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[[File:C64 ASSY NO 250425 motherboard 1984.jpg|thumb|359x359px|Commodore 64 (250425) Motherboard]]
[[File:Commodore_64_(photo).jpg|thumb|400px|Commodore 64 in the original case]]
The '''Commodore 64''' (C-64, C64 C, “bread bin”, 64C, Aldi C64, SX-64) is now 40-plus years old.  Aging electrolytic capacitors, brittle plastics, and the infamous “death‐brick” PSU all put these machines at risk.
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>


This page collects '''best-practice hardware care''', preventive service, fault-pre-emption, and periodic checks for '''every board revision from 1982 ASSY 250407 to the late 250469-05 short board'''. Follow the sections below to keep a C-64 healthy for the next four decades.
== 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>


== Identify Your Board Revision ==
{| class="wikitable styled-table" style="width:100%; text-align:left;"
{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+'''C64 and C64C board assemblies'''
|+'''C-64 Motherboard Generations'''
! Version !! Identifying factor (Commodore) !! PCB assembly !! Schematic
! ASSY !! Year(s) !! Key Differences
|-
|-
| 326298 (“Rev A”) || 1982 || 5-pin A/V DIN, 6567R56A / 6569R1 VIC-II, ceramic SID (6581R1), fully socketed
| Original || 5-pin video connector (CN5) || 326298-01 || 326106
|-
|-
| 250407 / 250425 || 1983-84 || 8-pin A/V DIN, original long board, PLA 906114-01, discrete DRAM (8×4164)
| A (CR) || 8-pin video connector || 250407 || 251138
|-
|-
| 250466 || 1984-85 || Long board, HMOS-II 6510 CPU & 8701 clock chip, “short” color RAM, improved RF modulator
| B || 8-pin; reduced oscillator circuit || 250425 || 251469
|-
|-
| 250469 rev 3/4/5 (“64C short”) || 1986-92 || Short board, 64-pin CSG 8500 CPU + PLA inside 64-pin “Super PLA” (Chips #252535-xx), HMOS-II 8565 VIC-II, 8580 SID, 2×128-kbit DRAMs
| B-2 || 8-pin; reduced oscillator with component changes || 250441-01 || 251469
|-
|-
| SX-64 portable || 1984-86 || 5″ RGB CRT, 1541 mechanism inside; logic equivalent to 250425
| 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.<ref name="smid" /> 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.<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.
 
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>
 
{| class="wikitable styled-table" style="width:80%; text-align:left;"
|+'''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
|-
| 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
|}
|}


''Find the '''ASSY''' number silkscreened near the cartridge port or RF modulator.''
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 ===
Measure the supply's +5 V output before connecting it to the computer. The C64-Wiki's guide to the reading:<ref name="c64wps" />
 
{| class="wikitable styled-table" style="width:80%; text-align:center;"
|+'''+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
|}


== Regular Cleaning ==
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. Commodore's diagnostic manual expects 9 to 12 V AC at CN7 pins 6 and 7 and +5 V at CN7 pin 5, all within 10 %.<ref name="dm">Commodore Business Machines, ''C64 Diagnostic Instruction and Troubleshooting Manual, Assembly #326070-01'', 1992, Section 2, pp. 2-1, 2-3 and 2-4, hosted at [[Commodore 64 Diagnostic Instruction and Troubleshooting Manual (326070-01)]].</ref> 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" />
=== Case & Keyboard ===
* Wipe ABS plastic with a '''damp micro-fiber cloth & mild dish-soap'''.
* Keys pop off vertically; clean plungers with 99 % IPA. De-yellow with '''retro-brite''' only if you can monitor surface temp.
* Lube space-bar stabiliser wire with plastic-safe grease.


=== PCB Dust & Oxidation ===
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)]].
* Disconnect PSU; wait 5 min for the +5 V rail to drain.
* Blow dust with compressed air, brush gently with anti-static brush. 
* Re-seat every socketed IC to wipe oxidised contacts.


