I have a Sony VGA Monitor CPD-E540.

The screen has uneven brightness. The left edge is brighter than the rest of the screen. Does anyone know what causes this issue? Or which circuit I should inspect?

Looks like this except no retrace lines and left hand edge that is brighter rather than right hand edge.
https://www.youtube.com/watch?v=qBzTAul-Gvw&t=34s

Generally speaking, this is caused by the video or kine b+ being modulated at the horiz rate. Any secondary voltage supply sourced from the horizontal output or HV transformer will cause this if the bypass filter is failing. Usually, you can find an electrolytic cap in the sweep area. If you have a scope, set the timebase to the horiz frequency and check the caps for noise. If this monitor uses a smps type low voltage power supply, a bad cap there can cause this as well.

If you have an esr meter, look for a bad electrolytic.

John

Some monitors fed the video directly to the neck board. The filter cap for the video amp B+ rail could also be on the neck board instead of the sweep section.

Some monitors fed the video directly to the neck board. The filter cap for the video amp B+ rail could also be on the neck board instead of the sweep section.

Or you could add one right there even if it was in the sweep section..:D

John