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Commodore 64 General Maintenance: Difference between revisions

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Correct the VIC-II row to what Commodore's parts lists actually print; the 6569 designation is not in this manual
Re-cite to service manual pages; replace invented tolerances (4.9-5.1 V, 9-11 V AC) with C64-Wiki and 78xx datasheet figures; drop invented symptom column, annual reseat interval, heatsink and Retrobrite advice; fix C90 role; remove self-reference and bold
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[[File:Commodore_64_(photo).jpg|thumb|400px|Commodore 64 "bread bin".]]
[[File:Commodore_64_(photo).jpg|thumb|400px|Commodore 64 in the original case]]
Routine care and preventive checks for the '''[[Commodore 64]]''' long boards. The machine's weak point is its external power supply, whose +5 V output reaches the logic without further regulation. For fault-finding see [[Commodore 64 Troubleshooting Guide]]; for capacitor values see [[Commodore 64 Capacitor Replacement Guide]]. The figures below come from Commodore's ''Service Manual C64/C64C'' (March 1992) unless another source is cited.<ref name="sm">Commodore Business Machines, ''Service Manual C64/C64C'', March 1992, PN-314001-03, hosted at [[Commodore 64 / 64C Service Manual]].</ref>


The '''[[Commodore 64]]''' is forty years old and the failure modes are well understood: an external power supply that cannot be inspected, a handful of electrolytics carrying the whole ripple load, and a set of custom chips that do not forgive an over-voltage. This page covers routine care and preventive checks. For fault-finding see [[Commodore 64 Troubleshooting Guide]]; for component values see [[Commodore 64 Capacitor Replacement Guide]].
== Identify the board ==
 
The assembly number is printed on the board. Commodore's identification table:<ref name="smid">''Service Manual C64/C64C'' (1992), "C64 Board Identification" and "C64C Board Identification", p. 17.</ref>
'''The electrical figures on this page are taken from Commodore's own [[Commodore 64 / 64C Service Manual|C64 / C64C Service Manual]]''' (March 1992, PN-314001-03), hosted here.<ref name="sm">Commodore Business Machines, ''C64 / C64C Service Manual'', March 1992, PN-314001-03, hosted at [[Commodore 64 / 64C Service Manual]]. Contains the board-identification tables, the circuit-theory sections describing the power supply, reset and clock circuits, component-level parts lists for every mainboard assembly, Commodore's symptom-to-component troubleshooting table, and schematics 252278 (B-3) and 252312 (E).</ref>
 
== Identify your board ==
 
The assembly number is silkscreened on the board. Commodore's own identification tables:<ref name="sm" />


{| class="wikitable styled-table" style="width:100%; text-align:left;"
{| class="wikitable styled-table" style="width:100%; text-align:left;"
|+'''C64 and C64C mainboard assemblies'''
|+'''C64 and C64C board assemblies'''
! Version !! Identifying factor !! PCB assembly !! Schematic
! Version !! Identifying factor (Commodore) !! PCB assembly !! Schematic
|-
|-
| Original || 5-pin video connector (CN5) || 326298-01 || 326106
| Original || 5-pin video connector (CN5) || 326298-01 || 326106
|-
|-
| A (CR) || 8-pin video connector || 250407-01 || 251138
| A (CR) || 8-pin video connector || 250407 || 251138
|-
|-
| B || 8-pin, reduced oscillator circuit || 250425 || 251469
| B || 8-pin; reduced oscillator circuit || 250425 || 251469
|-
|-
| B-2 || 8-pin, reduced oscillator with component changes || 250441-01 || 251469
| B-2 || 8-pin; reduced oscillator with component changes || 250441-01 || 251469
|-
|-
| B-3 || Only 2 RAM chips, U9 and U10 || 250466-01 || 252278
| B-3 || Only two RAM chips, U9 and U10 || 250466-01 || 252278
|-
|-
| E || KERNAL and BASIC ROMs combined into U4, new memory controller at U8 || 250469-01 || 252312
| E || KERNAL and BASIC ROMs combined into U4; new memory controller at U8 || 250469-01 || 252312
|}
|}


Commodore records two specific changes on the '''B-2''': resistors R28, R29, R30, R36 and R48 were replaced by resistor pack RP5, and diodes CR100&ndash;CR105 are no longer piggybacked but fitted at CR9 and CR12&ndash;CR16.<ref name="sm" />
On the B-2, resistors R28, R29, R30, R36 and R48 became resistor pack RP5 and diodes CR100-CR105 moved from piggyback positions to CR9 and CR12-CR16.<ref name="smid" /> The B-3 and E are listed under C64C identification; see [[Commodore 64C]] and [[Commodore 64C General Maintenance]].
 
The B-3 and E boards are the two Commodore lists under ''C64C'' identification; see [[Commodore 64C]] and [[Commodore 64C General Maintenance]].
 
