DIY microwave repair: step-by-step workshop

We bring to your attention another article to help the home master. Its theme will be the repair of microwave ovens Samsung, LG, Panasonic, as well as other popular brands. In the introduction, we will briefly talk about the principle of operation and design features of microwave ovens. After that, we give a list of typical malfunctions, an algorithm for diagnosing breakdowns and methods for solving the problem. Next, consider the real case of repairing a microwave, using the LG-4022G as an example.

Content

  1. The device and principle of operation
  2. Preparatory stage
  3. Typical malfunctions and solutions
  4. An example of a step-by-step repair of the microwave LG MB-4022G
  5. Video Guide

The device and principle of operation

We will superficially consider this issue so as not to get away from the main topic. The information will be simplified as much as possible, since not all home masters have deep knowledge in electrical engineering. Let's start with the description and purpose of the main structural elements, they are presented below in the figure.

How is the microwave
Fig. 1. Microwave device

Designations:

  1. Door latches serve both for fixing the latter, and for the system of blocking work in the open position.
  2. A rotating pan on which dielectric dishes are mounted.
  3. Separator equipped with rollers, driving the pan.
  4. Drive, rotating separator.
  5. The backlight turns on depending on the operating mode.
  6. Ventilation (usually forced).
  7. A magnetron is a microwave generator, in fact, this is the main structural element. How it is arranged, and the principle of its action, you can find out by reading an article on our website dedicated to this issue.
  8. The waveguide provides the movement of microwave waves to the microwave chamber. It is a hollow metal pipe of rectangular cross section.
  9. High voltage diode.
  10. Capacitor.
  11. Transformer of a waveguide power supply circuit and control circuit.
  12. Control block.

We will not give a complete schematic diagram of the device, since they can differ greatly in different models of microwave ovens. In our case, the magnetron power circuit will suffice. As a rule, it has a typical structure.

Typical magnetron power circuit
Typical magnetron power circuit

Briefly describe the principle of operation of the above scheme. Power to the primary winding of the transformer (I) comes from an external control circuit that regulates the power and duration of microwave radiation. One of the secondary windings (II) provides voltage to the magnetron filament. Winding II is made of 2-4 turns of thick wire, since the current in the filament circuit can reach 10.0 A at a voltage of about 3 volts.

Another secondary winding (III), providing a high voltage level (up to 3.0 kV), is usually called the anode. As can be seen from the figure, in this circuit, a rectifier and a voltage multiplier are built on the basis of a high-voltage diode (VD1) and a capacitor (C1). In this case, VD1 is turned on so that the opening occurs with a positive half-cycle, as a result, the capacitor begins to charge. When the negative half-cycle begins, the diode VD1 closes and the voltage is supplied to the magnetron M1 together with the charge accumulated on the capacitor. This leads to a doubling of the voltage and the formation in the magnetron of an electric field of the desired intensity.

Resistance R1 in this case is necessary for discharge C1. Typically, this resistor is located in the capacitor housing. As for VD2, it provides protection in the event of an increase in voltage at the capacitance C1 or the occurrence of a short circuit in the magnetron M1.

Preparatory stage

Before proceeding with the repair, it is necessary to collect as much information as possible about the failed device. Ideally, this is a service manual for a particular model. In this document, the manufacturer provides all the necessary data, starting from the assembly drawing (exploded view, literally from the English explosion-scheme) and ending with the troubleshooting algorithm.

Fragment of a microwave explosion diagram
Fragment of a microwave explosion diagram

Unfortunately, manufacturers are in no hurry to share this information, distributing it only among networks of certified service centers. If you can find the technical documentation for the repair, get ready for the fact that it will be in English.

If the documentation could not be found, and this will happen in most cases, do not worry, typical malfunctions of the microwave oven can be determined without a circuit diagram. It is enough to know how the basic elements look and where they can be located. A photo of the microwave with the cover removed will help you with this.

Appearance and arrangement of the main elements in the microwave housing
Appearance and arrangement of the main elements in the microwave housing

The intuitiveness of the process in most cases allows you to remove the casing even without the assembly drawing and get to the main structural elements. But in this case, it is necessary to remember the sequence of actions and try not to leave “extra” parts after reassembling.

What tools are needed?

In most cases, you can do with a Phillips screwdriver and multimeter. In some cases, you may also need a soldering iron. Accordingly, spare parts will also be needed, which will be clear after the diagnosis.

Typical malfunctions and solutions

Before considering in detail the elimination of the malfunctions listed below, we consider it necessary to warn that before diagnostics and repair it is necessary to physically disconnect the device from the power supply, that is, pull the plug from sockets.

