Editing Commodore VIC-20 Troubleshooting Guide (section)

== Typical symptoms and first suspects ==
{| class="wikitable styled-table" style="width:100%;"
! Symptom !! Most probable culprits (in order) !! Diagnostic clue
|-
| Black screen, no chirp || PSU / fuse ▸ 7805 ▸ 6502 ▸ VIC ▸ KERNAL ROM || 5 V missing, φ2 clock absent or RESET stuck low
|-
| Random or “checkerboard” characters || 1 K SRAM(s) ▸ character ROM ▸ VIC || Pattern repeats every N bytes ⇒ single data bit bad
|-
| Stable screen, wrong colours || Colour SRAM (2114) || Characters correct but always same or flickering colour
|-
| No sound, video OK || VIC audio DAC ▸ coupling capacitor ▸ RF modulator transistor || Scope shows silence on VIC pin; A/V hum test silent
|-
| No keyboard at all || VIA #1 ▸ keyboard ribbon || System boots, cursor blinks but no input accepted
|-
| Some keys/joystick directions dead || Matrix row/col break ▸ VIA port line || Entire row/column missing in key-test PEEK(37151)
|-
| Intermittent resets / flashing || Power switch ▸ cracked solder on 7805 ▸ dried electrolytics || Wiggle switch recreates fault; volt-meter shows 5 V dip
|}