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 — 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 | 1.02 MHz (NTSC) · 1.108 MHz (PAL) | CPU of the 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 6502 clock follows the TV standard — 1.02 MHz NTSC, 1.108 MHz PAL: on a frequency counter that is the figure for the machine in front of you, not a tolerance.
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.
Procedure — a dead computer, step by step ✅¶
There is a CRT inside this machine
The tube holds its charge with the machine unplugged, and on these all-in-ones the analogue board sits right where your hands go. Discharge it, or work on the logic board out of the case. → CRT safety
What this adds
Ordered for a home computer: what kills these is its power supply, its battery, its capacitors, its socketed RAM and its keyboard — roughly in that order, and well before the CPU. → PCB diagnosis
1. The power supply — measured before it is plugged in. This is the one step not to swap around. On a machine of this age a supply does not simply stop: a 5 V regulator that fails high puts 7, 9, 12 V onto the bus and takes the RAM and the custom chips with it. Measure every rail off the machine, under a dummy load if you can, and only then plug it in.
2. The battery, if it has one. Any RTC or backup cell soldered to a board is a future leak. If yours has one and it has never been changed, take it out before it takes the tracks with it. → Backup batteries
3. Capacitors. Recap is the default on these machines, not a last resort — and on the supply first, since that is what protects everything else. → Recap
4. RAM, on its sockets. These machines were built to be repaired: the RAM, and often the logic, sit in sockets. Reseat everything first — thirty years of oxide is a classic no-boot. Then swap chips: a bad RAM chip usually gives a repeatable pattern of garbage or a screen frozen at a fixed character.
5. The keyboard. A console does not have one; a computer's is a wear part. A membrane keyboard fails as whole dead rows or columns — that is the membrane, not the machine. A mechanical one wants its contacts cleaning, not replacing. Check this page: the failure mode is often documented per model.
6. Storage — tape or disk. Both often come down to the same things: a dirty head, and for tape the azimuth and a belt turned to tar. ⚠️ On the floppy side, check whether the mechanism actually has a belt before looking for one: many do, but some drives are direct drive, and hunting a belt that isn't there means stripping a drive for nothing. A machine that boots but loads nothing is a drive fault, not a logic fault — do not go looking on the board for it.
7. Video output. RF, composite, RGB or a dedicated monitor, depending on the machine (see Identification). Test with something else before you conclude: a period modulator and a modern TV disagree far more often than either is broken.
8. Still nothing. Now the logic: rails on the board, reset, clock, bus. → PCB diagnosis
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.
Archived technical documentation ✅¶
Manufacturer documentation archived locally: schematics, block diagrams, expected voltages, parts lists. The reference when a symptom matches no known fault.
Status
Manufacturer documents kept for preservation (machines out of production for decades), with no explicit licence — the same position as for the cabinet manuals already archived here.
See also¶
- Commodore 64 (the flagship) · Amiga · CRT safety · Recap.
Sources & attribution¶
- Photo Commodore VIC-20 — Evan-Amos, domaine public, Wikimedia Commons — https://commons.wikimedia.org/wiki/File:Commodore-VIC-20-FL.jpg
- Wikipedia — VIC-20 / Commodore 128 / Commodore PET / Commodore Plus/4 — CC-BY-SA — https://en.wikipedia.org/wiki/VIC-20
-
Established Commodore community knowledge (killer PSU, PLA) — synthesis. See C64 for PSU/PLA detail.
-
Wikipedia — VIC-20 (MOS 6502 clock: 1.02 MHz NTSC, 1.108 MHz PAL — the clock follows the TV standard, as on every Commodore of that generation) — CC BY-SA — https://en.wikipedia.org/wiki/VIC-20
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.