== The power supply ==
 
=== What the brick actually does ===


'''The C64's +5&nbsp;V rail is regulated inside the brick and is not regulated again on the board.''' Commodore's circuit-theory section is explicit: the external supply generates a regulated 5&nbsp;V DC and a 9&nbsp;V AC output; the 5&nbsp;V is applied to the power connector, filtered by L5, C97 and C100, switched by the power switch, and then supplies the microprocessor logic directly.<ref name="sm" />
== Power supply ==
=== How the board is powered ===
The external supply provides regulated +5 V DC and 9 V AC. The +5 V enters at CN7, is filtered by L5, C97 and C100, passes the power switch and supplies the processor logic directly.<ref name="smps">''Service Manual C64/C64C'' (1992), Circuit Theory, "The Power Supply", p. 4.</ref> The board makes its other supplies from the 9 V AC:<ref name="smps" />
* +12 V: the 9 V AC is "added to 9VDC through CR6, and rectified by CR5", filtered by C88 and C89 and regulated by VR1 (MC7812CT). It supplies the VIC-II, the SID and the audio amplifiers.
* +5 V CAN: rectified by CR4, filtered by C19 and C95 and regulated by VR2 (MC7805CT), then split into Vvid for the video circuits and Vc for the clock circuits.
* +9 V unregulated: from CR4, for the cassette motor transistors and, on the B board, the RF modulator.


Everything else is made on the board from the 9&nbsp;V AC:<ref name="sm" />
Because nothing on the board regulates the logic supply, a power supply that fails with a high output puts that voltage on the processor logic. The C64-Wiki records that the original supplies tend to rise as they age, and that in most cases where a supply killed a C64 its output had passed 6 V, with the RAM usually failing first.<ref name="c64wps">C64-Wiki, [https://www.c64-wiki.com/wiki/Power_Supply "Power Supply"], sections "Power Supplies of the Commodore 64" and "Failure". Retrieved 2026-10-01.</ref>
 
* '''+12&nbsp;V DC''' — 9&nbsp;V AC is added to 9&nbsp;V DC through CR6, rectified by CR5, filtered by C88 and C89, and regulated by '''VR1 (MC7812CT)'''. It supplies the VIC-II, the SID and the audio amplifiers.
* '''+5&nbsp;V CAN''' — 9&nbsp;V AC rectified by CR4, filtered by C19 and C95, regulated by '''VR2 (MC7805CT)''', then split into Vvid for the video circuits and Vc for the clock circuits.
* '''+9&nbsp;V unregulated''' — rectified from the 9&nbsp;V AC; powers the cassette-motor transistor amplifier and, on the C64B, the RF modulator.
 
That is why an over-voltage in the brick is so destructive: '''nothing on the board stands between the brick's 5&nbsp;V output and every logic device in the machine.''' The original supplies are potted in epoxy, cannot be inspected or serviced, and give no warning before they fail.


{| class="wikitable styled-table" style="width:80%; text-align:left;"
{| class="wikitable styled-table" style="width:80%; text-align:left;"
|+'''Power connector CN7 (7-pin DIN), from Commodore's schematics'''<ref name="sm" /><ref group="note">The circuit-theory text says 5&nbsp;V DC is applied to "pins 5 and 1"; the schematic drawings show pins 1, 2 and 3 grounded, +5&nbsp;V on pin 5 (long boards) and on pins 4 and 5 (E board). The drawings are used here. Whichever is right, '''pin 1 is not a 5&nbsp;V pin''', and measuring between pins 1 and 2 measures nothing.</ref>
|+'''Power connector CN7 (7-pin DIN)'''<ref name="smps" /><ref name="sme">''Service Manual C64/C64C'' (1992), E schematic 252312, p. 42.</ref>
! Pin !! Function
! Pin !! Function
|-
|-
| 1, 2, 3 || Ground
| 1, 2, 3 || Ground
|-
|-
| 4 || Not connected on long boards; +5 V on the E board
| 4 || Not connected on the long boards; +5 V on the E board
|-
|-
| 5 || +5 V DC, regulated, from the brick
| 5 || +5 V DC from the supply
|-
|-
| 6, 7 || 9 V AC
| 6, 7 || 9 V AC
|}
|}
The circuit-theory text says the 5 V is applied to "pins 5 and 1"; the power-input drawing on the same page, and schematic 252312, show pins 1, 2 and 3 as ground.<ref name="smps" /><ref name="sme" />


=== Checking the supply ===
=== Checking the supply ===
Measure the supply's +5 V output before connecting it to the computer. The C64-Wiki's guide to the reading:<ref name="c64wps" />


