How many motherboard revisions did the Commodore 64 have?

The Commodore 64 had four main PCB revisions: Assy 326298 (Rev A, 1982), Assy 250407 (Rev B), Assy 250425 (Rev C, the most common) and Assy 250466 (Rev D, found in later C64C machines). Each revision reduced the component count and fixed known issues from the previous version.

How do I identify which motherboard revision my C64 has?

Look at the ASSY number printed on the underside of the PCB, usually stamped near the cartridge port edge. The number will be one of 326298, 250407, 250425 or 250466. Some boards also have a handwritten or stamped revision letter (A, B, C or D) nearby.

What does ASSY mean on a Commodore 64 motherboard?

ASSY stands for Assembly. It is Commodore's internal board identifier, used to track which version of the PCB design was manufactured. The full ASSY number combined with the revision letter uniquely identifies the board version.

What is the difference between the SID 6581 and SID 8580?

The SID 6581 is found in earlier C64 boards and runs at 12V for the filter circuit, giving it a warmer, slightly distorted sound. The SID 8580 is found in later C64C machines and runs at 9V, producing a cleaner sound with a different filter response. Many musicians prefer the 6581 for its character.

What is the PLA chip and why does it fail?

The PLA (Programmable Logic Array) is the address decoder chip that controls how the C64 routes memory and I/O. The original ceramic package degrades over time as the capacitor inside the chip dries out, causing random crashes, graphics glitches or a completely dead machine. Modern solid-state PLA replacements are widely available and strongly recommended for restored machines.

Can I replace the C64 motherboard with a modern one?

Yes. The Ultimate 64 Elite, EVO64 and C64 Reloaded MK2 all fit inside an original C64 or C64C case and use the original keyboard matrix and ports. The Ultimate 64 Elite uses an FPGA to emulate the full C64 hardware in one chip. The EVO64 and C64 Reloaded MK2 use modern discrete components with original or compatible chips.

What chips are on a Commodore 64 motherboard?

The main chips on a C64 motherboard are the MOS 6510 CPU, the VIC-II video chip, the SID sound chip (6581 or 8580), two CIA interface adapter chips (6526 or 8521) and the PLA logic chip. Together they handle all processing, video output, sound synthesis, keyboard scanning and memory management.

Which C64 motherboard revision is considered the best?

Revision C (Assy 250425) is generally considered the most reliable and is the version most repair documentation is written for. It is also the most commonly found board. Rev D (Assy 250466) is equally reliable but less common. Earlier revisions (Rev A and B) are rarer and can have more age-related issues due to their higher component count.

Commodore 64 Motherboard

PCB revisions, chips and modern replacements

Commodore 64 Motherboard: Inside the World's Best-Selling Home Computer

The Commodore 64 motherboard, also known as the PCB (Printed Circuit Board), is the central circuit board that connects every chip, port and component in the machine. Between 1982 and 1994, Commodore produced four main board revisions, each identified by a unique ASSY number printed on the PCB itself.

Understanding which revision is inside a C64 matters to collectors, repair technicians and enthusiasts. The ASSY number determines which chips are fitted, which bugs are present and how the machine behaves with certain cartridges and upgrades. Revision C (Assy 250425) is the most commonly found board and is generally considered the most reliable.

Commodore 64 PAL motherboard PCB showing the main chips and circuit layout

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PCB Revisions: Assy 326298 to 250466

Commodore released the C64 with the Assy 326298 board in 1982 and continued refining the design over the following years. Each revision addressed known bugs, reduced the component count and improved manufacturing reliability. The revision number is stamped on the underside of the board near the cartridge port edge.

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Assy 326298 (Rev A):The original 1982 board. Highest component count of all revisions. Fitted with the earliest SID 6581 chips and the first VIC-II variants. Often found in very early breadbin C64 units.
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Assy 250407 (Rev B):First revision of the original board. Introduced minor circuit improvements and some component consolidation. Still a large board with many discrete components.
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Assy 250425 (Rev C):The most commonly found C64 board. Component layout optimised, fewer solder points, improved reliability over Rev A and B. This is the revision most repair guides are written for.
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Assy 250466 (Rev D):Later revision, often found in C64C cases. Further reduced component count. Some boards use the SID 8580 instead of the 6581, which has different filter characteristics.

PLA Chip Revisions

The PLA (Programmable Logic Array) chip controls the address decoding and memory map for the entire system. Two main versions exist: the 906114-01 (original) and the 251715-01 (later, with bug fixes). The PLA is one of the most common failure points in aging C64 machines. Modern solid-state replacements are widely available and are a recommended upgrade for any restored machine.

ICS64S Sixtyclone modern C64 motherboard clone using new components

Chips on the C64 Motherboard

Six main chips define the Commodore 64 as a machine. Each has a specific role, and understanding what they do explains both the capabilities and the limitations of the hardware.

Processor

Chip	MOS Technology 6510
Type	8-bit CPU
Clock	0.985 MHz (PAL) / 1.023 MHz (NTSC)
Feature	Built-in 6-bit I/O port for memory banking

Video

Chip	MOS Technology VIC-II
Resolution	320x200 pixels
Colours	16 fixed colours
Sprites	8 hardware sprites

Sound

Chip	MOS Technology SID 6581 / 8580
Voices	3 independent channels
Waveforms	Triangle, sawtooth, pulse, noise
Features	Programmable filter, ring modulation, sync

Interface (x2)

Chip	MOS Technology CIA 6526 / 8521
Function	Keyboard, joystick, serial, timers
Count	Two CIA chips per board
Ports	User port, datasette, IEC bus

Logic

Chip	PLA 906114-01 / 251715-01
Function	Address decoding and memory map
Revisions	906114-01 original, 251715-01 with fixes
Note	Common failure point in older machines

Memory

RAM	64 KB SRAM
ROM	BASIC 2.0 + KERNAL (16 KB each)
Addressing	64 KB flat address space
Banking	Controlled via 6510 I/O port

SID chip 6581 and 8580 side by side on a Commodore 64 motherboard

EVO64 complete motherboard build, a modern Commodore 64 clone using new components

Modern C64 Motherboards: FPGA and Clone Boards

For collectors and builders who want a reliable, modern-component C64 inside an original case, several drop-in replacement boards are available. Each takes a different approach to recreating the original hardware.

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Ultimate 64 EliteFPGA-based replacement that fits inside an original C64 case. The entire C64 hardware is emulated in a single programmable chip. Supports HDMI output, USB peripherals, dual SID and cartridge emulation.
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EVO64A from-scratch C64 clone built with modern components. Fully compatible with original C64 cartridges, peripherals and the keyboard matrix. Designed for builders who want a PCB made entirely from new parts.
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C64 Reloaded MK2Designed for longevity, using modern through-hole components in an original-style layout. Accepts real SID chips. Fits original C64 cases. Popular with collectors who want the authentic chip sound.
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ICS64S SixtycloneA faithful recreation of the original C64 PCB using modern components, closely matching the original board layout. Compatible with original chips and peripherals.

Building a C64 from Scratch with the EVO64

Perifractic (Christian Simpson) built a fully working Commodore 64 using 100% modern components, with the EVO64 as its core. The result is a machine that looks and behaves exactly like the original but uses no legacy parts. The build demonstrates how far modern reproduction boards have come.