Editing Commodore VIC-20 Troubleshooting Guide (section)

== Diagnostic procedure ==
=== 1&nbsp;·&nbsp;Machine dead / black screen ===
Confirm monitor & cable, then measure 5 V at pin 40 of the 6502 and 12 V at the VIC supply pin.  Check RESET on pin 1: it must pulse low-then-high at power-up.  Verify a 1 MHz square-wave on φ2 (pin 37).  With power proven, remove cartridges and swap in a known-good 6502, then VIC, then KERNAL ROM.  Boot with both VIAs pulled—BASIC will appear without them; if it does, test each VIA individually.  A persisting blank screen after those swaps implies address-decoder logic or a PCB trace fault; probe 74LS138 outputs for activity when the CPU fetches ROM.

=== 2&nbsp;·&nbsp;Partial boot / garbage display ===
Stable power but corrupted screen points to RAM.  The video matrix lives at \$0400–\$0FFF, so bad chips in that block produce immediate garbage.  Replace or piggy-back the lower SRAMs first.  If the banner occasionally appears yet the machine quickly freezes, suspect a VIA flooding IRQ or a BASIC ROM with marginal bits.  A cartridge that runs correctly while BASIC crashes confirms on-board ROM trouble.

=== 3&nbsp;·&nbsp;Good boot, bad graphics ===
Composite cable checked?  If characters themselves are wrong the 4 KB character ROM is likely defective.  Correct characters in the wrong hues implicate colour RAM.  Vertical stripes that repeat every 4 or 8 columns suggest a stuck address line or a dead 74LS157/257 multiplexer feeding the VIC.  Persistent jail-bars or dot-crawl after RAM/ROM replacement hint at dried capacitors in the RF modulator or a VIC analogue fault.

=== 4&nbsp;·&nbsp;No audio ===
Generate a tone in BASIC (`POKE 36878,15`).  Probe the VIC audio pin; if silent the VIC is faulty.  If the waveform is present, follow it through the 10 µF coupling capacitor into the modulator or audio transistor; replace whichever blocks the signal.

=== 5&nbsp;·&nbsp;Keyboard / joystick faults ===
If every key is dead, reseat the ribbon then swap the two 6522s—keyboard scanning will move with the faulty VIA.  Isolated columns/rows trace to breaks in the keyboard PCB or a single VIA port pin.  A joystick shares the same matrix lines; a faulty joystick direction AND its corresponding keyboard column failing confirm the VIA line.