Commodore 64C Troubleshooting Guide
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
[edit | edit source]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.
| 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
[edit | edit source]- 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
[edit | edit source]The C64C lacks a startup chime, so video output is your primary indicator. Use the following table to interpret common power-on symptoms:
| 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
[edit | edit source]- 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
[edit | edit source]The C64C uses two 41464 DRAM ICs (U10, U11) for main memory and a single 2114 for colour RAM.
| 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
[edit | edit source]- 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
[edit | edit source]The C64C uses the 8580 SID (U18) and two 6526A CIAs (U1, U2).
| 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
[edit | edit source]- 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
[edit | edit source]Clock & Reset
[edit | edit source]- 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
[edit | edit source]- 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
[edit | edit source]- 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
[edit | edit source]Dead-Test Cartridge Flash Codes
[edit | edit source]- 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
[edit | edit source]- 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