN-C circuit.

One probe of my pocket multimeter Mestek102 I crocodile placed on the tap of the faucet, and the second – connected to the body of the microwave. It is not heating food at this point, but its electronic clock shows the time.

The power cord, like all owners, is plugged into a two-wire outlet that has no protective zero. The multimeter shows a potential of 123 volts.

It is important for us to understand the causes of it. This will prevent cases of family members falling under the influence of current: all modern household appliances are created under a three-wire system, which provides increased safety for the user.

Digital devices are very sensitive to the quality of the power supply, and it is very far from ideal in our homes, saturated with all sorts of interference, both in-phase and differential signals.

Any disturbance is a current of uncertain frequency or kind, which always moves along a closed circuit. In a two-wire network such a signal (differential) can only flow through the phase and neutral conductor in some direction.

A three-wire circuit supplements these possibilities with protective conductor circuits through the potentials of the zero and the phase (common mode disturbances). When the body of the appliance is connected to the protective PE conductor, the in-phase signal flows to the ground circuit. Everything works perfectly correctly.

If the microwave is on a two-wire circuit, however, its case has an electrical connection to the ground contact of the power cord plug.But then it just breaks off.

The case potential is disconnected from the grounding system. At the same time it is connected to the artificially created by capacitors middle point of the filter.

It is technically difficult to make a ground loop according to science and connect appliances to it when living in old high-rise buildings. That’s why you have to:

  • Warn family members about the dangers of touching the microwave and grounded metal structures at the same time;
  • Use only resistant dielectric flooring;
  • Limit access of children;
  • Use other protective measures.

When repairing a microwave and checking it under voltage on a two-wire circuit, this hazard should always be considered.

Preparatory stage of repair: What is important to do before you start disassembling the microwave oven case

After the safety briefing, we do:

  1. Preparing the workplace and tools;
  2. Study of technical documentation;
  3. External inspection of the repaired equipment.

The first question should not cause problems: I’ll omit it. But the technical documentation can be a problem. That’s why I made a couple of simple schemes, which will help to understand the principles:

  • The work of the kinematic units;
  • The construction of the electrical connections.

How the mechanics of a microwave oven work

High-frequency radiation is directed in one direction, but is reflected inside the cooking chamber. It creates local heating of individual food areas. To increase the volume of the heated area, a rotating table is used, driven by its own drive motor.

The magnetron emitter is cooled by its own fan. The door curtains ensure smooth operation and the latches clearly lock the closed position controlled by the limit switches.

What a microwave oven circuit diagram consists of

Here it should be clear that the microwave can be controlled:

  • Digital technology;
  • Or through the use of electromechanical devices.

Each variant has different circuits with specific features. Therefore, to repair it is desirable to find the exact technical description from the manufacturer, which is quite difficult.

What to look for when externally inspecting the oven

Before disassembling the enclosure, it is necessary to evaluate:

  • Whether power is supplied to the internal circuitry;
  • How securely the door curtains and door locks work;
  • Whether the interlocking contacts are working properly;
  • Smoothness of the table tray rotation and the condition of its drive;
  • The work of the backlight;
  • State of cleanliness of the waveguide window, as well as the inner walls of the cooking chamber.

Next, if you can not find a specific scheme from the manufacturer, we will rely on intuitive thinking and common sense.

8 typical defects: detailed instructions

In order to exclude electric shock first of all we turn off the microwave oven: remove the power cord plug from the socket and wait for the time necessary to discharge the capacitors (if we do not remove the charge with a current-limiting resistor).

Where and which screws to unscrew to disassemble the case see for yourself beforehand. I will break down the typical malfunctions.

The microwave oven does not respond to the power button

At this point we should already understand: is there voltage applied to the internal circuitry. If not, then:

  1. We check if the socket is working properly with a multimeter in voltmeter mode. It should be present 220 volts;
  2. Probing with a tester of the supply chain potential phase, operating zero and ground from the contacts of the plug through the power cord to the input of the noise filter. There may be a break somewhere;
  3. When it is not there, assess the condition of the input mains fuse. It is a calibrated wire with contact pads, enclosed in a glass or porcelain case. Its serviceability should also be checked.

