Repair of the power supply of the microwave oven. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Power Supplies

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About a year ago, I had to repair a Bork microwave oven model MB IIEI 2623 S1, which failed due to a significant overvoltage in the mains. The malfunction was quite common - the power transformer of the control unit failed. Replace - half an hour, on the strength - an hour of work. But the main problem was the lack of the transformer I needed for repair on sale. I had to change the layout a bit. The work was facilitated by the fact that a diagram of its windings was applied to the transformer, indicating the value of their alternating voltage. True, their output current was not specified.

Figure 1 shows a diagram of this transformer with power supply rectifiers. It has the factory part numbering intact.

As can be seen from the circuit, it is very simple and does not contain voltage stabilizers. Apparently, the voltage under load of the upper rectifier according to the circuit is about 5 V, and the lower one is about 20 ... 22 V. Judging by the diameter of the secondary windings of the transformer wires, the output current of the five-volt rectifier is unlikely to exceed 0,5 ... 0,6 A, and the second - 0,1 A. In the course of further work, all these assumptions were fully confirmed.

The scheme of the new power supply is shown in Fig.2.

It was based on the quite "ancient" vertical scan output transformer TVK-110-LM, which is still widely used by many radio amateurs in their work. Pin 5 of this transformer is not used. Due to the different number of windings compared to the burnt one, it was necessary to change the rectifier circuit and introduce a voltage regulator of 5 V. The efficiency of the unit deteriorated somewhat, but the reliability increased many times over. The numbering of newly introduced parts in the diagram starts from 10. All other parts are taken from the old block.

Before use, remove the paper spacers between the halves of the transformer core, which form a non-magnetic gap. After a mandatory check for operability in idle mode, the transformer should not warm up at all, it is useful to impregnate its core with some kind of metal varnish. These measures will further completely relieve it of an unpleasant buzz and significantly reduce the no-load current. It is unacceptable to get varnish on the windings of the transformer, which can lead to damage to the insulation of the wires due to their dissolution in it. And this may well lead to a short circuit in the windings. If drying oil is used instead of varnish, then the transformer can be completely saturated with it. This will not damage the windings at all. But drying can take up to a week. If you expose the transformer impregnated with drying oil in the sun in the summer, the drying time will decrease somewhat.

Chip DA1 is mounted on a radiator with a cooling surface area of ​​at least 120 cm2. Instead of the KR142EN5A microcircuit, you can use imported ML7805A, L7805CV or any similar ones for an output voltage of 5 V. Diodes VD10 ... VD13 can be used silicon or germanium. They must withstand an average forward current of at least 0,5 A and a reverse voltage above 40 V. Capacitor C10 can be used with any small-sized capacitor with a capacity and voltage not less than those indicated in the circuit (Fig. 2).

Instead of the TVK-110-LM transformer, it is quite acceptable to take TVK-110-L-2. The numbering of the conclusions is exactly the same. Before installation, it is advisable to disassemble the core of this transformer and assemble its U- and W-shaped plates with overlap. It is also desirable to impregnate the core of this transformer after the performance test with some kind of varnish for metals.

With serviceable parts, an unmistakably assembled adjustment scheme will absolutely not require. There are no problems with the placement of a fairly large power transformer and other parts. There is more than enough free space inside the microwave oven housing.

Literature

1. TVK transformers. - Radio designer, 2002, No. 8, C 47.
2. G.P. Veresov. Power supply of household radio-electronic equipment. - Moscow. Radio and communications. 1983.

Author: M.Ryndenkov, Republic of Buryatia, Barguzinsky district, Suvo village

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