There is no reaction to the power button.

In this case, diagnostics and repairs must obey the following algorithm of actions:

  1. We check the presence of voltage in the power supply. If it is not there, we solve the problem with the power source, otherwise we proceed to the next step.
  2. Check the BP of the control module. We start with the fuse. If it is burned, we make a replacement. After that, turn on the device, and try to heat, for example, a glass of water. If everything works, the repair is complete. If the fuse blows, the problem is in the control module; it must be repaired or replaced.

In order to repair the control module yourself, it is necessary to have certain skills in radio electronics, without them it is not recommended to start independent repair of the control module without them.

Example fuse arrangement on the control module
Example fuse arrangement on the control module

The microwave oven does not turn off after working out the mode.

In most cases, this problem indicates a malfunction of the door position microswitch. To fix the problem, we find, check and, if necessary, replace the circuit breaker.

If the microswitches are normal, then the problem may be with the relay providing voltage to power transformer in the magnetron power circuit. We “call” the relay contacts with a multimeter, if they are “stuck”, we change the electrical switch to a new one.

When no problems are detected with the relay, it means that the malfunction is connected with the control unit, we change or repair it.

Mild heat.

Most often, this malfunction is associated with a voltage drop in the household electrical network. If it falls below 205.0-210.0 V, a sharp decrease in the intensity of the microwave stream occurs. This problem is typical for private houses in rural areas, where regularly occurs power line overvoltage, and as a result, a voltage drop.

If the multimeter shows the permissible voltage level of the household network, then you should check the magnetron power circuit, as we will describe in the next section.

When the diagnostics of the magnetron circuit did not give results, then everything indicates problems with the control module.

No heat.

Such a malfunction clearly indicates a malfunction in the magnetron power circuit. Diagnosis is as follows:

  1. We check with a multimeter the presence of voltage on the primary winding T1 (see fig. 1). If not, the problem should be sought in the control module.
  2. The presence of voltage indicates that it is necessary to check the fuse of the high-voltage circuit, the transformer T1, the fuse, the capacitance C1, the diode VD1 and the magnetron itself. Checking the listed items is performed when the power is off!
  3. The fuse is "called" by a multimeter, switching it to the mode of checking diodes or measuring resistance. If the device shows an open, we replace the fuse.
  4. Check T1, open circuit and short circuit of the primary and secondary windings.
  5. We test the capacity of C1, how to check the capacitor with a multimeterhas been described on our website.
  6. “Call” VD If anyone is unfamiliar with the technology of testing diodes, then it can be found in a previously published article.
  7. Check the magnetron. There are certain features with its testing, so let us consider this process in more detail:
  • First of all, it is necessary to “ring” the filament if the multimeter shows a resistance close to zero (see and in fig. 6), then everything is fine with her, we continue testing. If the device shows an open, we check the contact of the filter coils (marked with yellow arrows in b fig. 6). If there is a problem with the coil mounting contact, the device can be restored, otherwise, everything indicates the need for replacement.
  • After testing the thread, we check the passage tanks for a breakdown. To do this, put the multimeter in the “dialing” mode, touch the case with one probe, and touch the magnetron contacts one by one (b in Fig. 6). Infinite resistance will be a normal indicator, otherwise everything indicates that the capacitance is broken, which means that a magnetron needs to be replaced.
  • If the magnetron test does not give a result, it is necessary to measure the input voltage on the device. If they are below the permissible parameters, then this can be caused by an interturn circuit in a high-voltage transformer or by a lowered level of power in a household electrical network.
Magnetron test
Fig. 6. Magnetron test

Important! The magnetron must be changed to the same type. This is due to the fact that the parameters of the high-voltage transformer and the control circuit are calculated based on the specific model of the microwave generator.

There is sparking.

Such a malfunction may be caused by the following reasons:

  1. Burning mica plate isolating the waveguide from splashes and pieces of food. The plate is located inside the chamber on the side of the magnetron. The condition is determined visually. If the problem is with the plate, just replace it.
  2. During operation, the cover of the coupler burned out. This is such a plastic cap that rotates the pan. In this case, only a replacement will help. Naturally, it is necessary to install a coupler from the same type of models, since the design of such a cover can be different even from one manufacturer.
  3. The “wrong” dishes are installed in the camera. We remind you that metal devices, as well as those on which metallized dyes are applied, cannot be used in microwave ovens.

The pan does not rotate.

