| name | c64-hardware |
| description | The physical Commodore 64 and its servicing: system specs, the chip complement (6510, VIC-II, SID, two CIAs, PLA, ROMs, DRAM) and each chip's role, the circuit theory (power, reset, clock, video/audio, ports), board revisions and how to ID them, connector pinouts, and troubleshooting. Use this skill WHEN a question is about C64 hardware, a board, or repair — "what chip is U17", "why won't my C64 boot", "blank screen on power-up", "power-brick voltage", "which board revision", "5-pin vs 8-pin video", "expansion port pinout", "PLA failure". Pairs with c64-memory-map, c64-vic-ii / c64-sid / c64-cia, and c64-io / c64-game-ports. |
C64 Hardware & Servicing
This is the repair / hardware-internals skill: the physical machine, its chips,
its circuit blocks, its connectors, and how it fails. For programming a chip's
registers go to the per-chip skill instead (c64-vic-ii, c64-sid,
c64-cia); for the ROM/RAM/IO banking the PLA implements go to
c64-memory-map; for using the ports from BASIC/assembly go to c64-io,
c64-game-ports, c64-disk, c64-tape.
Schematics in this skill's sources are the 1985 Commodore Service Manual, whose
figures were too complex to render as ASCII in the etext and are marked
[Figure: ...]. For trace-level work (component placement, exact net routing)
you need the original scanned schematics (#326106, #251138, #251469).
System at a glance
| Spec | Value |
|---|
| CPU | 6510 (6502 core + 6-bit I/O port at $0000/$0001), ~1.02 MHz NTSC / ~0.985 MHz PAL |
| RAM | 64 KB DRAM (8× 4164) + 0.5 KB color RAM (2114, only 4 bits used) |
| ROM | 20 KB: 8 KB BASIC ($A000–$BFFF), 8 KB KERNAL ($E000–$FFFF), 4 KB character ($D000–$DFFF) |
| Video | VIC-II: 40×25 text, 320×200 hires, 16 colors, 8 sprites |
| Sound | SID 6581: 3 voices, ADSR, filters, two paddle A/D inputs |
| Power in | external brick: regulated +5V DC and 9V AC |
Chip complement (what each chip is and where it sits)
Board positions are the Un designators silk-screened on the PCB. Detailed
pinouts are in references/board-identification.md; per-board parts lists there
map every Un to its exact part number.
| Pos | Part | Role |
|---|
| U7 | 6510 (906107-01) | CPU; its on-chip port $00/$01 banks ROM/RAM and runs the cassette motor/sense/write |
| U19 | 6567 (NTSC) / 6569 (PAL) VIC-II (906109) | video; also generates DRAM RAS/CAS/refresh and the system PHI0 clock |
| U18 | 6581 SID (906112-01) | sound; also the A/D for paddles |
| U1, U2 | 6526 CIA (906108-01) | U1 = keyboard matrix + joysticks + cassette read (FLAG); U2 = serial bus + user port + RS-232 |
| U17 | 82S100 PLA (906114-01) | address-decode / bank-switch logic (the ROM/RAM/IO selector) |
| U3 | 2364 BASIC ROM (901226-01) | BASIC 2.0 interpreter |
| U4 | 2364 KERNAL ROM (901227-03) | OS / KERNAL |
| U5 | 2364 character ROM (901225-01) | character generator font |
| U6 | 2114 RAM (901453-01) | 0.5 KB color/nybble RAM |
| U9–U12, U21–U24 | 4164 DRAM | the 64 KB main memory (eight chips) |
| U8 | 7406 (or 7416) | inverting buffer — drives the serial bus (ATN/CLK/DATA) and reset |
| U13, U25 | 74LS257 | address multiplexers feeding the DRAM |
| U14 | 74LS258 | address mux |
| U15 | 74LS139 | I/O device decode ($D000-block chip selects) |
| U16, U28 | 4066 | analog switches (color RAM gating; paddle routing to SID) |
| U20 | 556 (LM556) | reset one-shot and RESTORE-key NMI generator |
| U26 | 74LS373 | data/address latch |
| U29–U32 | 74LS74 / 193 / 629 / MC4044 | the discrete clock-generator/PLL (original & A boards); the B board folds this into one clock IC (8701) |
Power supply (read this before you suspect a chip)
The external brick supplies a regulated +5V DC and 9V AC. Inside the C64:
- 5V DC (via CN7 pins 5/1, filtered, switched by S1) runs all the digital logic.
