Editing Commodore 64C

{{Infobox computer
| name         = Commodore 64C
| image        = [[File:C64c system.jpg|250px]]
| caption      = Commodore 64C in the low-profile case
| manufacturer = Commodore Business Machines (CBM)
| type         = Home computer
| release date = 1986
| discontinued = 1994
| cpu          = MOS 8500 (6510-compatible) @ 1.023&nbsp;MHz (NTSC) / 0.985&nbsp;MHz (PAL)
| memory       = 64&nbsp;KB RAM, 20&nbsp;KB ROM<br />(8&nbsp;KB BASIC 2.0 + 8&nbsp;KB KERNAL in one 16&nbsp;KB device, 4&nbsp;KB character ROM)
| storage      = External cassette (1530 Datasette), 5.25&Prime; floppy drive (1541 / 1541-II), 3.5&Prime; (1581)
| display      = 320 &times; 200 pixels, 40 &times; 25 text, 16 colours
| sound        = MOS 8580 SID (some boards built with the 6581): 3 voices, 4 waveforms, programmable filter, ADSR
| os           = Commodore BASIC 2.0 (in ROM)
| predecessor  = [[Commodore 64]]
| successor    = [[Commodore 128]]
| model        = C64C, C64G
| logo         = [[File:Commodore 64 logo-00.svg|250px]]
}}
<templatestyles src="Template:StyledTable/styles.css" />

The '''Commodore 64C''' is the cost-reduced, slim-cased redesign of the original [[Commodore 64|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 [[Commodore 64 / 64C Service Manual|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.<ref name="sm">Commodore Business Machines, ''C64 / C64C Service Manual'', March 1992, PN-314001-03, hosted at [[Commodore 64 / 64C Service Manual]]. Contains the C64 and C64C board-identification tables, component-level parts lists for every mainboard assembly, the circuit-theory sections, Commodore's symptom-to-component troubleshooting table, and schematics 252278 (B-3) and 252312 (E).</ref> 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:<ref name="sm" />

{| class="wikitable styled-table" style="width:100%; text-align:left;"
|+'''C64C board identification, from the service manual'''
! Version !! Identifying factor !! PCB assembly !! Schematic
|-
| '''B-3''' || Only 2 RAM chips, U9 and U10 || 250466-01<ref group="note">Commodore's identification table prints "250446"; the parts list two pages later is headed "PCB ASSEMBLY #250466-01 C64-B3", and the board layout photograph in the same manual shows "ASSY NO. 250466" silkscreened. 250466 is the number on real boards.</ref> || 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.&nbsp;3'''.<ref name="sm" />

'''250425 is not a C64C board.''' Commodore lists it under ''C64'' board identification as revision B, an 8-pin long board.<ref name="sm" />

== Architecture ==

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

On the E board, timing comes from the '''8701 clock generator at U20''' with crystal Y1 and trimmer CT-1 (7.7&ndash;40&nbsp;pF). Y1 is '''14.31818&nbsp;MHz''' on NTSC machines and '''17.734472&nbsp;MHz''' on PAL, and the dot clock is 8.18&nbsp;MHz NTSC / 7.88&nbsp;MHz PAL. '''Jumper J3 sets the standard''' — open for NTSC, shorted for PAL.<ref name="sm" />

=== Chip positions on the E board (250469-01) ===

{| class="wikitable styled-table" style="width:100%; text-align:left;"
|+'''250469-01 integrated circuits'''<ref name="sm" />
! Position !! Part !! Commodore part no.
|-
| '''U6''' || 8500 CPU || 318012-01
|-
| '''U7''' || VIC-II &mdash; 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 &mdash; the "Super PLA" || 251715-01
|-
| '''U4''' || Combined BASIC + KERNAL ROM, 128&nbsp;kbit (23128B) || 251913-01
|-
| '''U5''' || Character ROM, 2332C || 901225-01
|-
| '''U10, U11''' || DRAM, 64K &times; 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.<ref name="sm" />
* '''The character ROM is U5, not U3.''' U3 is a 74LS08.<ref name="sm" />