'''Measure the brick before you plug it into the computer.''' Its 5&nbsp;V output is what is going to reach the CPU, the RAM, the PLA and the SID unchanged.
{| class="wikitable styled-table" style="width:80%; text-align:center;"
 
|+'''+5 V output, C64-Wiki interpretation'''
The '''user port''' is the easiest place to measure on a running machine: +5&nbsp;V is on '''pin 2''', ground on pins 1 and 12, and the 9&nbsp;V AC is brought out on '''pins 10 and 11'''.<ref name="sm" /> Measure the AC '''between pins 10 and 11''', not from one of them to ground.
! Reading !! Meaning
|-
| 4.95-5.10 V || Normal
|-
| 5.10-5.20 V || Minor concern; measure more often
|-
| 5.20-5.50 V || Failing; replace or repair the supply
|-
| 5.50-6.00 V || The C64 is in danger
|-
| Above 6 V || The level at which most supply-related C64 deaths occurred
|}


If your supply is original, fit protection. See [[Commodore 64 Power-Supply Protector (C64 Saver)]] for how an over-voltage crowbar works and how to install one. A modern replacement supply does the same job by not failing in the first place.
On a running machine, the user port carries +5 V on pin 2 (ground on pins 1, 12, A and N) and the 9 V AC on pins 10 and 11.<ref name="c64wup">C64-Wiki, [https://www.c64-wiki.com/wiki/User_Port "User Port"], pin assignment table. Retrieved 2026-10-01.</ref><ref name="smps" /> Measure the AC between pins 10 and 11. The C64-Wiki gives about 8.1 V AC as the lowest the machine will run on, and notes that the transformer reads higher unloaded than loaded.<ref name="c64wps" />


'''You are allowed to open a power supply''' — but the original Commodore bricks are potted solid, so there is very little to see and nothing to recap. The hazard, if you cut one open, is mains voltage on the primary side and a charged reservoir after disconnection. Treat it as you would any mains-powered equipment.
Some original supplies can be opened and repaired; others are glued shut and potted.<ref name="c64wps" /> The primary side of any of them runs at mains voltage. Unplug the supply from the wall before opening it. For an over-voltage cut-out see [[Commodore 64 Power-Supply Protector (C64 Saver)]].


== Test points ==
== Test points ==
{| class="wikitable styled-table" style="width:100%; text-align:left;"
{| class="wikitable styled-table" style="width:100%; text-align:left;"
|+'''Go/no-go checks, from Commodore's schematics'''<ref name="sm" />
|+'''Long-board test points'''
! Signal !! Where !! Expected
! Signal !! Where !! Expected
|-
|-
| '''+5 V''' || 6510 / 8500 '''pin 6''' (VCC). Pin 21 is GND || 4.9 &ndash; 5.1 V
| +5 V || 6510 (U7) pin 6; ground is pin 21<ref name="smpin">''Service Manual C64/C64C'' (1992), pin assignments for U7 6510 and U17 PLA, p. 27.</ref> || Same as the supply reading above
|-
|-
| '''RESET''' || 6510 / 8500 '''pin 40''' || Low for roughly 0.5 s at power-up, then +5 V
| RESET || 6510 pin 40<ref name="smpin" /> || Low for about 0.5 s after power-on, then high<ref name="smrst">''Service Manual C64/C64C'' (1992), Circuit Theory, "Reset Logic Circuits", p. 5.</ref>
|-
|-
| '''&Phi;0 in''' || 6510 / 8500 '''pin 1''' || 1.023 MHz NTSC / 0.985 MHz PAL
| Phase 0 clock in || 6510 pin 1<ref name="smpin" /> || About 1.02 MHz (NTSC) or 0.985 MHz (PAL)<ref name="smclk">''Service Manual C64/C64C'' (1992), Circuit Theory, "The C64 Clock Circuits", p. 6; dot clock frequencies in the notes to schematic 252312, p. 42.</ref>
|-
|-
| '''&Phi;2 out''' || 6510 / 8500 '''pin 39''' || Same frequency
| Phase 2 clock out || 6510 pin 39<ref name="smpin" /> || Same frequency
|-
|-
| '''+12 V''' (long boards) || VR1 output, or VIC-II '''pin 13''' (Vdd) and SID '''pin 28''' (Vdd) || 11.5 &ndash; 12.5 V
| +12 V || VR1 output; SID pin 28 (VDD) || 11.5-12.5 V (7812 limits)<ref name="ti78">Texas Instruments, ''LM340, LM340A and LM7805 Family Wide VIN 1.5-A Fixed Voltage Regulators'', SNOSBT0L (revised September 2016), tables 6.6 (7805) and 6.7 (7812), output voltage at TJ = 25 °C.</ref>
|-
|-
| '''+5 V''' (user port) || CN2 '''pin 2'''; ground on pins 1 and 12 || 4.9 &ndash; 5.1 V
| +5 V CAN || VR2 output || 4.8-5.2 V (7805 limits)<ref name="ti78" />
|-
|-
| '''9 V AC''' || CN2 '''pins 10 to 11''' || 9 &ndash; 11 V rms
| SID audio || Pin 27 out, pin 26 in<ref name="smav">''Service Manual C64/C64C'' (1992), Circuit Theory, "8 Pin Video and Audio Output Circuits", p. 10.</ref> || Signal while a voice is playing
|-
|-
| '''SID audio out''' || SID '''pin 27'''; audio in is pin 26 || Signal present when a voice is running
| VIC-II video || Pin 15 sync/luminance, pin 14 colour<ref name="smav" /> || Composite sync with luminance; colour signal
|-
| '''VIC-II video''' || Sync/luminance on '''pin 15''', colour on '''pin 14''' || Composite sync and luma
|}
|}


