LCD screen buzzing tone

[fonograph]

I obtained old 1280x1024 LCD screen,its Belinea brand,it looks like something from 2002-2006 year.There is one very weird thing about this acient LCD,it starts to make buzzing tone once its brightness setting is set bellow 73.

Literaly you can set it to any brightness from 73 to 100 its dead quiet,but once its bellow it starts to make buzzing tone,it sound like 1000 Hz square wave.As you set it to lower and lower brightness,the tone gets lower and between 73 and 60 it gets louder.Is this perhaps some old capacitor used in PWM control of the brightness that due to age gets piezoelectric,or starts to vibrate due to age?

What component,probably in PWM circuit can start buzzing at switching frequency due to age? Correct me if I wrong,but only widely used component that buzz is there cheap high capacitance ceramic capacitors,but I doubt its been buzzing like this from factory.

EDIT: Using the monitor right now,its smells little bit like burning plastic around it,kind of like before my previous monitor started burning.

[TMM]

It is the CCFL backlight tubes or CCFL inverter. The tubes are driven by high voltage AC and often the inverter circuit has an oscillation frequency which varies with the output current. If the load changes from nominal the oscillator may run at a frequency in the audible range and you'll hear a buzzing either from the inverter (usually inductor/transformer) or tubes themselves. By adjusting the brightness you're probably causing the oscillation frequency to change to an inaudible frequency or at least a frequency which doesn't hit a resonant mode of the components. The burning plastic smell is probably a sign that something has been or still is arcing over - check the plugs and cables going from the inverter board to the tubes. Often the contacts inside the plugs make bad contact and an arc forms, melting the plastic housing

[mariush]

Since it's so old, it's probably using CFL tubes instead of led for backlight.
So there's a high voltage transformer involved used to power the fluorescent tubes. These fluorescent tubes will often use more power as they age so i don't know, maybe they're "power consumption profile" has changed.
There's also electrolytic capacitors involved, which can give bad power to the circuit which creates the high voltage using that transformer. So it could be lower quality input voltage coupled with different power consumption from tubes could make the transformer to work in a range where it normally wouldn't and therefore vibrate.
Check the electrolytic capacitors on the board and also check the transformer for weak soldering joints (due to cold/hot cycles).

Also regarding the smell ... a common problem with fluorescent tubes is that sometimes the wires disconnect from the ends, due to repeated hot/cold cycles .. so there's either some weak contact there, at one of the tubes, or there's sparks between the lead (the wire) and the metal shielding that's over the area where the wire was soldered to the tube end.
You may want to check that out.


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