Commodore 64C

Commodore 64C

Commodore 64C in the low-profile case
Specifications
Manufacturer	Commodore Business Machines (CBM)
Type	Home computer
Released	1986
Discontinued	1994
CPU	MOS 8500 (6510-compatible) @ 1.023 MHz (NTSC) / 0.985 MHz (PAL)
Memory	64 KB RAM, 20 KB ROM (8 KB BASIC 2.0 + 8 KB KERNAL in one 16 KB device, 4 KB character ROM)
Storage	External cassette (1530 Datasette), 5.25″ floppy drive (1541 / 1541-II), 3.5″ (1581)
Display	320 × 200 pixels, 40 × 25 text, 16 colours
Sound	MOS 8580 SID (some boards built with the 6581): 3 voices, 4 waveforms, programmable filter, ADSR
OS / Firmware	Commodore BASIC 2.0 (in ROM)
Predecessor	Commodore 64
Successor	Commodore 128
Model no.	C64C, C64G

The Commodore 64C is the cost-reduced, slim-cased redesign of the original Commodore 64 "bread bin", introduced in 1986. It kept full software and peripheral compatibility while lowering manufacturing cost, power consumption and heat output.

The hardware figures on this page are taken from Commodore's own C64 / C64C Service Manual (March 1992, PN-314001-03), which is hosted on this wiki and carries the board-identification tables, the component-level parts lists and the E-board schematic 252312. Figures that could not be traced to a Commodore document have been removed rather than repeated.

Which board is a C64C?

Commodore's own C64C Board Identification table lists exactly two assemblies:

C64C board identification, from the service manual

Version	Identifying factor	PCB assembly	Schematic
B-3	Only 2 RAM chips, U9 and U10	250466-01	252278
E	KERNAL and BASIC ROMs combined into U4; new IC memory controller at U8	250469-01	252312

The E board is the one usually meant by "C64C short board"; its layout page records PCB assy no. 250469, PCB no. 252311 Rev. 3.

250425 is not a C64C board. Commodore lists it under C64 board identification as revision B, an 8-pin long board.

Architecture

The 8500 is the HMOS shrink of the 6510 and runs from a single +5 V rail. Clock rates are unchanged from the bread bin: 1.023 MHz NTSC and 0.985 MHz PAL.

On the E board, timing comes from the 8701 clock generator at U20 with crystal Y1 and trimmer CT-1 (7.7–40 pF). Y1 is 14.31818 MHz on NTSC machines and 17.734472 MHz on PAL, and the dot clock is 8.18 MHz NTSC / 7.88 MHz PAL. Jumper J3 sets the standard — open for NTSC, shorted for PAL.

Chip positions on the E board (250469-01)

250469-01 integrated circuits

Position	Part	Commodore part no.
U6	8500 CPU	318012-01
U7	VIC-II — the parts list gives two fits, printed "8567" and "8562"	318014-01 / 318027-01
U9	8580 SID	318013-01
U9	6581 SID (alternative fit)	906112-01
U1, U2	6526 CIA	906108-01
U8	Memory controller — the "Super PLA"	251715-01
U4	Combined BASIC + KERNAL ROM, 128 kbit (23128B)	251913-01
U5	Character ROM, 2332C	901225-01
U10, U11	DRAM, 64K × 4 (50464-150)	390083-07
U19	Colour RAM, 2114	901453-07 / -09
U20	8701 clock generator	251527-08 / -09
U3 / U22 / U23 / U18, U21	74LS08 / 7406 or 7416 / 74LS14 / 4066

Two points worth correcting, because they are widely repeated:

• The Super PLA does not contain the colour RAM. Commodore's parts list gives a separate 2114 at U19, and the E schematic labels it COLRAM.
• The character ROM is U5, not U3. U3 is a 74LS08.

Memory map

The address decoding comes from the PLA. Commodore's circuit-theory section gives the I/O map explicitly:

Commodore's own address map

Region	Device
$A000–$BFFF	BASIC ROM (PLA output F1)
$D000–$D02E	VIC-II
$D000–$DFFF	Character generator ROM (PLA output F3), banked against I/O
$D400–$D7FF	SID
$D800–$DBFF	Colour RAM
$DC00–$DC0F	CIA 1
$DD00–$DD0F	CIA 2
$DE00–$DEFF	I/O 1 (expansion port)
$DF00–$DFFF	I/O 2 (expansion port)
$E000–$FFFF	KERNAL ROM (PLA output F2)

Everything not banked in above is RAM, including $C000–$CFFF, which is plain RAM and not an I/O window.

