Editing Commodore VIC-20 Troubleshooting Guide

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This page concentrates on '''hardware faults''' that affect every VIC-20—NTSC models built around the MOS 6560 and PAL models that use the MOS 6561.  Step-by-step diagnosis is given for the two major board families (ASSY 324003 and the cost-reduced ASSY 250403/VIC-20CR), together with tables of the most frequent symptoms, IC failure modes and electrical test points.  Follow anti-static precautions at all times and remember that the early two-prong machines contain a live 9 VAC / linear-regulated 5 V supply.

== Model overview ==
The entire VIC-20 range shares a '''6502 CPU clocked at ≈ 1 MHz, 5 KB of on-board SRAM, twin 6522 VIAs and a single VIC chip that generates 22 × 23 text, colour, sound and the φ2 CPU clock.'''  
* '''NTSC''' boards: MOS 6560, 14.318 MHz crystal, 60 Hz video.  
* '''PAL''' boards: MOS 6561, 17.734 MHz crystal, 50 Hz video.  

{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+'''Major motherboard revisions'''
! Assembly !! Power connector !! Regulator inside computer !! RAM layout !! Notable changes
|-
| 324003 (early) || 2-pin AC || 7805 + fuse || 11 × 2114 (1 K × 4) || Large PCB, nearly every major IC socketed
|-
| 250403 “CR” || 7-pin DIN || External 5 V, internal 12 V doubler || 2–4 larger SRAMs (e.g. 6116) || Smaller PCB, fewer sockets, cassette motor driven by Q3 + Q4 + CR1 zener
|}

== Safety notes ==
Disconnect the PSU before opening the case; early machines contain a linear 5 V regulator that can be hot and hold charge on large electrolytics.  
Use a grounded wrist-strap when handling the MOS 6502, VIC or 6522s.  When probing a powered board keep multimeter tips clear of adjacent pins; slipping across the user-port or cartridge edge '''will''' short 5 V onto delicate lines.

== 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
|}

== IC failure reference ==
{| class="wikitable styled-table"
! IC !! Function !! Failure behaviour
|-
| MOS '''6502''' (40 pin) || CPU, executes KERNAL/BASIC || Black screen, no bus activity; may overheat
|-
| MOS '''6560/6561 VIC''' || Generates φ2, video, 3-voice audio || No video + no sound (total); or colour gone, rolling picture, silent audio
|-
| MOS '''6522 VIA × 2''' || Keyboard, joystick, IEC, timers, cassette || Dead keyboard, motor won’t run, “DEVICE NOT PRESENT”, stuck IRQ
|-
| '''SRAM''' 2114 / 6116 || 5 KB main + colour memory || Garbled text, random lock-ups, wrong colours
|-
| '''ROMs''' (BASIC / KERNAL / CHAR) || System firmware & font || Black screen (BASIC/KERNAL) or correct screen with wrong glyphs (CHAR)
|-
| 74LS logic (138, 245, 157 …) || Decode & bus buffers || Black/garbled screen, often one enable line missing
|}

== 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.

== Electrical test points ==
{| class="wikitable styled-table" style="width:80%;"
! Signal !! Location (any board) !! Expected value
|-
| +5 V DC || 6502 pin 40 or C15 positive lead || 4.90 – 5.15 V
|-
| +12 V DC || VIC pin (schematic) or 470 µF 16 V cap || 11 – 13 V (absent → black screen / no colour)
|-
| φ2 CPU clock || 6502 pin 37 || ≈ 1.02 MHz (PAL) / 1.11 MHz (NTSC) square wave
|-
| RESET || 6502 pin 1 || Low <100 ms on power-up then +5 V
|-
| Composite video || Centre pin of A/V DIN || 1 V p-p with –0.3 V sync
|}

== Internal jumpers ==
Board links that differ between NTSC and PAL set the crystal and VIC clock wiring; do '''not''' alter them unless performing a full conversion (VIC + crystal + jumpers together).  The VIC-20 has no DIP switch for memory or ROM selection, and cassette-motor voltage on the CR board is fixed by Q3/Q4/CR1.

== Preventive maintenance ==
Replacing the large electrolytics (5 V filter ≈ 2200 µF 16 V and 12 V filter ≈ 470 µF 25 V) cures many stability and video-noise issues.  An early two-prong machine benefits from a modern Cool-Running 5 V DC/DC regulator in place of the hot 7805.  Keep the VIC heat-sink intact and renew thermal compound.  Clean cartridge and user-port edge connectors with IPA; oxidised gold fingers often masquerade as cartridge “no boot” faults.

== IC location cheat-sheet ==
=== ASSY 324003 (“two-prong”) ===
* '''UB2''' or '''UE1''' – MOS 6502  
* '''UC1''' – MOS 6560/6561 VIC (under RF can)  
* '''UB3 & UB4''' – 6522 VIA A / VIA B  
* '''UD3 (KERNAL), UD4 (BASIC), UD5 (Character)''' – 8 KB, 8 KB, 4 KB mask ROMs  
* '''Row UE? / UF?''' – 1 K × 4 SRAMs, colour RAM near VIC  
All above are socketed; SRAMs and 74LS logic are soldered.

=== ASSY 250403 (“VIC-20 CR”) ===
CPU, VIC and often one VIA remain socketed, ROMs still grouped together (UA14-UA16).  SRAM count drops to three larger 6116-type ICs.  Transistors '''Q3 & Q4''' with zener '''CR1''' sit next to the cassette/user-port edge and control motor +6 V.

[[Category:Commodore Systems]]