Editing Sinclair ZX Spectrum+ Troubleshooting Guide (section)

== Reading the screen ==

The failure pattern on screen is the fastest route to the fault:<ref name="ian" />

{| class="wikitable styled-table" style="width:100%; text-align:left;"
|+'''Faulty screen designs'''
! What you see !! Likely cause and action
|-
| '''Alternate thick black and white vertical bars''', with or without flashing || Check the CPU (IC2) is seated correctly and any cut legs resoldered; check the ROM (IC5). Test or change '''TR6''' and '''TR3''' (both ZTX313). Then re-measure legs 2–14 of IC6–IC13
|-
| '''Bright, shimmering, fine herring-bone pattern''' filling the screen || Check for −5 V on pin 1 of IC6 — it may have vanished. '''Change TR4'''
|-
| '''Black screen with one or more thin vertical dashed lines''' || Almost certainly one or more 4116 lower RAM chips. Also check the ULA for straddling, bent or damaged legs
|-
| '''Blank black screen, nothing else''' || Likely one of the upper RAM chips IC15–IC22
|-
| '''Flashing blocks''', or a mixture of the above || Check both legs of any upper-memory chip that was cut are resoldered; check the ROM is the correct type for the fitted links and correctly seated
|-
| '''Character break-up when the machine warms up''' || Nearly always the ULA (IC1)
|-
| '''Black dashes or dots appear when warm, then the machine locks up''' || Typical of the lower RAM (IC6–IC13), or the ULA. Do a resistance check on pin 1 while the machine is still warm
|}

The Service Manual adds that '''various faults which appear only when warm''' should start with a thorough heatsink check; if the heatsink is sound, the ULA is the cause, and if the ULA has been replaced before it may be worth adding extra heatsinking to it.<ref name="sm48" />