Sound — the 8580 SID and its supply

The 64C introduced the HMOS MOS 8580 SID, which runs from a lower supply than the NMOS 6581. On the E board that supply is made by a zener, CR7, with a pass transistor Q5 from the 9 V unregulated rail — not by a three-terminal regulator. Commodore's note on schematic 252312 is explicit:

CR7 selection by SID type

SID fitted	CR7
6581	12.6 V
8580	9.6 V

Two further build differences follow from which SID is fitted:

Component	With 8580	With 6581
C70, C71 (filter)	22000 pF	470 pF
C80, C81 (filter)	2200 pF	1800 pF
R8	Removed	Fitted

This matters if you are substituting one SID for the other. Dropping an 8580 into a board built for a 6581 puts 12.6 V on a 9 V part and leaves the wrong filter capacitors in circuit.

There is no 7809 on the board; earlier versions of this page said there was.

Display

Commodore's parts list gives two VIC-II fits for U7 without labelling either by television standard; the standard is set by jumper J3 and crystal Y1, not by a different part number in the list. The VIC-II fitted to the E board is the HMOS 856x family, which runs from +5 V alone — the E board generates no 12 V rail. Specifications are unchanged from the original C64: 320 × 200 bitmap or 40 × 25 text, 16 colours, eight hardware sprites of 24 × 21 pixels with up to four colours each, sprite and data collision detection, and raster interrupts.

Power and input/output

The 64C takes the same 7-pin DIN supply as the bread bin: +5 V DC regulated and 9 V AC. On the E board, pins 6 and 7 carry the 9 V AC through fuse F1 (1.5 A), pins 4 and 5 carry +5 V through the switch, and pins 1, 2 and 3 are ground. The 9 V AC is rectified by CR2 into C63 to make the 9 V unregulated rail, which feeds the SID supply, the cassette motor drive and the audio amplifier.

All the original ports are retained: cartridge/expansion port, serial IEC bus, user port, two 9-pin control ports, and an 8-pin A/V DIN (CN5) carrying composite, luma, chroma, audio out and audio in, alongside the RF modulator.

A note for French machines

Schematic 252312 carries a France version note: R2 becomes a 20 Ω 5 W wirewound resistor and C63 a 2200 µF capacitor instead of 1000 µF. This explains the two C63 entries in the printed parts list.

Maintenance and common faults

The external power supply is the prime suspect in any C64 or C64C power fault. Commodore's own troubleshooting table names "Check External Power Supply" as the first item for four separate symptoms, ahead of every chip: blank screen on power-up, a white band scrolling down the screen, a wavy screen after warm-up, and a black band scrolling through the screen after warm-up. The original bricks are potted and cannot be serviced or inspected; when they fail they can put an over-voltage onto the +5 V rail, which is directly connected to every logic device in the machine. A modern replacement supply, or an in-line over-voltage protector, is the cheapest insurance available — see Commodore 64 Power-Supply Protector (C64 Saver).

Symptoms Commodore attributes to specific parts

Symptom	Commodore's check
Blank screen on power-up	External power supply; U4 (KERNAL ROM), U17 (PLA), U7 (6510), U3 (ROM), U8 (7406), U19 (VIC-II), the RAM, VR2, CR4, VR1
Out of memory error on power-up	RAM
Abnormal colours in the letters	Colour RAM (2114), 4066
Powers up with all characters as blocks	74LS373
White band scrolling down the screen	External power supply; 5 V regulator
Black band scrolling through the screen after warm-up	External power supply; C90, C88, CR4; 5 V regulator
Intermittent display	C88, noted as a possible bad connection rather than a failed part

Those designators are the long board's; the equivalent parts on the E board are listed under Commodore 64C Capacitor Replacement Guide. The logic of the table — supply first, then decoding, then memory — holds for both.

What is realistically repairable

• The 74-series logic, the DRAM, the colour RAM and the discrete parts are ordinary components.
• The ROM is recoverable. The combined BASIC/KERNAL image at U4 and the character ROM at U5 are well-preserved data; a failed mask ROM can be replaced with an EPROM on an adapter.
• The PLA is the classic C64 failure and it is substitutable. Replacements built from programmable logic have been made by the community for years.
• The SID is substitutable but not identical. If you swap 6581 for 8580 or the reverse, change CR7, the filter capacitors and R8 as tabulated above, or the result will be wrong at best and destructive at worst.
• The VIC-II is the part with no substitute. It generates the video and the system clock; a dead VIC-II means a donor board.

Case and plastics

The 64C's lighter plastics yellow. Retrobrite restores the colour and embrittles the plastic; decide which you care about before starting.

For fault-finding see Commodore 64C Troubleshooting Guide and Commodore 64C General Maintenance.

Gallery

• 
  Front view of a 64C
• 
  Rear ports (8-pin A/V, RF, IEC, cartridge, user, cassette and power)
• 
  Assembly 250469 short board

Related pages

• Commodore 64 · Commodore 128 · Commodore 64C General Maintenance · Commodore 64C Troubleshooting Guide · Commodore 64C Capacitor Replacement Guide
• Commodore 64 / 64C Service Manual — the source for the figures on this page
• Commodore 64 Power-Supply Protector (C64 Saver) · Commodore Test Cartridge Index · Recommended Tools