Repair of the LG 42LM669T-ZC TV power module. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Power Supplies
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Apparently, the malfunction is not uncommon for this model, since it occurred after a year of operation of the new TV. He stopped turning on. Even with a cursory examination of the power module board, the name of which is MODEL: PLDF-L103B, the place of heating of the varnish was noticeable in the area of \u601b\u602binstallation of transistors Q3, Q60, designated L3NM60N. Apparently, these are n-channel STL600NM2,2N field-effect transistors (maximum drain-source voltage - XNUMX V, maximum drain current - XNUMX A). Neither the scheme of the TV, nor, at least, the scheme of its power supply module, could be found, so the section of the circuit where the faulty elements were identified was compiled from the printed circuit board of the power supply module.
A fragment of the circuit is shown in fig. 1. Apparently, these are the output circuits of the power factor corrector assembled on the FAN7930BG chip (read more about the power factor corrector in the article "Why is power factor correction necessary?", Published in "Radio" No. 1, 2006, p. 30. - Note ed.). The appearance of the power module board section from the side of mounting large elements is shown in fig. 2, and the view from the side of the printed conductors - in fig. 3.
Rice. 1. Fragment of the circuit
Rice. 2. Appearance of the power module board area
Rice. 3. View of the printed circuit board from the printed conductors
As a result of checking the elements, the following was revealed: the fusible insert F100 5A 250V at the mains voltage input failed, the Q601 and Q603 transistors were broken, the latter had the designation 2F on the case - this is the FMMT2907A transistor. In addition, the resistors R612 (100 Ohm), R608 (10 kOhm) burned out, as well as the F fuse for surface mounting, connected between the source terminals of transistors Q601, Q602, whose designation on the printed circuit board, unfortunately, could not be read. The FAN7930BG chip, oddly enough, turned out to be serviceable. Instead of the STL3NM60N field effect transistor, a commercially available P3NK90ZF transistor was installed. After replacing all the faulty elements, the TV started working, but the transistors Q601, Q602 got too hot, however, none of the numerous protections and fuses worked.
After a more thorough check, already of all the elements of the power module, it was found that the D603 MUR460AX diode was also broken, the failure of which, apparently, provoked all the other malfunctions. After replacing the diode with a MUR460 without the letters AX, the power module worked fine. So far, there are no complaints about the operation of the TV.
Author: P. Yudin
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