I am having trouble tracing the problem of a LG 37" LCD TV with conventional backlights. The power supply is providing the proper voltages to the main board. 20V, 12V, and 3.5V. also, the power on line goes to 3.23V when the power on button is activated.
I suspect a problem with the main board. I disconnected the inverter on line at the main board and supplied it with 3.3V. The inverters lit the backlight for about 2 seconds, and then shut off. It seems the main board never attempts to supply the inverter on voltage.
This TV is a bit newer than most I have salvaged, and I do not know how the protection circuits operate on this model. On older sets I am able to activate the backlights as long as the inverter on line is powered (This unit did the 2 seconds to black).
A quick check with the ohm meter did not show gross abnormalities in the inverter power switching transistors. I also traced the transistors that are associated with the inverter on circuit on the main board and found no gross problems with the switching transistors. (The basic check I do is for opens, shorts, and diode check mode for hi/lo resistance of semiconductor junctions between the 3 leads).
Since the backlights do not stay on for more than a few seconds when the inverter line is activated, The tubes, step-up transformers, switching transistors in the inverters, or switching logic could also be at fault. Puts me at square one (or two) for troubleshooting this unit. (It was working before this failure.)
Here are a few pics of the boards. The panel is a LG type LC370WUG.
I suspect a problem with the main board. I disconnected the inverter on line at the main board and supplied it with 3.3V. The inverters lit the backlight for about 2 seconds, and then shut off. It seems the main board never attempts to supply the inverter on voltage.
This TV is a bit newer than most I have salvaged, and I do not know how the protection circuits operate on this model. On older sets I am able to activate the backlights as long as the inverter on line is powered (This unit did the 2 seconds to black).
A quick check with the ohm meter did not show gross abnormalities in the inverter power switching transistors. I also traced the transistors that are associated with the inverter on circuit on the main board and found no gross problems with the switching transistors. (The basic check I do is for opens, shorts, and diode check mode for hi/lo resistance of semiconductor junctions between the 3 leads).
Since the backlights do not stay on for more than a few seconds when the inverter line is activated, The tubes, step-up transformers, switching transistors in the inverters, or switching logic could also be at fault. Puts me at square one (or two) for troubleshooting this unit. (It was working before this failure.)
Here are a few pics of the boards. The panel is a LG type LC370WUG.
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