== Power Supply: The “Death Brick” ==
== Test points ==
[[File:Internals of C64 Power Supply (8595594936).jpg|thumb|Commodore 64 Power Supply|250x250px]]
{| class="wikitable styled-table" style="width:100%; text-align:left;"
Original Commodore linear bricks often drift above '''5.5 V DC''', frying PLA, DRAM and SID. 
|+'''Long-board test points'''
{| class="wikitable styled-table" style="width:70%;"
! Signal !! Where !! Expected
! Pin !! Function !! Healthy Range
|-
| +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 pin 40<ref name="smpin" /> || 0 V at power-on, rising to +5 V; about 0.5 s in the service manual, about 1 s in the diagnostic manual<ref name="smrst">''Service Manual C64/C64C'' (1992), Circuit Theory, "Reset Logic Circuits", p. 5.</ref><ref name="dm" />
|-
|-
| 1 (red) || +5 V DC || 4.95 – 5.20 V
| 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>
|-
|-
| 2 (white) || GND || 0 V
| Phase 2 clock out || 6510 pin 39<ref name="smpin" /> || Same frequency
|-
|-
| 6 (yellow) || 9 VAC @ 1 A || 9.0 – 11.0 V rms
| +12 V || VR1 output; VIC-II (U19) pin 13; 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 CAN || VR2 output; VIC-II (U19) pin 40 || 4.8-5.2 V (7805 limits)<ref name="ti78" />
|-
| 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
|-
| VIC-II video || Pin 15 sync/luminance, pin 14 colour<ref name="smav" /> || Composite sync with luminance; colour signal
|}
|}


''Measure with a multimeter before every session.''
Commodore fitted Motorola MC7812CT and MC7805CT regulators. The limits above are the generic 78xx datasheet figures; Commodore's diagnostic manual allows 10 % on all its readings.<ref name="dm" />
'''Safe replacements:''' modern switch-mode “C64RMK2”, Ray Carlsen’s CR adapter, or over-voltage-protected DIY bricks.
 
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" />
 
== 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:<ref name="smts">''Service Manual C64/C64C'' (1992), "64 Troubleshooting Guide", pp. 14-16.</ref>
* 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>


== Capacitor Health ==
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]].
C-64 boards generally fare better than Amigas, but '''filter cans in the RF modulator''' and '''5 V/12 V regulators''' dry out. 
Early long boards: replace the '''1000 µF / 16 V +5 V filter, 470 µF / 25 V +12 V''', and the two 220 µF caps in the cassette/9 VAC rectifier path. 
Short board 250469-05 uses SMD tantalums—rarely fail, but the modulator still contains 22 µF / 16 V electrolytics. See [[Commodore 64 Capacitor Replacement Guide]] for more detailed replacement information.


== Known Failure-Prone ICs ==
== Chip positions ==
{| class="wikitable styled-table" style="width:100%; text-align:center;"
{| class="wikitable styled-table" style="width:100%; text-align:left;"
|+'''C-64 Common Chip Failures'''
|+'''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>
! IC !! Part # !! Typical Symptom !! Quick Test / Hint
! 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
|-
|-
| '''PLA''' || 906114-01<br/>251064-01 || Black screen, random colors, crash on cartridge || Replace with GAL-based PLAnkton or EPROM “PLA20V8”
| Main RAM || U9-U12, U21-U24 || 4164, 200 ns || 901505-01
|-
|-
| '''SID''' || 6581 / 8580 || No sound, stuck keys, overheating || Touch pin 28 audio-out—hear hum? Swap with known good SID
| Colour RAM || U6 || 2114 || 901453-01
|-
|-
| '''VIC-II''' || 6567/6569 / 8565 || Black screen, jail-bars, bad color, sparkle pixels || Check 12 V @ pin 13 (long board)
| BASIC ROM || U3 || 2364 || 901226-01
|-
|-
| '''CIA #1 / #2''' || 6526 || Dead keyboard / joystick / IEC, time -outs || Swap the two CIAs—fault moves? Easy socket test
| KERNAL ROM || U4 || 2364 || 901227-03
|-
|-
| '''DRAM''' || 4164 ×8 or 41464 ×2 || Random chars, “OUT OF MEMORY”, intermittent crash || Dead-test cart pinpoints bad bit
| Character ROM || U5 || 2332 || 901225-01
|-
| Reset timer || U20 || LM556 || 901523-03
|-
| Serial bus driver || U8 || 7406 || 901522-06
|}
|}


