c64-hardware

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).