'''Pin 40 of the 6510 is RESET, not Vcc.''' Earlier versions of this page gave +5&nbsp;V at pin 40, &Phi;2 at pin 2 and reset at pin 3. Pin 2 is RDY and pin 3 is IRQ; probing them proves nothing and shorting them can hang the machine.
Commodore fitted Motorola MC7812CT and MC7805CT regulators; the voltage limits above are the generic 78xx datasheet figures, as Commodore's manual gives none.<ref name="smps" />
 
The reset itself comes from '''U20, an LM556 timer''' configured as a one-shot; Commodore gives the pulse width as 1.1 &times; R34 &times; C24, about half a second, with the active-high output on pin 9.<ref name="sm" />
 
== Capacitor health ==
 
Do not work from a value list on a maintenance page. '''See [[Commodore 64 Capacitor Replacement Guide]]''', which carries Commodore's per-assembly parts lists; the values differ between board revisions and the wrong voltage rating in the wrong position is how machines get damaged.


What is worth knowing here is which capacitors Commodore itself associates with faults:<ref name="sm" />
The reset pulse comes from U20, an LM556 timer wired as a one-shot. Commodore gives the pulse width as 1.1 × R34 × C24, about half a second, with the timer's output on pin 9 inverted by U8 to drive the active-low reset.<ref name="smrst" />


* '''C88''' — "intermittent display", listed as a '''possible bad connection''' rather than a failed part. Reflow its joints before replacing it.
== Capacitors ==
* '''C90, C88, CR4, VR2''' — a black band scrolling through the screen once the machine warms up, '''after''' checking the external supply.
Values differ between board revisions; use the [[Commodore 64 Capacitor Replacement Guide]], which gives each board's list. Commodore's troubleshooting guide names three capacitors:<ref name="smts">''Service Manual C64/C64C'' (1992), "64 Troubleshooting Guide", pp. 14-16.</ref>
* '''C70''' — "wrong frequency".
* C88: "intermittent display", marked as a possible bad connection.
* C90 and C88, with CR4 and VR2: a black band scrolling through the screen when the system warms up, after checking the external supply.
* C70: "wrong frequency". C70 is a 10 pF mica capacitor in the clock oscillator on the 326298.<ref name="sm326">''Service Manual C64/C64C'' (1992), "Parts List - PCB Assembly #326298", pp. 19-20.</ref>


On the long boards the main reservoir is C19, 2200&nbsp;&micro;F 16&nbsp;V; C88 and C90 are the supply-filter cans; C91 is 100&nbsp;&micro;F 16&nbsp;V. There are no 220&nbsp;&micro;F parts in the 9&nbsp;V AC rectifier path and there are no SMD tantalums on any C64 board.
On the long boards C19 (2200 µF 16 V) is the reservoir for VR2, C88 (470 µF 50 V on the 326298, 1000 µF 25 V on later lists) is the reservoir for VR1, and C91 (100 µF 16 V) sits on the incoming +5 V.<ref name="smps" /><ref name="sm326" /> See [[Capacitor Failure Symptoms]] and [[Battery Explosion, Capacitor or Corrosion Damage]].
 
See [[Capacitor Failure Symptoms]] for recognising a failed part and [[Battery Explosion, Capacitor or Corrosion Damage]] for cleaning up after a leak.
 
