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Encyclopedia of radio electronics and electrical engineering / Телевидение

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The hot summer of 1999, when the temperature in the room exceeded 30 degrees, was difficult not only for users, but also for computers. It was at this time that two 14-inch "Abion" monitors of the Taiwanese company ABI OVERSEAS LTD broke down with a difference of several days. The first explanation suggested itself - voltage surges in the power supply network were to blame. But the replaced fusible link burned out with a blinding flash. This means that the power elements inside the monitor are faulty.

Usually, a powerful key transistor in the horizontal scanning output stage does not withstand severe operating conditions first. When it fails, it overloads the power supply. Alas. testing disproved this assumption. Transistor Q303 (hereinafter, the names of the elements are given in accordance with the designations on the printed circuit board of the monitor and the markings on the housings of the elements) turned out to be absolutely serviceable.

Another energy-intensive element is a powerful field-effect transistor in the power supply. Its verification according to the method described in the article by V. Kosenko, S. Kosenko and V. Fedorov "Reverse pulse IP" ("Radio", No. 12 for 1999 and No. 1 for 2000), confirmed the unfavorable prognosis. Transistor Q101 2SK1118 turned out to be faulty. The reason for the rejection was also clear. The monitor manufacturers installed the power element in the TO220ML plastic case on the heat sink without heat-conducting paste. Obviously, thermal overload led to the breakdown of the transistor.

The protection device built into the integrated PWM controllers used in switching power supplies responds to overload in the secondary power supply circuits, but, unfortunately, is powerless against thermal breakdown of the power transistor - an avalanche-like increase in current in its drain circuit, when the transistor actually becomes uncontrollable.

The consequences of thermal breakdown of a switching transistor are known. The rectified mains voltage through a faulty transistor disables the low-power protective zener diode ZD101 in its gate circuit, and then the integrated circuit of the V101 KA3842A PWM controller. After that, one of the diodes of the mains rectifier D101-D104 is damaged. Lastly, the fusible link burns out.

The repair was short lived. True, I had to install a microcircuit with the letter index B instead of A, but this did not affect the performance of the power supply. It was not even necessary to adjust the output voltage with a variable resistor VR101 (the voltage across the oxide capacitor of the C123 filter should be 90 V). The transistor, in principle, could be replaced by any one with a slope of the drain-gate characteristic of 3 A / V. maximum drain-to-source voltage of 600 V, maximum drain current of 6 A, and power dissipation of 45 W. The closest domestic analogue is KP707V2. But for the purity of the experiment, the faulty transistor was replaced with the same type, however, it was installed on the heat sink using heat-conducting paste.

The zener diode was replaced by the domestic KS218Zh. and the mains rectifier diode is KD202R, although it would be better to replace the latter with KD257V or KD257G. As already mentioned, after turning on the device, no adjustment was required.

The second monitor was repaired even faster. The serviceability of the transistor in horizontal scanning was not even checked. As an experiment, instead of a 2SK1118 transistor, a 2SK1117 was installed. which has a steepness of the drain-gate characteristic of 2,5 A / V, but the power dissipation is 100 watts. The metal plate-heat sink of the 2SK1117 transistor (it is made in a standard metal-plastic case TO-220), connected to the drain terminal, had to be insulated with a mica plate. Thermally conductive paste was applied to both sides. The self-tapping screw was also insulated. fixing transistor. Before connecting the drain output of the transistor to the printed circuit board, the quality of its isolation from the negative power bus was checked with an ohmmeter. The repair of the second video monitor was also successful.

If you use an "Abion" monitor or similar, help him. Otherwise, sooner or later, the failure of the power elements is inevitable. Loosen the four screws securing the monitor's plastic back cover and remove it. Look at how a powerful field-effect transistor of the power supply is installed on the heat sink. If heat-conducting paste was used during assembly, traces of it are clearly visible. For comparison, you can see how the horizontal scan output stage transistor is installed.

If you find that the paste is missing, eliminate the marriage. To do this, with a well-heated soldering iron and a rosin-covered "braid" removed from the shielded wire, "suck" the solder from the middle (drain) output of the transistor. Then, unscrewing the fixing screw, take the transistor housing a few millimeters away from the heat sink and inject a small amount of paste into the gap formed, evenly lubricating the surface of the transistor housing with it. When screwing in the screw, excess paste will be squeezed out. It is not necessary to remove them.

It remains to solder the drain output of the transistor in place and close the monitor case. Now you can be sure that there will be no failure of the mains power supply due to thermal breakdown of the switching transistor.

Author: S. Kosenko, Voronezh

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