== 📏 Voltage & Clock Test Points ==
=== Commodore's fault table ===
{| class="wikitable styled-table" style="width:80%; text-align:center;"
A selection from the manual's troubleshooting guide; the [[Commodore 64 Troubleshooting Guide]] carries the full table.<ref name="smts" />
! Node !! Board Pad / IC Pin !! Expected Reading
 
{| class="wikitable styled-table" style="width:100%; text-align:left;"
! 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)
|-
|-
| '''+5 V DC''' || 6510 pin 40 || 4.95 – 5.20 V
| White band scrolls down the screen (60 Hz hum) || External power supply; VR2
|-
|-
| '''+12 V DC''' (long boards) || 7812 VR1 out or VIC pin 13 || 11.8 – 12.3 V
| Wavy screen after warm-up || External power supply; U31 (74LS629), U30 (74LS193)
|-
|-
| '''9 VAC''' || User-port pin 10 vs 12 || 9–11 V rms (scope shows 50/60 Hz sine)
| Cursor disappears after the system warms up || U1 (6526)
|-
|-
| '''CPU φ2 clock''' || 6510 pin 2 || 985 kHz (PAL) / 1.022 MHz (NTSC)
| Cassette motor keeps running; TIP29 very hot; fuse may blow || Cassette port for shorts; R4 possibly open
|-
|-
| '''Reset''' || 6510 pin 3 || Low ≤ 100 ms then High (+5 V)
| DEVICE NOT PRESENT with a disk drive || U1, U7, R28, R29, R30
|}
|}


== Essential Tools ==
PLA replacements are covered in [[PLA Replacement Options]].
* ESD wrist-strap & soft bristle brush 
* Digital multimeter (check PSU every session) 
* Solder station + flux & desolder braid (for PLA/SID sockets) 
* Diagnostic cartridges: '''C-64 Dead Test 781220''' & '''Diagnostic Rev. 586220'''
* Freeze spray or hot-air pencil for thermal fault hunts


== Preventive Maintenance Checklist ==
== Cleaning and inspection ==
# '''Test PSU''' – verify +5 V & 9 VAC before connecting computer.
# Disconnect the power supply before opening the case.
# '''Inspect regulators''' 7805 / 7812 for cracked solder joints; re-flow if dull.
# Remove dust with dry air and a soft anti-static brush.
# '''Re-seat chips''' annually; apply DeoxIT to sockets.
# Clean the cartridge and user-port edge connectors and the keyboard contacts with isopropyl alcohol.
# '''Add heat-sinks''' to VIC-II, SID, PLA (especially early ceramic packages).
# Inspect the joints on VR1, VR2 and the bridge rectifier CR4, the power parts Commodore names for a blank screen.<ref name="smts" />
# '''Clean cartridge & expansion edge-connectors''' with isopropyl + pink eraser.
# 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" />
# '''Replace RF modulator electrolytics''' if video shows shimmering border.
# Measure the supply's +5 V output before reconnecting it.
# '''Ventilation''' – leave 2 cm clearance above vents; consider modern transparent top-shell or fan for hot climates.


== Quick-Fix Flowcharts ==
== Tools ==
=== No Video / Black Screen ===
* Digital multimeter; oscilloscope or logic probe
* Check PSU rails → OK? 
* Temperature-controlled soldering iron, flux, desoldering braid and pump
* Feel PLA & VIC: burning hot = replace. 
* Isopropyl alcohol, lint-free wipes, soft brush
* Swap PLA → CPU / VIC → both CIAs → ROMs. 
* A diagnostic cartridge: see [[Commodore Test Cartridge Index]] and [[Commodore 64 Diagnostic Instruction and Troubleshooting Manual (326070-01)]]
* Still black? probe φ2 clock & Reset; if missing, suspect 8701 clock generator or bad crystal.


=== Garbled Characters / Checkerboard ===
See also [[Recommended Tools]].
* Run Dead Test cart: code flashes DRAM bank; count flashes to identify bad 4164. 
* If only colors wrong → replace 2114 color RAM. 
* If only certain glyphs wrong → character ROM (901225-01) socket issue.


