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Commodore 8-bit — VIC-20 · C128 · C16/Plus4 · PET

The Commodore 8-bit family around the C64: the PET (1977, the very first), the VIC-20 (1981, first sub-$300 color micro, 5M units), the C16/Plus4 (1984, 264 series) and the C128 (1985, tri-mode Z80/C64/C128). Standard Commodore repair: "killer" PSU, PLA, RAM, contacts.

Commodore VIC-20

Commodore VIC-20 — photo by Evan-Amos, domaine public (Wikimedia Commons).

Identification

Model Year CPU Video / Sound Note
PET / CBM 1977 6502 integrated mono screen 1st Commodore, all-in-one CRT
VIC-20 1981 6502 VIC (6560/6561) first mass-market color micro
C16 / Plus4 1984 7501/8501 TED 264 series, C64-incompatible
C128 1985 8502 + Z80 VIC-IIe + VDC (80 col) 3 modes: C128/C64/CP-M

Clocks & frequencies ✅

Component Frequency Role / scope
MOS 6502 (non relevée dans notre source) CPU du VIC-20
MOS 8502 1 MHz ou 2 MHz CPU principal du C128 en mode natif — version améliorée du 6510
Zilog Z80 — second CPU du C128, utilisé en mode CP/M (conjointement au 8502)

The C128 is a dual-processor machine: the 8502 in native mode (1 or 2 MHz, with both 40- and 80-column display) and the Z80 for CP/M. ⚠️ The VIC-20's exact 6502 clock is not given by our source and is therefore not stated here.

Why this matters in repair

A clock that is missing or off-value on a frequency counter points straight at the crystal, its divider chain or the component it feeds — the fastest starting point on a machine that will not boot. Source: Wikipedia (CC-BY-SA).

Repair (technician)

  • "Killer" PSU (VIC-20/C64/C128 brick): the 5 V regulation fails and over-volts → fried chips. Use a modern regulated/protected supply → see also C64.
  • PLA / PLA-like: on the C128, programmable logic → substitution (modern equivalents).
  • RAM: failing DRAM → test / replace.
  • VDC (C128, 80 col): the 80-column mode has its own chip + video RAM → isolate.
  • All-in-one PET: it is an integrated CRT → CRT safety before working; classic PSU/deflection faults.
  • Contacts: cartridge / keyboard ports → clean.

Common faults

Symptom Likely cause Lead
Fried chips killer PSU (over-volt) Modern regulated supply
Black screen / checkerboard RAM / PLA / CPU Test, substitute
No 80-col mode (C128) VDC / video RAM Isolate the VDC
Dead picture (PET) CRT / PSU / deflection CRT safety first

Modernisation & mods — what to buy, how

VIC-20 / C128 'killer' PSUs: replace BEFORE

Like the C64, the VIC-20 and C128 use "killer" PSUs that over-volt when regulation fails → fry the CPU/PLA/RAM. A modern regulated protected supply is essential before any power-up (see C64).

Modern storage (the IEC bus is shared across the line) - SD2IEC 🟢: a cheap SD drive on the IEC port → replaces the 1541; Pi1541 🟢 emulates a real 1541 (high compat); 1541 Ultimate II+ 🟢 = disk + cartridge + REU in one. No soldering. - Datassette → SD: Tapuino / TapeCart 🟢 to drop the tapes. - JiffyDOS 🟡 (replacement/socketed ROM) greatly speeds up disk access.

Video - Composite/S-Video outputs → image-quality mods 🟡; the C128 80-column mode (VDC) is a digital RGBI signal → an RGBI→VGA/HDMI adapter (RGB2HDMI handles it) 🟡 for crisp 80 columns.

Expansion - VIC-20: a RAM expansion is near-mandatory for games 🟢; C128: REU, cartridges 🟢.

⚠️ Do not host ROMs/games — method only.

See also

Sources & attribution

Review & corrections

How this space is used

Spotted a wrong value, an outdated procedure, a chip reference that does not match your board? Say so here, with what you observed (model, board revision, serial number, measurement). Every report is cross-checked against a source before anything changes — an unverifiable correction is published as “reported by …, not cross-checked” rather than silently applied.

Reading is open to everyone; posting requires signing in with Discord. Reports from the wiki's declared reviewers are handled first; anyone else's are read too, but go through a human before anything is changed.