== Known failure-prone ICs ==


== Chip positions ==
{| class="wikitable styled-table" style="width:100%; text-align:left;"
{| class="wikitable styled-table" style="width:100%; text-align:left;"
|+'''Long-board positions, from Commodore's parts lists'''<ref name="sm" />
|+'''Long-board ICs (326298 and 250407 lists)'''<ref name="sm326" /><ref name="sm407">''Service Manual C64/C64C'' (1992), "Parts List - PCB Assembly #250407-04", pp. 24-25.</ref>
! Function !! Position !! Part !! Typical symptom
! Function !! Position !! Part !! Commodore part no.
|-
| CPU || U7 || 6510 || 906107-01
|-
| PLA || U17 || 82S100 || 906114-01
|-
| VIC-II || U19 || 6567 (NTSC) || 906109-04
|-
|-
| PLA || '''U17''' || 82S100, Commodore 906114-01 || Black screen, random colours, crashes with a cartridge fitted
| SID || U18 || 6581 || 906112-01
|-
|-
| CPU || '''U7''' || 6510, 906107-01 || Blank screen; cursor locks up; cassette motor faults
| CIA || U1, U2 || 6526 || 906108-01
|-
|-
| VIC-II || '''U19''' || 6567, Commodore 906109-04 (the B-3 list marks it "VIC NTSC", 906109-01) || Blank screen, graphic characters instead of letters
| Main RAM || U9-U12, U21-U24 || 4164, 200 ns || 901505-01
|-
|-
| SID || '''U18''' || 6581, 906112-01 || No sound
| Colour RAM || U6 || 2114 || 901453-01
|-
|-
| CIA 1 / CIA 2 || '''U1''', '''U2''' || 6526, 906108-01 || Dead keyboard or joystick; DEVICE NOT PRESENT; cursor vanishes when warm
| BASIC ROM || U3 || 2364 || 901226-01
|-
|-
| Main RAM || '''U9&ndash;U12''', '''U21&ndash;U24''' || 4164, 901505-01 || OUT OF MEMORY on power-up, random characters
| KERNAL ROM || U4 || 2364 || 901227-03
|-
|-
| Colour RAM || '''U6''' || 2114, 901453-01 || Abnormal colours in the letters
| Character ROM || U5 || 2332 || 901225-01
|-
|-
| ROMs || '''U3''' BASIC, '''U4''' KERNAL, '''U5''' character || 2364 / 2332 mask || Various; see the fault table below
| Reset timer || U20 || LM556 || 901523-03
|-
|-
| Reset timer || '''U20''' || LM556, 901523-03 || No reset, RESTORE key dead, cursor locks on power-up
| Serial bus driver || U8 || 7406 || 901522-06
|}
|}


=== Commodore's own fault table ===
=== Commodore's fault table ===
 
A selection from the manual's troubleshooting guide; the [[Commodore 64 Troubleshooting Guide]] carries the full table.<ref name="smts" />
The manual's troubleshooting guide is a symptom-to-component list of about thirty entries. A representative selection:<ref name="sm" />


{| class="wikitable styled-table" style="width:100%; text-align:left;"
{| class="wikitable styled-table" style="width:100%; text-align:left;"
! Symptom !! Commodore's check
! Symptom !! Commodore's check
|-
|-
| Blank screen on power-up || '''External power supply'''; U4, U17, U7, U3, U8, U19, the RAM, VR2, CR4, VR1
| Blank screen on power-up || External power supply; U4, U17, U7, U3, U8, U19, U9-U12, U21-U24, VR2, CR4, VR1
|-
|-
| Out of memory error on power-up || U9&ndash;U12, U21&ndash;U24
| Out of memory error on power-up || U9-U12, U21-U24
|-
|-
| Powers up with a graphic display and a blinking cursor || U14 (74LS258)
| Powers up with graphic display and blinking cursor || U14 (74LS258)
|-
|-
| All characters displayed as blocks || U26 (74LS373)
| All characters displayed as blocks || U26 (74LS373)
Line 153: Line 151:
| Abnormal colours in the letters || U6 (2114 colour RAM), U16 (4066)
| Abnormal colours in the letters || U6 (2114 colour RAM), U16 (4066)
|-
|-
| White band scrolling down the screen (60 Hz hum) || '''External power supply'''; VR2
| White band scrolls down the screen (60 Hz hum) || External power supply; VR2
|-
|-
| Wavy screen after warm-up || '''External power supply'''; U31, U30
| Wavy screen after warm-up || External power supply; U31 (74LS629), U30 (74LS193)
|-
|-
| Cursor disappears after the system warms up || U1 (CIA)
| Cursor disappears after the system warms up || U1 (6526)
|-
|-
| Cassette motor keeps running, TIP29 gets very hot, fuse may blow || Cassette port for shorts; R4 possibly open
| Cassette motor keeps running; TIP29 very hot; fuse may blow || Cassette port for shorts; R4 possibly open
|-
|-
| DEVICE NOT PRESENT when a disk drive is used || U1, U7, R28, R29, R30
| DEVICE NOT PRESENT with a disk drive || U1, U7, R28, R29, R30
|}
|}


Note how often the first item is the external supply. Commodore's own ordering is the right ordering.
PLA replacements are covered in [[PLA Replacement Options]].
 
== What is realistically repairable ==
 
* The 74-series logic, the DRAM, the colour RAM and the discrete parts are ordinary components.
* '''ROMs are recoverable''' — the BASIC, KERNAL and character images are well preserved, and a failed mask ROM can be replaced with an EPROM on an adapter.
* '''The PLA at U17 is the classic C64 failure, and it is substitutable.''' Community replacements built from programmable logic have existed for years.
* '''The SID is substitutable but not interchangeable.''' A 6581 wants 12&nbsp;V and an 8580 wants 9&nbsp;V, and the filter capacitors differ. Do not swap one for the other without changing the supply and the filter parts — see [[Commodore 64C]] for the E board's table.
* '''The VIC-II is the part with no substitute.''' A dead VIC-II means a donor board.
 
