Editing Commodore 64C Troubleshooting Guide

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The Commodore 64C, a late-model revision of the classic C64, has an improved motherboard design—but failures still occur. This guide details systematic troubleshooting for the C64C, covering common symptoms, diagnostic steps, and component-level remedies.

== Preliminary & Power-up Checks ==

Before investigating deeper faults, always confirm the basics:

# Disconnect all peripherals (cartridges, datasette, drives).
# Inspect the board for burnt, cracked, or leaking components—especially capacitors and voltage regulators.
# Check the power supply brick with a multimeter:
#* '''+5 V DC''' (±5%) between pin 2 (5V) and pin 1 (GND) of the power DIN.
#* '''9 V AC''' (rms) between pins 6 and 7 of the DIN.
# Inspect and reflow the power jack solder joints if needed.
# Confirm the power switch is not intermittent or oxidised.

{| class="wikitable styled-table" style="text-align:left"
! Test Point !! Expected Value !! Notes
|-
| Power DIN pin 2 ↔ pin 1 || +5 V DC || Main logic supply
|-
| Power DIN pin 6 ↔ pin 7 || ~9 V AC || SID/VIC-II analogue, time-of-day
|-
| VIC-II Vdd (pin 28, 8565) || +5 V DC || Video IC supply
|-
| SID Vdd (pin 28, 8580) || +9 V DC || Audio IC supply
|-
| RESET (expansion port pin C) || Low → High (5 V) || Must release high after power-on
|}

=== Common Power Faults ===
* '''5 V missing/high''' → no boot, possible chip damage.
* '''9 V AC missing''' → no sound, black screen, no colour.
* '''Intermittent power''' → cracked solder, faulty switch, or bad PSU.
* '''Hum or buzzing''' → dried-out PSU capacitors.

Never proceed with an unstable or suspect power supply.

== Display & Chime Diagnostics ==

The C64C lacks a startup chime, so video output is your primary indicator. Use the following table to interpret common power-on symptoms:

{| class="wikitable styled-table"
! Symptom !! Likely Cause !! Action
|-
| Black screen, no border || PLA, VIC-II, or CPU failure; no clock; bad ROM || Check voltages, swap VIC-II, PLA, CPU; test with Dead-Test cartridge
|-
| Blank screen with border || BASIC ROM failure || Swap BASIC ROM (U3)
|-
| Coloured border, garbage text || RAM or character ROM fault || Test RAM (U10–U17), swap character ROM (U5)
|-
| Solid white/grey screen || VIC-II alive, but no bus access || Check PLA, address lines, CPU
|-
| Rolling/distorted image || Wrong VIC-II or bad clock circuit || Confirm VIC-II type, check 8701/oscillator
|-
| No video, but power LED on || Dead VIC-II or missing 5 V/9 V || Confirm supply rails, swap VIC-II
|}

=== Minimal Boot Procedure ===
# Remove SID, both CIAs, and 4066 ICs (if socketed).
# Power up: if BASIC screen appears, one removed IC was dragging the bus.
# If still dead, proceed to chip substitution (see below).

== Memory & ROM Faults ==

The C64C uses two 41464 DRAM ICs (U10, U11) for main memory and a single 2114 for colour RAM.

{| class="wikitable styled-table"
! Symptom !! Probable Fault !! Diagnostic Action
|-
| Black screen || Lower RAM or PLA || Swap/test RAM, try Dead-Test cartridge, check PLA
|-
| Garbage on screen, freezes || Upper RAM or 74LS257 mux || Swap/test RAM, replace 74LS257 (U13/U25)
|-
| Wrong "BASIC bytes free" || Partial RAM failure || Confirm both 41464s are good
|-
| Legible layout, corrupt characters || Character ROM (U5) || Swap character ROM
|-
| No boot, no border || KERNAL ROM (U4) || Swap KERNAL ROM
|}

=== RAM Testing ===
* Use a Dead-Test cartridge: border flashes indicate bad RAM bit/chip.
* Piggy-back a known-good 41464 onto each RAM IC; if behaviour changes, replace the underlying chip.
* If RAM replacement does not resolve Dead-Test errors, replace both 74LS257 multiplexers.

== Audio & I/O Failures ==

The C64C uses the 8580 SID (U18) and two 6526A CIAs (U1, U2).

{| class="wikitable styled-table"
! Symptom !! Likely Fault !! Action
|-
| No sound at all || SID (U18) or missing +9 V || Confirm +9 V at SID pin 28, swap SID
|-
| One voice/noise/distortion || Partial SID failure || Replace SID
|-
| No keyboard/joystick || CIA-1 (U1) || Swap CIA-1
|-
| No IEC/serial devices || CIA-2 (U2) || Swap CIA-2
|-
| No RESTORE key || CIA-2 or 556 timer || Replace CIA-2, check 556
|-
| Joystick port issues || CIA-1, port traces, or resistor array || Test continuity, swap CIA-1
|}

== Connector & Socket Issues ==

* Inspect all IC sockets for corrosion or poor contact—especially after prior repairs.
* Edge connectors (cartridge, user, cassette) may develop cracked solder joints; reflow as needed.
* Joystick and power jacks are prone to mechanical stress; check for fractured pins or PCB traces.

== Component-level Tests ==

=== Clock & Reset ===
* System clock: 1.0227 MHz (PAL) or 1.023 MHz (NTSC) at CPU pin 6 (φ2).
* VIC-II clock: 17.734472 MHz (PAL) or 14.31818 MHz (NTSC) at VIC-II pin 22.
* RESET: should be low for ≈½ s at power-on, then high (5 V).

=== Chip Substitution ===
* Swap socketed chips (PLA, VIC-II, SID, CPU, CIAs) one at a time with known-good parts.
* Always power off before removing/inserting ICs.
* Never piggy-back custom MOS chips (PLA, VIC-II, SID).

=== Thermal Checks ===
* After 1–2 minutes, gently touch chips: a ''too-hot-to-touch'' PLA, SID, or RAM usually indicates failure.
* Use freeze spray: if behaviour changes when cooling a chip, suspect that IC.

== Error & Code Tables ==

=== Dead-Test Cartridge Flash Codes ===
* Each border flash = failed RAM bit (see [[Commodore 64 Dead Test Cartridge]] for full mapping).
* No flashes, no display: suspect PLA, CPU, or VIC-II.

== Storage/Subsystem Failures ==

* Cartridge port: If autostart cartridges work but BASIC does not, suspect BASIC/KERNAL ROMs.
* Datasette: No response may indicate CIA-1 failure or bad 4066 switch.
* IEC serial: No disk drive detection = CIA-2 or 7406 buffer fault

[[Category:Commodore Systems]]
[[Category:Troubleshooting Guides]]