Hitachi LCD TV 32LD390TC normally starts backlighting after 20 seconds or so, no backlight common problem - android - Phones Developers

Photocoupler

I encountered a common issue with the Hitachi 32LD390TC TV. After about 20 seconds of startup, the backlight would not turn on. This type of problem is often related to the power supply circuit. During my diagnosis, I noticed that the voltage on the 24V rail fluctuated between 24V and 25.3V when there was no load. However, once I connected a 40W bulb, the voltage stabilized at 24V, which indicated that the optocoupler-based voltage regulation circuit was functioning properly. The real issue turned out to be in the peripheral components of the power IC. I suspected that resistors R549 and R545 might have degraded over time. These resistors are part of the feedback network for the power IC, and if their resistance increases, it can cause the internal voltage to rise beyond normal levels. This leads to instability in the high-voltage board, causing the transformer to struggle and potentially resulting in the backlight failure. To confirm this, I tested the resistors using a multimeter. Their color codes were brown-black-brown-gold, which corresponds to 10kΩ with a 5% tolerance. I also checked the 5V standby IC and found similar resistors with the same color code, suggesting a possible pattern of degradation. Replacing both R549 and R545 resolved the issue, and the TV started up normally with full backlight functionality.