Hot summer of 1999, when the room temperature is above 30 degrees, it was hard not only to users and computers. At this time with a difference of a few days went down two 14-inch monitor "Abion" Taiwanese firm ABI OVERSEAS LTD. The first explanation suggests itself - fault throws of voltage in the mains. But replaced fuse burned with a blinding flash. So, a faulty power elements inside monitor.
Usually heavy duty first not withstand powerful key the transistor in the output stage of line scanning. Reaching down, he overloads the power supply unit. Alas. check disproved this assumption. Transistor Q303 (here and below the names of the elements given in accordance with the notation on the PCB of the monitor and the markings on the hulls of elements) was completely defective.
Another high-energy element power MOSFET in the power supply. Its validation by the method described in article V. Kosenko, S. Kosenko and V. Fedorov Flyback pulse SP" ("Radio". No. 12 of 1999 and No. 1 of 2000 G.), confirmed the poor prognosis. Transistor Q101 was 2SK1118 defective. Understandable was the cause of the failure. Force element in plastic the housing TO220ML manufacturers monitor mounted on a heat sink without heat-conductive paste. Obviously, thermal overload and led to the breakdown of a of the transistor.
The security device embedded in used in pulsed power supplies integrated PWM controllers, responds to an overload in the secondary circuit power, but, unfortunately, powerless against thermal breakdown of the power transistor - avalanche-like increase of the current in its stock chain when the transistor actually becomes unmanageable.
Effects of thermal breakdown of the switching transistor is known. Rectified mains voltage through faulty transistor disables low-power a protective Zener diode ZD101 in the chain of his shutter, and then an integrated circuit A PWM controller V101 CAA. After that damages a diode network rectifier D101-D104. In the last turn blows fuse.
The repair was short lived. However, I had to install the chip with an alphabetical index In instead of And, but on the efficiency of the power supply is not affected. Not it took even adjust output voltage variable resistor VR101 (the voltage on the oxide filter capacitor C should be 90). Transistor, in principle, you can replace any with the steepness of the drain-gate characteristics 3 A/V. the maximum voltage of the source-drain 600 V, maximum drain current 6 A and power dissipation of 45 watts. The nearest domestic analogue - KW. But for the purity of the experiment faulty transistor was replaced with the same type, but the heat sink is installed with the use of a heat-conducting paste.
A Zener diode has been replaced by domestic XJ. a diode rectifier network - CDR, although better would be to replace the last KDV or CDG. As already mentioned, after switching adjustment was not required.
A second monitor was repaired even faster. The serviceability of the transistor in the line scan was not even checked. As an experiment instead transistor 2SK1118 was installed 2SK1117. which the steepness of the drain-gate features 2.5 A/V, but the power dissipation of 100 watts. Metal plate-heat sink transistor 2SK1117 (it is made in standard plastic case TO-220) connected to the output of the drain, had insulating mica plate. Heat-conductive paste was applied on both sides. Was isolated and the tapping screw. fasten the transistor. Before connecting to PCB output transistor's drain, the quality of its isolation from the sub-zero tires power was checked with an ohmmeter. The second monitor was also successful.
If you use the monitor "Abion" or similar, help him. Otherwise, before or later, failure of power elements is inevitable. Remove the four screws that secure the back cover of the plastic case of the monitor, and remove it. See how installed on the heat sink power MOSFET a power supply. If the Assembly was used a heat-conducting paste, traces of it visible. For comparison, you can see how you installed the transistor horizontal output stage sweep.
Having found that the toothpaste is missing, repair the marriage. For this well-heated a soldering iron and procritline "twist", shot with a shielded wire, "suck" the solder from the middle (drain) of the output transistor. Then, unscrewing the attaching screw, pull the body of the transistor by a few millimeters from heat sink and into the crack, enter a small amount of paste, evenly brushing the surface of the body of the transistor. When screwing the screw excess paste will be squeezed out. To remove them is not necessary.
It remains to be soldered in place of stock output transistor and close the Cabinet. Now you can be sure that the failure power supply unit due to thermal breakdown of the switching transistor will not.
Author: S. Kosenko, Voronezh