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it works perfectly fine however a slight buzzing sound keeps coming from tv’s back side, it reduces on stand by mode. My laptop charger also produces this sound but very low as compared to TV. No such sound comes when lamp or speaker is connected.
Of course you are right. This is due to the fact that it is a PWM waveform which your UPS is generating. The magnetic components inside your LCD power supply (basically filters / inductors) don’t like the non-sine wave. These components make buzzing sound when used on non-sine wave power supply.
As such you don’t need to worry about it.