- 9V AC (CN7 pins 6/7) is rectified on-board into +12V DC (VR1 = 7812, feeds
VIC, SID, audio amps), a separate +5V CAN (VR2 = 7805, split into Vvid for
video and Vc for the clock), and 9V DC unregulated (cassette motor, RF
modulator on the B board). The raw 9V AC is also exposed on user-port pins
10/11.
The classic killer: as the original brick ages, its internal 5V regulator
can drift high. Because that regulated 5V goes straight to the logic rails, an
overvolting brick can take out the RAM, PLA, and other chips at once. A
"60 Hz hum" white band scrolling down the screen points at the supply or VR2;
a black band on warm-up points at the supply / C90 / C88 / CR4 / VR2. Suspect
the power supply first on multi-chip or intermittent failures.
Connectors / ports
Full pin tables (with the physical connector drawings) are in
references/connector-pinouts.md. Roster:
| Connector | What | Notes |
|---|
| Power (CN7) | 7-pin DIN | +5V DC + 9V AC from the brick |
| Audio/Video (CN5) | 5-pin DIN (orig board) or 8-pin DIN (A/B boards) | 8-pin breaks out separate luma + chroma (S-Video) + composite + audio out + audio in; 5-pin only carries composite + audio |
| RF out | coax | channel-switched TV signal from the modulator |
| Serial bus (CN4) | 6-pin DIN | Commodore IEC: ATN/CLK/DATA/SRQ/RESET — disk & printer |
| Cassette (CN3) | 6-pin edge | MOTOR/READ/WRITE/SENSE + 5V + GND |
| Control Port 1 / 2 | 9-pin D | joystick (4 dir + button) + paddle pots; port 1 = player 2 joystick on CIA1 PB, port 2 = player 1 joystick on CIA1 PA per the service manual wiring |
| User port (CN2) | 24-pin (12+12) edge | CIA2 port B (PB0–7), serial SP/CNT, /FLAG2, /PC2, 9V AC, RESET |
| Expansion / cartridge (CN6) | 44-pin edge | full CPU bus: A0–A15, D0–D7, R/W, PHI2, dot clock, /IRQ, /NMI, /DMA, BA, /ROML, /ROMH, I/O1, I/O2, /GAME, /EXROM, /RESET |
Quick answers:
- Which video pins are S-Video? On the 8-pin DIN, pin 1 = LUMINANCE, pin 6 =
CHROMINANCE (pin 2 GND). The 5-pin board does not break chroma out.
- Does the user port have power? Yes — +5V (pin 2, max 100 mA) and raw 9V AC
(pins 10/11).
- What resets the machine from the bus? /RESET is on serial pin 6, user-port
pin 3, and expansion pin C — pulling it low resets the whole machine.
Reset, RESTORE, NMI (hardware level)
- Power-on reset: U20 (a 556) is a one-shot; pulse width ≈ 1.1 × R34 × C24 ≈
0.5 s. Its active-high output is inverted (U8) to the active-low /RES that
hits the 6510 (pin 40), which then vectors through $FFFC/$FFFD into the
KERNAL. If the machine won't reset and RESTORE is dead, suspect U20.
- RESTORE key: wired straight to hardware, not through the keyboard
matrix. Pressing it triggers U20 → U8 pulls /NMI (6510 pin 4) low; the CPU
vectors through $FFFA/$FFFB. RUN/STOP+RESTORE additionally re-inits I/O and
BASIC pointers (the KERNAL NMI handler checks STOP).
- NMI vector $FFFA/$FFFB, IRQ vector $FFFE/$FFFF, RESET vector $FFFC/$FFFD.
Clock / dot clock
Original & A boards: crystal Y1 = 14.31818 MHz (NTSC color-clock fundamental)
feeds a discrete VCO/PLL (U29–U32) that produces the 8.18 MHz dot clock; the
VIC-II divides the dot clock by 8 to make PHI0, the ~1 MHz system clock, and
locks the loop. The C64B board replaces all that with a single clock IC
(crystal becomes 16 MHz, the 8701 outputs dot + color clocks). PAL machines use
the 6569 VIC-II and a 17.734472 MHz crystal; the divide-by-8 gives the ~0.985 MHz
PAL system clock. "Wavy screen on warm-up" → clock chain (U30/U31); "wrong
frequency" → C70.