=== No Sound ===
== Related pages ==
* Confirm volume & cable → check SID pin 27 audio-out with scope; flat-line = dead SID. 
* [[Commodore 64]] · [[Commodore 64C]] · [[Commodore 64 Troubleshooting Guide]] · [[Commodore 64 Capacitor Replacement Guide]]
* Audio present before modulator but silent on TV = RF modulator electrolytic or bad audio-mix resistor.
* [[Commodore 64 / 64C Service Manual]]
* [[Commodore 64 Power-Supply Protector (C64 Saver)]] · [[Capacitor Failure Symptoms]] · [[Recommended Tools]]
* [[Floppy Drive Repair]]


=== Keyboard / Joystick Dead ===
== References ==
* Swap CIAs; if fixed, replace bad 6526. 
<references />
* If row/column missing → inspect keyboard ribbon; repair broken trace with conductive ink.


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

Latest revision as of 09:52, 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

[edit | edit source]

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

[edit | edit source]

How the board is powered

[edit | edit source]

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

[edit | edit source]

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. Commodore's diagnostic manual expects 9 to 12 V AC at CN7 pins 6 and 7 and +5 V at CN7 pin 5, all within 10 %.[7] 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

[edit | edit source]
Long-board test points
Signal Where Expected
+5 V 6510 (U7) pin 6; ground is pin 21[8] Same as the supply reading above
RESET 6510 pin 40[8] 0 V at power-on, rising to +5 V; about 0.5 s in the service manual, about 1 s in the diagnostic manual[9][7]
Phase 0 clock in 6510 pin 1[8] About 1.02 MHz (NTSC) or 0.985 MHz (PAL)[10]
Phase 2 clock out 6510 pin 39[8] Same frequency
+12 V VR1 output; VIC-II (U19) pin 13; SID pin 28 (VDD) 11.5-12.5 V (7812 limits)[11]
+5 V CAN VR2 output; VIC-II (U19) pin 40 4.8-5.2 V (7805 limits)[11]
SID audio Pin 27 out, pin 26 in[12] Signal while a voice is playing
VIC-II video Pin 15 sync/luminance, pin 14 colour[12] Composite sync with luminance; colour signal

Commodore fitted Motorola MC7812CT and MC7805CT regulators. The limits above are the generic 78xx datasheet figures; Commodore's diagnostic manual allows 10 % on all its readings.[7]

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.[9]

Capacitors

[edit | edit source]

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:[13]

  • 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.[14]

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][14] See Capacitor Failure Symptoms and Battery Explosion, Capacitor or Corrosion Damage.

Chip positions

[edit | edit source]
Long-board ICs (326298 and 250407 lists)[14][15]
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

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A selection from the manual's troubleshooting guide; the Commodore 64 Troubleshooting Guide carries the full table.[13]

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

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  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.[13]
  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.[14][15]
  6. Measure the supply's +5 V output before reconnecting it.

Tools

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See also Recommended Tools.

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References

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  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 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 Commodore Business Machines, C64 Diagnostic Instruction and Troubleshooting Manual, Assembly #326070-01, 1992, Section 2, pp. 2-1, 2-3 and 2-4, hosted at Commodore 64 Diagnostic Instruction and Troubleshooting Manual (326070-01).
  8. ↑ 8.0 8.1 8.2 8.3 Service Manual C64/C64C (1992), pin assignments for U7 6510 and U17 PLA, p. 27.
  9. ↑ 9.0 9.1 Service Manual C64/C64C (1992), Circuit Theory, "Reset Logic Circuits", p. 5.
  10. ↑ Service Manual C64/C64C (1992), Circuit Theory, "The C64 Clock Circuits", p. 6; dot clock frequencies in the notes to schematic 252312, p. 42.
  11. ↑ 11.0 11.1 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.
  12. ↑ 12.0 12.1 Service Manual C64/C64C (1992), Circuit Theory, "8 Pin Video and Audio Output Circuits", p. 10.
  13. ↑ 13.0 13.1 13.2 Service Manual C64/C64C (1992), "64 Troubleshooting Guide", pp. 14-16.
  14. ↑ 14.0 14.1 14.2 14.3 Service Manual C64/C64C (1992), "Parts List - PCB Assembly #326298", pp. 19-20.
  15. ↑ 15.0 15.1 Service Manual C64/C64C (1992), "Parts List - PCB Assembly #250407-04", pp. 24-25.
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