I can't really afford to replace the monitor - and it does 'work' but perhaps with high - eye strain. this is not a graphics card issue - but something with the monitor). Unplugging the HDMI leaves the checkboard no matter which input you pick (HDMI, Display Port, USB3 etc.) - Swapping the cables or using a different computer does not affect the display (i.e. It seems the screen is flipping between two images, the checkerboard and the computer's screen/HDMI input.
However, after unplugging the HDMI the checkboard dimmed significantly and when I reconnected the computer's background came back but now with a superimposed checkboard. At first the checkerboard was all that was displayed. I have dual Samsung s24h850qfn displays and today one of them began to display a flickering BW checkerboard pattern after a misterious laptop crash (laptop is ok and seems to work fine).
