Many common problems can be seen by having a specific type of screen output.

Below are some common errors that might be quicker identified by their visual appearance.

Inverted / Red Overlay Appearance

Initially looking as if there is an over-wash of red, or lack of blue/green (although blue and green do still both appear), this has been known to be caused by a bad Hu6270 (CPU), specifically the pin 32 (VD2) that was faulty and required a new CPU.

Notice how it looks compared to a working CPU below.