=== Memory map ===

The address decoding comes from the PLA. Commodore's circuit-theory section gives the I/O map explicitly:<ref name="sm" />

{| class="wikitable styled-table" style="width:100%; text-align:left;"
|+'''Commodore's own address map'''
! Region !! Device
|-
| $A000&ndash;$BFFF || BASIC ROM (PLA output F1)
|-
| $D000&ndash;$D02E || VIC-II
|-
| $D000&ndash;$DFFF || Character generator ROM (PLA output F3), banked against I/O
|-
| $D400&ndash;$D7FF || SID
|-
| $D800&ndash;$DBFF || Colour RAM
|-
| $DC00&ndash;$DC0F || CIA 1
|-
| $DD00&ndash;$DD0F || CIA 2
|-
| $DE00&ndash;$DEFF || I/O 1 (expansion port)
|-
| $DF00&ndash;$DFFF || I/O 2 (expansion port)
|-
| $E000&ndash;$FFFF || KERNAL ROM (PLA output F2)
|}

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

== Sound &mdash; 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&nbsp;V unregulated rail — '''not''' by a three-terminal regulator. Commodore's note on schematic 252312 is explicit:<ref name="sm" />

{| class="wikitable styled-table" style="width:80%; text-align:left;"
|+'''CR7 selection by SID type'''<ref name="sm" />
! SID fitted !! CR7
|-
| 6581 || 12.6 V
|-
| 8580 || 9.6 V
|}

Two further build differences follow from which SID is fitted:<ref name="sm" />

{| class="wikitable styled-table" style="width:80%; text-align:left;"
! 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&nbsp;V on a 9&nbsp;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&nbsp;V alone — the E board generates no 12&nbsp;V rail. Specifications are unchanged from the original C64: 320&nbsp;&times;&nbsp;200 bitmap or 40&nbsp;&times;&nbsp;25 text, 16 colours, eight hardware sprites of 24&nbsp;&times;&nbsp;21 pixels with up to four colours each, sprite and data collision detection, and raster interrupts.<ref name="sm" />

== Power and input/output ==

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

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.<ref name="sm" />

=== A note for French machines ===

Schematic 252312 carries a '''France version''' note: R2 becomes a 20&nbsp;&Omega; 5&nbsp;W wirewound resistor and C63 a 2200&nbsp;&micro;F capacitor instead of 1000&nbsp;&micro;F.<ref name="sm" /> 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.<ref name="sm" /> The original bricks are potted and cannot be serviced or inspected; when they fail they can put an over-voltage onto the +5&nbsp;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)]].

{| class="wikitable styled-table" style="width:100%; text-align:left;"
|+'''Symptoms Commodore attributes to specific parts'''<ref name="sm" />
! 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 ==

<gallery mode="packed" heights="180">
File:Commodore 64C - PD.jpg|Front view of a 64C
File:Commodore 64C - PD - tył.jpg|Rear ports (8-pin A/V, RF, IEC, cartridge, user, cassette and power)
File:Commodore64C_Motherboard_250469.jpg|Assembly 250469 short board
</gallery>

== Notes ==

<references group="note" />

== Related pages ==

* [[Commodore 64]] &middot; [[Commodore 128]] &middot; [[Commodore 64C General Maintenance]] &middot; [[Commodore 64C Troubleshooting Guide]] &middot; [[Commodore 64C Capacitor Replacement Guide]]
* '''[[Commodore 64 / 64C Service Manual]]''' &mdash; the source for the figures on this page
* [[Commodore 64 Power-Supply Protector (C64 Saver)]] &middot; [[Commodore Test Cartridge Index]] &middot; [[Recommended Tools]]

== References ==

<references />

[[Category:Commodore Systems]]