Replace a blown fuse only with a fusible link of the same rating. If it is overrated, it will remain intact in the next fault, but the working internal circuit will burn out.

Replacing the fuse may not solve the problem, for the simple reason that it often burns out some sensitive electronic components along with it. This necessitates a further close inspection of all parts of the electrical circuitry.

If no other defects are detected, it remains to restore the disassembled housing and check the operation of the oven by heating, for example, a glass of water.

Closing the door is accompanied by a blown fuse and a power failure

I have already shown the door interlock options in the two diagrams above. Here it is important to understand that they must be very clearly and synchronously set.

Incorrect adjustment, mismatched operation will result in a short circuit and blown fuse or cause electromagnetic smog.

The microwave oven continues to work after the set time is over

Here, first of all, we pay attention to the operation:

  • The contact groups of the control module;
  • Timer;
  • Pulse shaping relays.

HF pulse shaper

Electromagnetically controlled designs use a relay to generate HF pulses to the primary winding of a high voltage transformer. It differs in design from model to model.

It is checked:

  • First, by checking the state of the contacts with a multimeter to make sure they are not sticking;
  • Then outside the circuit by taking electrical characteristics by applying a control signal to the winding.

The second method requires assembling a test circuit from various power sources, a set of measuring tools, and practical skills of the worker. This is a problematic issue for novice electricians.

Timer malfunction

It will be necessary to check the operation of the timer motor unit T and its output contact SW1. See typical wiring diagram.

In microprocessor microwave ovens, if the contact group is working properly, the defects should be looked for in the control unit.

A noticeable reduction in food heating

The defect is associated with a decrease in microwave power. Three causes can be seen here:

  • Contamination of the waveguide window of the cooking chamber;
  • Decrease of output signals of the high voltage transformer;
  • Violation of the control module algorithms.

Dirty cell

The condition of the mica coating of the waveguide window is clearly visible visually. There should be no traces of burnt food on it, hindering the passage of microwave radiation.

All dirt must be cleaned gently with a cleaner without damaging the outer surface of the mica layer.

Undervoltage in the apartment and private house

On long rural power lines, it is still quite common to see cases where the power supply company is not able to supply quality electricity to all consumers. Some houses get less than 200 volts.

To do this, you should use a multimeter to assess the input voltage in the socket. When the level is low, it must be raised in various ways.

Otherwise, the appliances will work intermittently, and the microwave transformer will produce reduced energy. It will not be enough for optimal operation of the magnetron, good heating of food.

What to do if the supply voltage is normal

In this situation it will be necessary to evaluate:

  • The proper functioning of the high voltage path that produces the input signals to the magnetron;
  • How the Magnetron works;
  • Reliability of the control unit algorithms.

These cases are discussed below together with other HF emitter malfunctions further on.

When you turn on the microwave oven in operation, the food is not heated

This is an obvious defect in the magnetron control circuitry or its internal design. The fault may be hidden in the voltage supply circuits or inside the high voltage path.

If it is necessary to energize the circuit, it is important to completely eliminate all five hazards of injury described at the beginning of this article.

Step 1: Measure the voltage on the input winding

First of all, we should take care of our own safety and disconnect the wires coming to the transformer input terminals. In this way we exclude a possible power supply to the high-voltage path.

We will measure the voltage safely on these leads. Set the multimeter to AC voltmeter mode and determine the value in volts.

A zero reading indicates a defect on the side of the interlocking contacts or the pulse shaping device. The search is carried out by consecutive measurements from the input filter to the winding of Tr1 through all switching points.

Eliminate the defect.

Step 2. Checking elements of the high voltage path

There are a lot of electrical components that require a detailed check of their operability. We evaluate their condition in series.

