Core tips£ºPanasonic TH-42PA20C plasma TV black screen and intermittent synchronization
This machine is cooler, the boot image and sound normal. But normal work after about 10 minutes, periodic appears instantaneous black screen, and then into the whole image about flashing, and the loss of step full screen image into black level, and to restore the normal image, then enter the next cycle of instantaneous black, and back again. Careful observation, found that each cycle a black line to instantaneous step is about 20s. Which black screen flashes about 1s, lost step salty horizontal black treaty in 4S, each cycle about 15s normal phase images. It was also found that failure when, the menu is not normal.
A failure occurs, which indicates that the performance of a part is poor, but it is not completely damaged. Black, yet further, that is circuit in video signal processing circuit is normal, and D board, logic board) has nothing to do with the DG on the plate (DG board: where the whole digital image processing and control system circuit board). IC is IC1301, IC1302, and DG in the row. Because of the normal image, most of the time, but only periodic appear black and lost step, description of the image of the synchronous circuit and does not damage but are subject to interference.
Image is not normal, is certainly DG in image processing contactors abnormal caused, either is the image processing IC internal adverse, may also be the image processing IC power supply side and the peripheral circuits of the IC has open circuit and short circuit. For many years and a lot of maintenance experience, the power supply circuit is not normal, often can cause the fault phenomenon of all sorts of strange things. Because the detection power supply voltage is the most simple and fast, it is decided to check the power supply from the DG board.
The DG board is powered by the socket DG2 introduced, which is supplied by the PA socket PA2. Pa2 11, 13 feet, it should be is a 3.3V power supply output, but the actual detection found only 2.4V voltage, IC7504 PA board is a 3.3V power supply IC, which is a DC-DC converter circuit, the input 12V transform into 3.3V, in fact, is a small switching power supply, input 12V electric pressure applied to the IC7504 input end of the 1 foot (switch tube D is IC of foot, foot is switch tube s pole), the foot is the control input terminal of the switch power is turned on, 4 feet external the pressure resistance is used to set the switching power supply output voltage value. L7511 is the energy storage inductor, the D7515 is the switching power supply of the continued flow diode, C7517 is the switching power supply filter capacitor (see figure below). The input pin is normal, while the output voltage of the C7517 is only 2.4V. Measurement ALCL (2 feet have normal 12Vp-p square wave, digital million with diode block detection D7515 found forward voltage drop is 1.1V and compared to measurements of SUB5VDC-DC freewheeling diode D7507 and main 2.5VDC-DC freewheeling diode D7512 and found that the voltage is 0.18V because the freewheeling diode are HF Schottky diode. Use the digital form diode retaining measure their forward voltage drop should 0.18V. Therefore, the decision of the D7515 forward on the resistance of the big change. After the replacement of D7515 boot normal.