Universal Smart LED TV Mainboard Save by a Small Capacitor
Today, a TV repairer sent a malfunction LED/LCD TV Mainboard asked for help. He said this TV Mainboard some time can work fine but some time not. When the TV mainboard is not working, try to read their “Start-up Log” (for more details, please read my V5.0-OLED/LED TV Repair Tips ebook) through RT809F programmer, but nothing at all. If shorted the 5V_STB & 5V_M, the Start-up Log can printing/show normal information. Disconnect the 5V_STB & 5V_M, press the power on button, the TV Mainboard can operate and working fine.
After testing this TV mainboard, the TV Mainboard symptom is like he said exactly same. The TV mainboard when supplied voltage, their 5V_M no any voltage appear at all. Normally it will appear 5V_M a moment then off when TV is in Standby Mode. Before we analyze the failure of the Mainboard, let’s look at the power start-up sequence of the whole TV mainboard. The first stage of TV Mainboard starting up requires the CPU to be ready. For the CPU to be ready, the first working condition is their power supply supplied. For example power supply like standby voltage, CPU kernel voltage and digital circuit power supply. When the TV Mainboard is powered on, in fact, all the power supply is not been controlled. Which means the CPU gets all the power supply at the moment of power on, their reset circuit and clock circuit are work at the same time. After meeting all the condition of the CPU, the CPU begins to read the BootLoader program/firmware & transfer the control right, and then it starts to initialize the hardware device. If all normal, the “Start-up Log” or print information will appear on RT809F program. After the hardware device initialization is completed, it returns to the standby mode and the control circuit that the output of 5V_M is cut off and waiting for the boot command (press power on button from TV or Remote Control).
From the malfunction of the TV Mainboard, their 5V_M is always be in no output state. So the power supply of CPU is not enough at the moment of power supply supplied and it will not or can’t start-up the TV mainboard. So, first of all, we need to check their control circuit state, 5V_M circuit. Measured the QM3 pin-G (Gate pin) is 5.1V and QM4 pin-B (Base pin) is 0.3V. Check QM3 & QM4 transistor in diode range, both are normal. So confirm that the 5V_M circuit is in closed state.
From the above circuit diagram, we can see that the on/off signal is from main chip (CPU). Measure the left side of RM8 resistor is about 0.23V. Try to cut off the printed circuit line (on/off control signal) from main chip. After that, their 5V_m voltage output is normal now. The conclusion is the main chip (CPU) damaged! For this TV Mainboard, if just because of this problem to replace whole mainboard, it is not cost-effective and not worth to do that. As mentioned earlier, as long as this mainboard 5V_STB and 5V_M shorted a while then released it, the TV Mainboard can working perfectly. That’s mean the 5V_M need some voltage output a moment only, then this TV Mainboard can normal operate. What we want is that there is output of 5V_M when it is energized. So modify the 5V_M circuit to add a small SMD capacitor 10uF to QM3 pin-G (Gate pin) to ground (GND), like the photo below:
After added a small smd capacitor on the 5V_M circuit, supplied the voltage to Mainboard, press power on button, it can start-up and working properly. Press again power button, it can shutdown normal. The TV Mainboard after modify, everything is normal now. So the TP.MS628.PC821 Universal LED/LCD TV Mainboard is modifying successfully!
Best Ebooks to learn Troubleshooting & Repairing LED/LCD TV Mainboard
2018 LCD/LED Screen Panel Repair Guide
Learn the Basic info for LCD & LED TV T-con Repair and TV Mainboard repairing information. It will helps to you in LCD & LED TV Repair!
PV1-Plasma TV Repair Tips Ebook by Kent Liew:
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