How to Repair LCD TV No Display, No Backlight But Sound ok
Model: Hisense TLM-55V88GP LCD TV Chassis: MST6M68FQ-LF-S1 Symptom: When power on the tv, the screen no display and no backlight. But the sound is normal.
After power on the Tv, it is no display and their back light also not work. But the sound is working properly. I can hear the Tv different channel sound after changed the Tv channel.
This lcd tv was using the CMO VIT70077.00 Ver.3 inverter board. This inverter board using the OZ964SN IC as their inverter board controller. When checking the OZ964SN pin 5 (VDDA, voltage supply pin), its 5V supply is missing. After refer to the OZ964SN datasheet and actual inverter PCB board design, found out their 5V was using 24V (inverter voltage supply) line through Q4 to control like a switch to send to U2 (78M05D). After that the U2 will generate 5V and send to U1 (OZ964SN) inverter IC pin5.
The Q4 pin E (Emitter) has voltage input, but the pin C (Collector) no any voltage output to U2. The Q4 pin B (Base) was connected to Q1, that’s mean the Q4 was control by the Q1 too. Check the inverter board connector CN1, the BL-ON signal pin has about 3.4V (active high), that’s mean the mainboard was sent out this backlight on signal to inverter board. But the backlight was not working now. So checking the Q1 now, because it is control the Q4 to working or not. Found the Q1 pin B (Base) had connected to a ceramic cap C8 and it was leakage. This C8 ceramic cap is connected to ground and function as a filter cap.
Because of the C8 just function as a filter cap, so try to remove it and give this TV a try. Finally this Tv was brought back to life now. Find a junk inverter board and get the same size and color SMD ceramic cap to replace it. After testing 7 hours, this Hisense TLM-55V88GP LCD TV is working properly.
Conclusion: Because of the C8 leakage and causing the Q4 can’t switch on, so no any voltage input to U2 (78M05D) to generate the 5V for U1 (OZ964SN) VDDA voltage supply pin (pin 5). So that the inverter board not working and causing backlight also not working too. After replaced C8, the TV is ok now. Extra Information: Thank you for all the subscriber email me. Even i'm not reply your email one by one, but i read all your email and really appreciate it. I had received some of the subscriber email me regarding this article, here are the questions: Q1: How do you know that the values of this cap (C8) is equal to that bad one? A1: Actually the SMD ceramic capacitor (non polarity type) their values are depends on their sizes. You can refer to the picture below:
So when find the same values of ceramic caps in the junk board was depends on their sizes and color too. For more information about SMD ceramic capacitor, you can visit to this Ceramic Capacitor Information page. Q2: How you found out C8 was leakaged? A2: When we don't have this inverter board schematice diagram, so we just can do analysis on their PCB circuit board only. Since in between the Q1 through Q4 components are not much, so we can checking the components one by one. The SMD caps and transistor need to remove to confirm that is ok or bad only. So found that C8 was leakaged. Q3: Which testing tool gave you this answer? A3: They are many tools can testing the capacitor, for example, Capacitance meter and Analogue multimeter. For me, i'm using the Analogue Multimeter to test it. Removed the C8 ceramic capacitor and using X10K range to measure it. Normally, when measuring the cap, the needle of meter will go to 0 ohm (charge), after that it will drop to infinity (discharge). When the capacitor can do these two steps, so it can say the capacitor is ok*. So when measuring this C8 cap, the meter needle of meter will not back to infinity and stay on the about 10 ohm position there. So confirm that this C8 was leakaged.
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