== Regular maintenance ==
 
=== Cleaning ===
 
* Wipe the case with a damp microfibre cloth and a drop of mild detergent. Key caps pull vertically; clean plungers and contacts with isopropyl alcohol.
* For yellowed plastics see [[Retrobrite]], and weigh the cosmetic gain against the embrittlement.
* Disconnect the supply and let the board sit before opening it. Blow dust out with dry air and brush with an anti-static brush.
* Reseat socketed ICs to wipe the contacts. This alone fixes a surprising number of intermittents.
* Clean cartridge and user-port edge connectors with isopropyl alcohol. '''Do not use an abrasive eraser''' on the board's gold fingers.
 
=== Thermal ===
 
* On long boards, check the solder joints at VR1 and VR2 and at the bridge rectifier CR4. These are heavy-current, thermally cycled joints and a cracked one produces an intermittent that mimics a failing chip.
* The PLA, VIC-II and SID all run hot, the early ceramic-packaged parts especially. Small self-adhesive heatsinks are cheap insurance.
* Leave clearance above the case vents.
 
=== Preventive checklist ===


# Test the brick's +5&nbsp;V output '''before''' connecting it, every session with an original supply.
== Cleaning and inspection ==
# Check the fuse rating if you have replaced it: Commodore specifies '''F1, 1.5&nbsp;A, 250&nbsp;V, normal blow'''.<ref name="sm" />
# Disconnect the power supply before opening the case.
# Inspect and reflow the regulator and rectifier joints.
# Remove dust with dry air and a soft anti-static brush.
# Reseat socketed chips annually.
# Clean the cartridge and user-port edge connectors and the keyboard contacts with isopropyl alcohol.
# Clean the edge connectors.
# Inspect the joints on VR1, VR2 and the bridge rectifier CR4, the power parts Commodore names for a blank screen.<ref name="smts" />
# Replace the supply electrolytics if the machine shows a scrolling band once warm — '''after''' clearing the brick.
# If the fuse has been replaced, check its rating: the long-board parts lists give F1 as 1.5 A, 250 V, normal blow.<ref name="sm326" /><ref name="sm407" />
# Measure the supply's +5 V output before reconnecting it.


== Tools ==
== Tools ==
 
* Digital multimeter; oscilloscope or logic probe
* Anti-static strap and mat; digital multimeter; oscilloscope or logic probe
* Temperature-controlled soldering iron, flux, desoldering braid and pump
* Temperature-controlled soldering station, flux, braid and a pump
* Isopropyl alcohol, lint-free wipes, soft brush
* Isopropyl alcohol, lint-free wipes, soft brush, contact cleaner
* A diagnostic cartridge: see [[Commodore Test Cartridge Index]] and [[Commodore 64 Diagnostic Instruction and Troubleshooting Manual (326070-01)]]
* Freeze spray for thermal fault hunting
* A diagnostic cartridge — see [[Commodore Test Cartridge Index]], and [[Commodore 64 Diagnostic Instruction and Troubleshooting Manual (326070-01)]] for Commodore's own documentation of the diagnostic harness


See also [[Recommended Tools]].
See also [[Recommended Tools]].
== Notes ==
<references group="note" />


== Related pages ==
== Related pages ==
 
* [[Commodore 64]] · [[Commodore 64C]] · [[Commodore 64 Troubleshooting Guide]] · [[Commodore 64 Capacitor Replacement Guide]]
* [[Commodore 64]] &middot; [[Commodore 64C]] &middot; [[Commodore 64 Troubleshooting Guide]] &middot; [[Commodore 64 Capacitor Replacement Guide]]
* [[Commodore 64 / 64C Service Manual]]
* '''[[Commodore 64 / 64C Service Manual]]''' &mdash; the source for the figures on this page
* [[Commodore 64 Power-Supply Protector (C64 Saver)]] · [[Capacitor Failure Symptoms]] · [[Recommended Tools]]
* [[Commodore 64 Power-Supply Protector (C64 Saver)]] &middot; [[Capacitor Failure Symptoms]] &middot; [[Battery Explosion, Capacitor or Corrosion Damage]] &middot; [[Recommended Tools]] &middot; [[Retrobrite]]
* [[Floppy Drive Repair]]
* [[Floppy Drive Repair]] for 1541 and 1581 mechanisms


== References ==
== References ==
<references />
<references />


[[Category:Commodore Systems]]
[[Category:Commodore Systems]]
[[Category:Maintenance Guides]]
[[Category:Maintenance Guides]]

Revision as of 09:45, 1 October 2026

Commodore 64 in the original case

Routine care and preventive checks for the Commodore 64 long boards. The machine's weak point is its external power supply, whose +5 V output reaches the logic without further regulation. For fault-finding see Commodore 64 Troubleshooting Guide; for capacitor values see Commodore 64 Capacitor Replacement Guide. The figures below come from Commodore's Service Manual C64/C64C (March 1992) unless another source is cited.[1]

Identify the board

The assembly number is printed on the board. Commodore's identification table:[2]

