Executive summary

Passed week I have been working on repairing electronics of household appliances. I repaired 4 items which generated ~7000 rubles for a business and saved ~25000 rubles for our clients.

Meanwhile I learned about how this household appliances are designed and work and what are their typical issues are. The publication is about this.

The cost of the service is calculated near 10%-20% to make it attractive to both client and business.

The cost calculation

Worth to note a few things about the cost calculation.

Seven years ago I already published we have to focus on RoI when do sales of our products or services, another publication shown how much business is ready to pay to mitigate potential losses caused by a breach in their information security.

The same approach is relevant here, the cost of the service is ~10%-20% of the cost of a new product, except relatively cheap devices which diagnostics and repair work cost nearly the same as the cost of a new device.

Cases

Microwave owen

How does it work?



The image above shows a simplified design of a microwave oven. It has a big capacitor to provide stable current and smooth it, a big diode to rectify a current, a transformer to increase voltage and a magnetron which transforms electricity into microwaves which heat your food. Signals from user interface are handled by a special control board

This particular microwave oven has a filter and a fuse at the beginning of a circuit smooth out noise and preserve the remaining elements of the circuit in the event of an electrical breakdown.

A microwave oven is a bit more sophisticated machine, but this general overview is enough to understand how it works.

The issue and a solution

The client complained it doesn't work. Primary diagnostics hadn't shown issues with key components of the circuit. It was a time to look at secondary components.

The issue was in one of the switches, you might see them as keys on the simplified circuit scheme above. Each oven has several of them (2 or 3) and they are used to open the electrical circuit if you open a door to prevent dangerous magnetron radiation into you.



The only one switch didn't work as it should be, but all three were replaced to make sure a client would not bring it again in a few months because of another switch breaks up. When it was done, oven started work as an expected.

Multicooker

How does it work?

A multicooker has a heating element, a power supply, a couple of thermosensors and a control board.

Thermosensors change their resistance based on temperature. The variation of the voltage gives a feedback to the control board how to adjust power of the heating element. Sensors are located on the top and on the bottom of the multicooker to make sure the most correct measurements of the temperature.

The issue and a solution

The client complained it stopped to turn on. Diagnostics shown there was a broken wire to the top thermosensor. After find a replacement and soldering it back to the thermosensor and the control board, device started function correctly.

Juicer

How does it work?

The most electronic devices I has repaired so far has two basic components -- a power board and a control board. The power board of this device is shown below.



This and many similar power boards works on principle described in this my publication

Worth to note this juicer has a mechanical lever which is pressed only when all parts are assembled together. In others cases, it the circuit will be turned off.

The issue and a solution

The client complained it stopped to work. Diagnostics shown obvious signs of a broken resistor which regulates a power voltage. After it was replaced device still didn't work. Further investigation and tracing the circuit shown there was another resistor which became broken. When it had been replaced, the device starts working.

Thermopot

How does it work?



The heating element, classics power and control boards, a pumping engine, a thermosensor and a thermofuse are main components of this device.

The power board has some elements of a control board, I never reached the control board near a control panel, so actually I don't have information on this wires goes from the control board or directly from buttons on the control panel.

The control board sends electric signals and either heating element or a pumping engine starts work.

The issue and a solution

The client complained it stopped work. Diagnostics shown flooded pumping engine, however after replacement device still wasn't working. Further investigation shown a broken stabilitron (Zenner's diode) and an a diode. When they had been replaced, device starts working.

A few related publications:
Electronics tech notes, part II: recover a manufactory machine
Electronics tech notes, part I: repair a power supply
Case study: flying drone DIY