First of all, it is necessary to check whether the pallet is not blocked by any foreign object, whether it is installed correctly or the separator. If everything is normal, then the reason lies in the drive. This may be due to the following reasons:

  1. Jammed engine (determined tactilely) or the breakage of one (a dialing is performed) from the windings. In these cases, a replacement drive is required.
  2. Gearbox problem. In this case, it all depends on the design. In some cases, the gearbox can be repaired. But, as practice shows, its replacement will be easier and cheaper.

There is no reaction to the control panel.

In modern electronic models, such a malfunction indicates problems with the control module. In products with an electromechanical control system, it makes sense to check the mechanical relays and / or switches, if necessary, replace defective parts.

When turned on, the scoreboard does not work.

If the power indicator lights up when turned on, but the digital display does not work, then everything indicates a problem with the control module. It must be repaired or replaced.

When the door is closed, the fuse blows

A characteristic indicator of faulty microswitches on the door position. One of them is “stuck” and does not switch; as a result, a short circuit occurs in the control circuit. Repair consists in replacing or cleaning the microswitches.

An example of a step-by-step repair of the microwave LG MB-4022G

The causes of breakdowns and malfunctions of the microwave oven, as you have already seen above, can be many. Most often, this is the failure of the magnetron itself, due to improper use of the device, namely the use of dishes not intended for cooking in a microwave oven. Also, various metal parts that may accidentally be inside during operation.

The failure of the magnetron can be considered the most unpleasant reason, since the replacement of this part is not worth the proof. In this case, it is easier to buy a new oven.

But sometimes there are minor breakdowns that can be easily fixed without special tools and without spending a lot of money.

Below, the article will describe one of these failures, and a way to resolve this malfunction. In the photo there is an oven that has stopped turning on and does not react at all to the manipulation of the control knobs.

LG MB-4022G
Fig. 7. LG MB-4022G

Before removing the protective cover from the furnace, it is necessary to carefully inspect the power cord and the plug itself for damage to breaks and cuts. If there are any, then you should disassemble the furnace.

To do this, you will need a Phillips screwdriver.

We unfold the furnace with the back to ourselves, and unscrewing the two screws of fastening, remove the ventilation cover. Next, remove the protective cover.

Remove the protective cover
Fig. 8. Remove the protective cover
Unscrew the cogs
Fig. 9. Unscrew the cogs

At the back, it is mounted on several screws. Unscrew them all.

When all the rear screws are unscrewed, go to the side, left side.

Turn off the left side
Fig. 10. Turn off the left side

There are three mounting screws, two from the bottom and one in the middle. They should also be unscrewed. As the screws are loosened, you can observe how the metal edges of the lid extend away from the chassis chassis.

We continue to unscrew the screws
Fig. 11. We continue to unscrew the screws and remove the covers

Next, lift the lid slightly up and pull it towards you. Thus, it comes out of the grooves located on the front of the case.

Inside lid
Fig. 12. Inside lid

You can notice traces of soot, which were formed due to the action of high temperatures.

The removed cover freed access to the main elements of the microwave oven (Fig. 13). At the top, you can see the elements for the grill - the heater is located in a special building.

Microwave internal device
Fig. 13. Microwave internal device

On the left is the magnetron itself (Fig. 14), namely its upper part.

Magnetron microwave oven
Fig. 14. Magnetron microwave oven

Bottom left is the power filter, from which the wire harness and network cable depart. Even on the top of the camera, you can notice two temperature sensors. They are attached to the housing and respond to temperature changes. Two wires are connected to them.

Power filter
Fig. 15. Power filter and 2 temperature sensors

If you look from the side, then here the gaze reveals other elements. For example, a power transformer having a boost winding.

Power transformer
Fig. 16. Power transformer

And also see the magnetron radiator.

Magnetron radiator
Fig. 17. Magnetron radiator

And inspect the power switch.

Power switch
Fig. 18. Power switch

Time relay with an audio signal, the role of which is played by a mechanical call (Fig. 19).

Time relay
Fig. 19. Time relay

Magnetron radiator fan (Fig. 20). It prevents overheating of this important expensive part.

Fan
Fig. 20. Magnetron Blow Fan

There is almost nothing on the right side (Fig. 21).

Right side
Fig. 21. Right side

Let's start by examining the power filter, because it is precisely on it that the network cable comes, and then the voltage from the board goes to other elements of the furnace. Therefore, the place where the voltage "disappears" must be sought from the side of its receipt, that is, the network cable.

So, on the power filter board we find the terminals where the power wires come, namely blue and brown (Fig. 22).