Board revisions and how to ID one
references/board-identification.md has the per-board parts lists; the quick
identification:
| Version | Tell | PCB assy # | Schematic # |
|---|
| Original | 5-pin A/V DIN | 326298-01 | 326106 |
| A (CR) | 8-pin A/V DIN | 250407-04 | 251138 |
| B | 8-pin DIN, reduced/single-IC oscillator | 250425 | 251469 |
| B-2 | 8-pin DIN; R28/29/30/36/48 collapsed into RP5, diodes CR100-105 relocated to CR9/12-16 | 250441-01 | 251469 |
So the fastest field ID is count the pins on the video DIN (5 = original,
8 = A or later) and then look at the clock section (discrete logic = original/A,
single clock IC = B). Boards are mechanically interchangeable, but match the
modulator and A/V cable to the board — modulators differ between revisions.
Common failure modes (from the service-manual symptom table)
The full symptom→suspect table is in references/troubleshooting.md. Highest-yield
patterns:
- Blank screen on power-up → power supply first, then KERNAL ROM (U4), PLA
(U17), CPU (U7), VIC (U19), the DRAMs (U9-12/U21-24), U8.
- "OUT OF MEMORY" / garbage characters / locked cursor at boot → bad DRAM
(U9-12, U21-24); run the diagnostic disk.
- Random crashes, wrong colors, fails only when warm → very often the PLA
(U17) or a marginal DRAM; PLAs (especially Commodore's HMOS replacement)
run hot and are a notorious failure point. Power supply is the other prime
warm-up suspect.
- Characters show as solid blocks → U26 (74LS373); graphics chars instead
of letters → VIC (U19); abnormal colors in letters → color RAM U6 / U16.
- Cassette motor never stops, TIP29 gets hot, fuse blows → cassette port
short / R4 open; check U7's port lines.
- DEVICE NOT PRESENT on disk → CIA U1/U2, U7, the serial bus resistors
R28/29/30 and the U8 buffer.
The single most-replaced parts on a dead C64 are, in rough order: the PLA
(U17), the DRAM chips, the failing power brick / 5V regulator, and the
CIAs (U1/U2, killed by hot-plugging joysticks/user-port devices).
How to read the references
references/specifications.md — read for the official spec sheet, the
product parts list (case/keyboard/accessories), and the block-diagram intro.references/circuit-theory.md — read when you need how a block works:
power supply rails, reset one-shot, clock/PLL, I/O & ROM address decode and
the expansion port, RAM control / refresh, the 5-pin vs 8-pin video & audio
output stages, cassette, keyboard/joystick/paddle, serial + user port. This is
the prose behind every schematic block.references/troubleshooting.md — read when diagnosing: the full
SYMPTOM → POSSIBLE SOLUTION table mapping a fault to the chips/parts to check.references/board-identification.md — read to identify a board revision,
for the per-board parts lists (every Un → part number, plus all R/C/CN/misc
values), and for the chip pinouts (CIA, SID, 6510, PLA, VIC-II, DRAM).references/connector-pinouts.md — read for the external port pin tables
and connector drawings: control ports 1/2, cartridge/expansion slot, A/V,
serial, cassette, user port.references/pla-82s100-datasheet.md — read for the 82S100 part itself
(the programmable-logic-array datasheet) and the provenance of the dumped C64
PLA. The actual banking equations live in c64-memory-map.
Cross-links
- Register-level programming of each chip → c64-vic-ii, c64-sid,
c64-cia.
- The ROM/RAM/IO banking the PLA implements (the $00/$01 port bits LORAM/HIRAM/
CHAREN, GAME/EXROM) → c64-memory-map.
- Driving the ports from software (joystick/paddle reads, user-port I/O, serial,
cassette) → c64-io, c64-game-ports, c64-disk, c64-tape.
- For deeper generic 6510/6502 CPU semantics you may optionally consult the
companion repo
https://github.com/sunsided/6502-skills (a suggestion, not a
dependency).