Power transformer

Look at the magnetron wiring diagram above. The voltage applied to the input winding means that it appears on the two output windings when the transformer is in working order.

But here you have to be careful to the limit: 2 kV is life-threatening. But 4 volts for heating can be measured.

Tip: You can assess the condition of a transformer by the value of the transformation ratio. After all, this device is reversible. This means that when 220V is applied to the input, the high-voltage winding produces 2000. Ktn=2000/220=9,09; if 220 at idle (all windings disconnected from the load) is applied to the high-voltage output, then the primary winding of a good transformer should have 220/9,09=24,2 volts.

With this simple measurement you can safely assess the condition of the high voltage output without having to carry an expensive kilovolt meter.

A winding failure, an inter-turn fault or an inter-phase short circuit will require the transformer to be replaced or repaired. In this situation you will need to perform a calculation.

The fuse for the high-voltage side is FU2

It is located next to the transformer and enclosed in a rugged dielectric cover.

It will need to be removed and tested with a multimeter, either by testing or by measuring resistance.

Replace a blown module only with a fusible link of the appropriate rating.

The voltage multiplier capacitor

At the beginning let me remind you again that there may be a charge on its tabs, which is very painful not only for your fingers: do not forget to take it off.

The capacitor capacity can be measured with a multimeter and compared with the factory reading. However, I advise you to consider its design beforehand, because often for a microwave multiplier it is produced in a common case with a current-limiting resistor. It will affect the accuracy of the reading.

How to check a high voltage diode

Ordinary resistance measurements with a multimeter will not help here. This semiconductor consists of a series of compound diodes, with a total voltage drop across them greater than the device can provide as a reference signal.

In principle, expensive multimeters have the ability to reliably measure large megohm circuits. But this is not available to everyone.

The Internet is full of bad tips that allow you to solve this problem with violations of safety measures, such as the use of a prohibited test lamp. I do not advise to use them and show the safest method.

We need to switch the multimeter to DC voltmeter mode and connect it to a 9-12 volt voltage source through our diode. The circuit is simple.

When the plus of the battery is connected to the anode of a good diode, the potential from the source will pass to the voltmeter and it will show a value reduced by the voltage drop across the semiconductor junctions.

If polarity is reversed, the diode blocks the negative potential from passing through. The voltmeter indicates zero.

A broken diode with a broken circuit will not let anything through in either case, but a shorted diode will work as a jumper.

How to check the magnetron

A real assessment of its condition can only be done under the load of a high-voltage circuit. For home conditions it is an impossible task. Therefore, we will use the indirect method of measurement, which allows you to identify only obvious defects.

Our device is an ordinary electronic lamp with heated cathode and grounded anode. At the input voltage it has a simple line filter, consisting of pass-through capacitors and chokes.

We start with the condition of the filament. It can be:

  • In one piece;
  • Or broken.

To find this out it is enough to remove the supply wires and use a tester to check its resistance. When it is small – the chain is serviceable, but not detectable value – breakage.

The mains filter of the magnetron is designed to suppress the noise coming from the magnetron into the mains. It does not perform other tasks, but the resulting damage to its capacitors or chokes can cause malfunction of the microwave oven.

The breakdown of the feed-through capacitor can be noticed visually by the formation of black dots of burnout on the input insulator.

However, for this purpose it is better to use a multimeter’s “Probing” mode. It is enough to connect one probe to the body of the device, and with the other to pass through the input terminals. The tester should not reveal low resistance on a faulty filter.

Turn the magnetron upside down, inspect the condition of its cooling radiators and permanent magnets. Everything should be intact here.

Cracks and chips on the magnets are not allowed.

The antenna cap, often called the electromagnetic pulse cannon, is also inspected. Its surface must be free of defects and cracks.

By the way, the faulty cap can be slightly squeezed with pliers, slightly turned and removed. In its place put another, serviceable in the same way.