Power wires
Fig. 22. Power wires

We plug the power cord into the network and measure the voltage in this section of the circuit. The device shows that the network voltage - 220 volts comes to the board. This means that the power cord is one hundred percent intact, and this indicates that the problem is further in the scheme.

We measure the voltage on the circuit
Fig. 23. We measure the voltage on the circuit

A fuse is installed on the power filter board, which may be the reason that the voltage does not pass further.

Checking the fuse
Fig. 24. Checking the fuse

With a circuit tester, measure the fuse integrity. You can do this without removing the fuse from the mounting clamps, only before that you need to disconnect the device by removing the power plug from the outlet.

The device shows the circuit, which means that the fuse is intact and the problem is not in it.

Next, pay attention to the brown wire (Fig. 25), through which the voltage passes further and enters the temperature sensor.

Brown wire
Fig. 25. Brown wire

Here, this device is responsible for disconnecting the microwave oven from the network if the case temperature is critical, that is, more than 150 degrees. At this temperature, the bimetallic contacts located in the device’s case open and interrupt the circuit. After cooling the case, they return to their original position.

You can check its integrity with the same device that measures the integrity of the circuit (Fig. 26).

Temperature Sensor Check
Fig. 26. Temperature Sensor Check

To do this, remove one end of the wire from the terminal so that the circuit does not introduce false readings. We connect the probes of the device with the terminals of the sensor and look at the result. As you can see, the device shows an open circuit, which means that the sensor is inoperative.

Now we can assume that the cause of the furnace malfunction is found, but the final result will be known only when the issue with the temperature sensor is resolved.

To restore the sensor, sometimes a sharp shake or blow to it with some object, for example, pliers or a screwdriver's tip, helps. But even if the device recovers, there is a risk that in the event of a critical overheating it will not work, and the consequences can be dire.

Therefore, in order not to take risks, it is better to replace the device with a new one, especially since its cost is approximately two dollars. You can find it easily in one of the online stores.

The photo shows the marking of the thermal switch.

Defective microwave thermal switch and its marking
Fig. 27. Thermal Switch Marking
Temperature Sensor: Rear View
Fig. 28. Temperature Sensor: Rear View

This thermostat is bimetallic KSD 201.

Marking indicates that it turns off at temperatures above 145 degrees and is restored when the temperature drops below 60 degrees.

It is easy to remove the sensor (Fig. 29), it is enough to screw one of the mounting lamellas with a screwdriver, and it can be easily removed. Before you remove it, you need to disconnect the wires. If you interchange the wires when installing a new sensor, this will not affect its operation.

remove the sensor
Fig. 29. Remove the sensor

After purchasing this device, install it in its original place, and connect the wire from the filter board to it. The second wire is not connected yet. The sensor head should fit snugly against the camera body. We connect the device to the terminals of the temperature sensor and check its integrity.

Now the device shows the circuit (Fig. 30), and this means that he is whole. Next, connect the second wire and check the quality of the connection by twitching it.

Check the integrity of the new sensor
Fig. 30. Check the integrity of the new sensor

Now we turn on the power plug in the network and check with a voltage indicator whether the voltage is coming to the sensor. To do this, we connect one probe of the device to the blue wire on the power filter board, and the second to the sensor - the near end - to which the brown wire comes.

Check if voltage comes to the sensor
Fig. 31. Check if voltage comes to the sensor

If the result is positive, but in the photo (Fig. 31) this is so, then measure the voltage at the output of the sensor (Fig. 32). The device shows that it is also present, as in the first case.

We measure the voltage at the output of the multicook temperature sensor
Fig. 32. We measure the voltage at the output of the multicook temperature sensor

Now we can assume that the sensor has been successfully replaced.

Although this is undesirable, without putting on a protective cover, apply voltage to the furnace, and setting the minimum temperature, turn on the device.

The furnace has started and is working (Fig. 33).

Microwave Oven Performance Test
Fig. 33. Microwave Oven Performance Test

Quickly turn off the device and put on a protective metal casing.

Install the housing cover in the reverse order, that is, by inserting the front part into the grooves and securing the rear one to the screws.

Put the cap back on
Fig. 34. Put the cap back on
Continue assembly
Fig. 35. Continue assembly

For a final check of the operation of the device, plug it into the network. We put some food inside in a glass jar and start the oven with the handle of a timer.

The oven works, the products heat up and produce steam.

Serviceable multicooker
Fig. 36. The slow cooker is working
The problem is fixed and it turned out to be not at all difficult and not expensive!

Video Guide

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