Some 48K Spectrums suffer from vertical ‘banding’ on plain, light colours that persists even after the capacitors have been changed. These are colloquially known as ‘jailbars’ and can be a quite annoying visual artifact.
The exact reason that some machines suffer more than others from this effect is not currently known, but it is suspected that it is caused by issues with the ground plane on the Spectrum PCB (see here for a slightly expanded explanation).
This effect can be somewhat alleviated by replacing capacitors C5 to C8 (4 ceramic caps on the south edge of the PCB, below the lower RAM) with 1uf ceramics. These capacitors filter the +12v voltage supply rail to the lower RAM IC’s, and help reduce noise on this rail.
Shown below is the screen display of the same machine after this modification. The jailbar effect is not completely eliminated but is significantly reduced.