Editing Sinclair ZX Spectrum 128 Troubleshooting Guide (section)

== Video faults ==

=== Vertical "jailbars" on RGB — a documented 128 fault ===

A distinctive Toastrack complaint: faint '''vertical banding on plain, light-coloured areas''' that appears '''on the RGB output but not over RF or composite'''. It is a noise/ground-plane artefact rather than a failing component, and it persists even after a routine recap.<ref name="jail">[https://spectrumforeveryone.com/technical/fixing-vertical-jailbars-48k-128k-spectrum/ "Fixing Vertical 'Jailbars' on your 48K, 128K and Grey Spectrum +2"], Spectrum for Everyone.</ref>

The established fix on a 128 is:<ref name="jail" />

* Replace '''C28''' with a '''47 µF axial''' capacitor. The original is 22 µF, located to the right of the 5-way keyboard connector.
* Replace '''C7''' and '''C8''' with '''1 µF ceramic''' capacitors. These sit next to the IC12 and IC13 RAM devices at the bottom left of the board.

This significantly reduces the effect, though it does not eliminate it completely. The equivalent fix on a grey [[Sinclair ZX Spectrum +2|+2]] is C52 instead.<ref name="jail" />

=== Other video symptoms ===

{| class="wikitable styled-table" style="width:100%; text-align:left;"
! Symptom !! Action
|-
| No picture at all, machine otherwise runs || Check the RGB connector outputs and LUM0 at IC36 pin 7; check the modulator drive waveform
|-
| Picture but no colour || Check the 8.8 MHz colour-encoder clock at IC1 pins 46/47; then the TEA2000 (IC36) and its surrounding components
|-
| No RF picture, RGB fine || Modulator or the MC1376 driver (IC38); check the sound carrier is within tolerance and try retuning the television
|-
| Corrupt or striped display || RAM; run a diagnostic ROM
|-
| Wavy lines across the screen || Press each electrolytic in turn — if the lines clear while pressing one, that capacitor is the fault
|}

Sources: Service Manual and the 48K field-service guide.<ref name="sm128" /><ref name="ian" />