C64 and C64C board assemblies
Version Identifying factor (Commodore) PCB assembly Schematic
Original 5-pin video connector (CN5) 326298-01 326106
A (CR) 8-pin video connector 250407 251138
B 8-pin; reduced oscillator circuit 250425 251469
B-2 8-pin; reduced oscillator with component changes 250441-01 251469
B-3 Only two RAM chips, U9 and U10 250466-01 252278
E KERNAL and BASIC ROMs combined into U4; new memory controller at U8 250469-01 252312

On the B-2, resistors R28, R29, R30, R36 and R48 became resistor pack RP5 and diodes CR100-CR105 moved from piggyback positions to CR9 and CR12-CR16.[2] The B-3 and E are listed under C64C identification; see Commodore 64C and Commodore 64C General Maintenance.

Power supply

How the board is powered

The external supply provides regulated +5 V DC and 9 V AC. The +5 V enters at CN7, is filtered by L5, C97 and C100, passes the power switch and supplies the processor logic directly.[3] The board makes its other supplies from the 9 V AC:[3]

  • +12 V: the 9 V AC is "added to 9VDC through CR6, and rectified by CR5", filtered by C88 and C89 and regulated by VR1 (MC7812CT). It supplies the VIC-II, the SID and the audio amplifiers.
  • +5 V CAN: rectified by CR4, filtered by C19 and C95 and regulated by VR2 (MC7805CT), then split into Vvid for the video circuits and Vc for the clock circuits.
  • +9 V unregulated: from CR4, for the cassette motor transistors and, on the B board, the RF modulator.

Because nothing on the board regulates the logic supply, a power supply that fails with a high output puts that voltage on the processor logic. The C64-Wiki records that the original supplies tend to rise as they age, and that in most cases where a supply killed a C64 its output had passed 6 V, with the RAM usually failing first.[4]

Power connector CN7 (7-pin DIN)[3][5]
Pin Function
1, 2, 3 Ground
4 Not connected on the long boards; +5 V on the E board
5 +5 V DC from the supply
6, 7 9 V AC

The circuit-theory text says the 5 V is applied to "pins 5 and 1"; the power-input drawing on the same page, and schematic 252312, show pins 1, 2 and 3 as ground.[3][5]

Checking the supply

Measure the supply's +5 V output before connecting it to the computer. The C64-Wiki's guide to the reading:[4]

+5 V output, C64-Wiki interpretation
Reading Meaning
4.95-5.10 V Normal
5.10-5.20 V Minor concern; measure more often
5.20-5.50 V Failing; replace or repair the supply
5.50-6.00 V The C64 is in danger
Above 6 V The level at which most supply-related C64 deaths occurred

On a running machine, the user port carries +5 V on pin 2 (ground on pins 1, 12, A and N) and the 9 V AC on pins 10 and 11.[6][3] Measure the AC between pins 10 and 11. The C64-Wiki gives about 8.1 V AC as the lowest the machine will run on, and notes that the transformer reads higher unloaded than loaded.[4]

Some original supplies can be opened and repaired; others are glued shut and potted.[4] The primary side of any of them runs at mains voltage. Unplug the supply from the wall before opening it. For an over-voltage cut-out see Commodore 64 Power-Supply Protector (C64 Saver).

Test points

Long-board test points
Signal Where Expected
+5 V 6510 (U7) pin 6; ground is pin 21[7] Same as the supply reading above
RESET 6510 pin 40[7] Low for about 0.5 s after power-on, then high[8]
Phase 0 clock in 6510 pin 1[7] About 1.02 MHz (NTSC) or 0.985 MHz (PAL)[9]
Phase 2 clock out 6510 pin 39[7] Same frequency
+12 V VR1 output; SID pin 28 (VDD) 11.5-12.5 V (7812 limits)[10]
+5 V CAN VR2 output 4.8-5.2 V (7805 limits)[10]
SID audio Pin 27 out, pin 26 in[11] Signal while a voice is playing
VIC-II video Pin 15 sync/luminance, pin 14 colour[11] Composite sync with luminance; colour signal

Commodore fitted Motorola MC7812CT and MC7805CT regulators; the voltage limits above are the generic 78xx datasheet figures, as Commodore's manual gives none.[3]

The reset pulse comes from U20, an LM556 timer wired as a one-shot. Commodore gives the pulse width as 1.1 × R34 × C24, about half a second, with the timer's output on pin 9 inverted by U8 to drive the active-low reset.[8]

Capacitors

Values differ between board revisions; use the Commodore 64 Capacitor Replacement Guide, which gives each board's list. Commodore's troubleshooting guide names three capacitors:[12]

  • C88: "intermittent display", marked as a possible bad connection.
  • C90 and C88, with CR4 and VR2: a black band scrolling through the screen when the system warms up, after checking the external supply.
  • C70: "wrong frequency". C70 is a 10 pF mica capacitor in the clock oscillator on the 326298.[13]

On the long boards C19 (2200 µF 16 V) is the reservoir for VR2, C88 (470 µF 50 V on the 326298, 1000 µF 25 V on later lists) is the reservoir for VR1, and C91 (100 µF 16 V) sits on the incoming +5 V.[3][13] See Capacitor Failure Symptoms and Battery Explosion, Capacitor or Corrosion Damage.