Replacement of the damaged magnetron with a new one should be carried out by the same brand with matching parameters of input and output power. Transformer control circuits and the rest of the circuit are adjusted for them.

Just in case, I warn you that the white magnetron insulator is made of beryllium oxide ceramic. Do not experiment with sawing it up, crushing it. Such dust is very toxic.

Sparks inside the microwave oven: what to do

Let me start by saying that the spark that occurs inside the electrical circuit is nothing but a current of a certain strength, flowing in a gas environment (air).

Take into account that our microwave oven can be without defects or with them. You just need to understand that even a faulty design will spark if you put metal objects or dishes that conduct electricity inside the confectionery chamber.

Due to the phenomenon of induction, the powerful electromagnetic radiation of the magnetron will induce an EMF in them, causing arcing discharges on the housing. Look at the wiring diagram: the high-voltage anode is shorted to it.

This arc is used in welding, but inside the microwave oven it is harmful. For this reason, only dishes with high dielectric properties without metallic dyes should be used for heating food.

What design faults contribute to arcing

Contamination of the waveguide

The first thing I recommend looking at is the condition of the waveguide window mica plate. When it gets splattered with grease and moisture, it will eventually burn through.

Through the resulting hole, dirt enters the waveguide, where it also creates burnouts. Dirt can reach the magnetron antenna cap, it will be damaged. All these places need to be inspected, cleaned thoroughly.

The repair of the mica plate is to replace it, although for short-term use it may be sufficient to turn it over after cleaning.

A new part can easily be cut to the old dimensions from a sheet of mica, which is sold in a construction store. It does not cost much.

A burnout in the body of the microwave oven

This occurs most often in the area of the bottom, where food fouling collects. The resulting gap becomes a siphon of electromagnetic radiation.

The burnout area should be carefully cleaned and mechanically joined tightly. It is possible that small holes will remain. Their size should be smaller than the holes in the mesh that blocks microwave radiation through the door.

It is acceptable to solder the gap in the case, using an overlay of brass mesh to clean the fuel, but this is a very time consuming process.

Painting traces of repair should be carried out with compounds that do not emit harmful substances when heated.

Hull wall burnout is also possible. In certain cases, it can be prevented by sanding down the dirt to the metal and then painting.

The tray began to rotate jerkily or stopped at all

The electric motor of the table is located at the bottom of the housing. It is secured with a screw through a rubber seal. To get to it, you will need to remove the bottom cover of the housing.

The motor, as well as the entire kinematics, must be inspected, cleaned, and lubricated.

On top of the plastic output shaft, a coupler is attached that transmits the torque of the shaft from the motor to the work table platform, which moves on three wheels on the circular recess of the housing.

If the motor windings are intact, a simple cleaning, lubrication of the assemblies and restoration of the fasteners will suffice. Otherwise, it will be necessary to replace the damaged part. Masters from service centers can select a suitable model from their stock.

When the power indicator is lit, the control panel does not work

Such a defect can occur in microwave ovens with both electromagnetic and microprocessor equipment.

In the first case, it makes sense to first check the operation of the relay part with contact switches, and in the second case – you will have to additionally deal with the electronic control circuit.

Time-tested advice from a home expert as a conclusion

If you’ve read the material I’ve outlined carefully, you should have noticed that the bulk of breakdowns are due to improper operation due to negligent handling.

When we bought our microwave oven 19 years ago, my wife noticed that cleanliness is not only the principle of maintaining good health, but also the competent maintenance of equipment.

However, regularly cleaning the insides of the cooking chamber is quite a time-consuming task. It can be avoided. We’ve made it easier.

Heating food in a dish with a lid is not accompanied by splashing its particles outside the plate. The latter can be easily washed after the end of the meal.

All this time our microwave oven serves us reliably in the kitchen, and its appearance and internal equipment remained in its original condition. Repair of the microwave is not required. Draw conclusions.