Chip positions

Long-board ICs (326298 and 250407 lists)[13][14]
Function Position Part Commodore part no.
CPU U7 6510 906107-01
PLA U17 82S100 906114-01
VIC-II U19 6567 (NTSC) 906109-04
SID U18 6581 906112-01
CIA U1, U2 6526 906108-01
Main RAM U9-U12, U21-U24 4164, 200 ns 901505-01
Colour RAM U6 2114 901453-01
BASIC ROM U3 2364 901226-01
KERNAL ROM U4 2364 901227-03
Character ROM U5 2332 901225-01
Reset timer U20 LM556 901523-03
Serial bus driver U8 7406 901522-06

Commodore's fault table

A selection from the manual's troubleshooting guide; the Commodore 64 Troubleshooting Guide carries the full table.[12]

Symptom Commodore's check
Blank screen on power-up External power supply; U4, U17, U7, U3, U8, U19, U9-U12, U21-U24, VR2, CR4, VR1
Out of memory error on power-up U9-U12, U21-U24
Powers up with graphic display and blinking cursor U14 (74LS258)
All characters displayed as blocks U26 (74LS373)
Abnormal colours in the letters U6 (2114 colour RAM), U16 (4066)
White band scrolls down the screen (60 Hz hum) External power supply; VR2
Wavy screen after warm-up External power supply; U31 (74LS629), U30 (74LS193)
Cursor disappears after the system warms up U1 (6526)
Cassette motor keeps running; TIP29 very hot; fuse may blow Cassette port for shorts; R4 possibly open
DEVICE NOT PRESENT with a disk drive U1, U7, R28, R29, R30

PLA replacements are covered in PLA Replacement Options.

Cleaning and inspection

  1. Disconnect the power supply before opening the case.
  2. Remove dust with dry air and a soft anti-static brush.
  3. Clean the cartridge and user-port edge connectors and the keyboard contacts with isopropyl alcohol.
  4. Inspect the joints on VR1, VR2 and the bridge rectifier CR4, the power parts Commodore names for a blank screen.[12]
  5. If the fuse has been replaced, check its rating: the long-board parts lists give F1 as 1.5 A, 250 V, normal blow.[13][14]
  6. Measure the supply's +5 V output before reconnecting it.

Tools

See also Recommended Tools.

References

  1. ↑ Commodore Business Machines, Service Manual C64/C64C, March 1992, PN-314001-03, hosted at Commodore 64 / 64C Service Manual.
  2. ↑ 2.0 2.1 Service Manual C64/C64C (1992), "C64 Board Identification" and "C64C Board Identification", p. 17.
  3. ↑ 3.0 3.1 3.2 3.3 3.4 3.5 3.6 Service Manual C64/C64C (1992), Circuit Theory, "The Power Supply", p. 4.
  4. ↑ 4.0 4.1 4.2 4.3 C64-Wiki, "Power Supply", sections "Power Supplies of the Commodore 64" and "Failure". Retrieved 2026-10-01.
  5. ↑ 5.0 5.1 Service Manual C64/C64C (1992), E schematic 252312, p. 42.
  6. ↑ C64-Wiki, "User Port", pin assignment table. Retrieved 2026-10-01.
  7. ↑ 7.0 7.1 7.2 7.3 Service Manual C64/C64C (1992), pin assignments for U7 6510 and U17 PLA, p. 27.
  8. ↑ 8.0 8.1 Service Manual C64/C64C (1992), Circuit Theory, "Reset Logic Circuits", p. 5.
  9. ↑ Service Manual C64/C64C (1992), Circuit Theory, "The C64 Clock Circuits", p. 6; dot clock frequencies in the notes to schematic 252312, p. 42.
  10. ↑ 10.0 10.1 Texas Instruments, LM340, LM340A and LM7805 Family Wide VIN 1.5-A Fixed Voltage Regulators, SNOSBT0L (revised September 2016), tables 6.6 (7805) and 6.7 (7812), output voltage at TJ = 25 °C.
  11. ↑ 11.0 11.1 Service Manual C64/C64C (1992), Circuit Theory, "8 Pin Video and Audio Output Circuits", p. 10.
  12. ↑ 12.0 12.1 12.2 Service Manual C64/C64C (1992), "64 Troubleshooting Guide", pp. 14-16.
  13. ↑ 13.0 13.1 13.2 13.3 Service Manual C64/C64C (1992), "Parts List - PCB Assembly #326298", pp. 19-20.
  14. ↑ 14.0 14.1 Service Manual C64/C64C (1992), "Parts List - PCB Assembly #250407-04", pp. 24-25.
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