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The legendary '''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. 
[[File:Commodore_64_(photo).jpg|thumb|400px|Commodore 64 in the original case]]
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.
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>


== 🔍 Identify Your Board Revision ==
== Identify the board ==
{| class="wikitable styled-table" style="width:100%; text-align:center;"
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>
|+'''C-64 Motherboard Generations'''
 
! ASSY !! Year(s) !! Key Differences
{| class="wikitable styled-table" style="width:100%; text-align:left;"
|+'''C64 and C64C board assemblies'''
! Version !! Identifying factor (Commodore) !! PCB assembly !! Schematic
|-
|-
| 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
|}
|}


''Find the '''ASSY''' number silkscreened near the cartridge port or RF modulator.''
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]].


== 🧼 Regular Cleaning ==
== Power supply ==
=== Case & Keyboard ===
=== How the board is powered ===
* Wipe ABS plastic with a '''damp micro-fiber cloth & mild dish-soap'''.
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" />
* Keys pop off vertically; clean plungers with 99 % IPA. De-yellow with '''retro-brite''' only if you can monitor surface temp.
* +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.
* Lube space-bar stabiliser wire with plastic-safe grease.
* +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.


=== PCB Dust & Oxidation ===
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>
* 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” ==
{| class="wikitable styled-table" style="width:80%; text-align:left;"
Original Commodore linear bricks often drift above '''5.5 V DC''', frying PLA, DRAM and SID. 
|+'''Power connector CN7 (7-pin DIN)'''<ref name="smps" /><ref name="sme">''Service Manual C64/C64C'' (1992), E schematic 252312, p. 42.</ref>
{| class="wikitable styled-table" style="width:70%;"
! Pin !! Function
! Pin !! Function !! Healthy Range
|-
| 1, 2, 3 || Ground
|-
|-
| 1 (red) || +5 V DC || 4.95 – 5.20 V
| 4 || Not connected on the long boards; +5 V on the E board
|-
|-
| 2 (white) || GND || 0 V
| 5 || +5 V DC from the supply
|-
|-
| 6 (yellow) || 9 VAC @ 1 A || 9.0 – 11.0 V rms
| 6, 7 || 9 V AC
|}
|}


''Measure with a multimeter before every session.''
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" />
'''Safe replacements:''' modern switch-mode “C64RMK2”, Ray Carlsen’s CR adapter, or over-voltage-protected DIY bricks.


== 💣 Capacitor Health ==
=== Checking the supply ===
C-64 boards generally fare better than Amigas, but '''filter cans in the RF modulator''' and '''5 V/12 V regulators''' dry out. 
Measure the supply's +5 V output before connecting it to the computer. The C64-Wiki's guide to the reading:<ref name="c64wps" />
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. 
Recap threads list exact values per ASSY.


== ⚠️ Known Failure-Prone ICs ==
{| class="wikitable styled-table" style="width:80%; text-align:center;"
{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+'''+5 V output, C64-Wiki interpretation'''
|+'''C-64 Common Chip Failures'''
! Reading !! Meaning
! IC !! Part # !! Typical Symptom !! Quick Test / Hint
|-
|-
| '''PLA''' || 906114-01<br/>251064-01 || Black screen, random colors, crash on cartridge || Replace with GAL-based PLAnkton or EPROM “PLA20V8”
| 4.95-5.10 V || Normal
|-
|-
| '''SID''' || 6581 / 8580 || No sound, stuck keys, overheating || Touch pin 28 audio-out—hear hum? Swap with known good SID
| 5.10-5.20 V || Minor concern; measure more often
|-
|-
| '''VIC-II''' || 6567/6569 / 8565 || Black screen, jail-bars, bad color, sparkle pixels || Check 12 V @ pin 13 (long board)
| 5.20-5.50 V || Failing; replace or repair the supply
|-
|-
| '''CIA #1 / #2''' || 6526 || Dead keyboard / joystick / IEC, time -outs || Swap the two CIAs—fault moves? Easy socket test
| 5.50-6.00 V || The C64 is in danger
|-
|-
| '''DRAM''' || 4164 ×8 or 41464 ×2 || Random chars, “OUT OF MEMORY”, intermittent crash || Dead-test cart pinpoints bad bit
| Above 6 V || The level at which most supply-related C64 deaths occurred
|}
|}


== 📏 Voltage & Clock Test Points ==
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" />
{| class="wikitable styled-table" style="width:80%; text-align:center;"
 
! Node !! Board Pad / IC Pin !! Expected Reading
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 ==
{| class="wikitable styled-table" style="width:100%; text-align:left;"
|+'''Long-board test points'''
! Signal !! Where !! Expected
|-
| +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" />
|-
| 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>
|-
| Phase 2 clock out || 6510 pin 39<ref name="smpin" /> || Same frequency
|-
| +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
|}
 
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" />
 
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>
 
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]].
 
== Chip positions ==
{| class="wikitable styled-table" style="width:100%; text-align:left;"
|+'''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 !! 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.<ref name="smts" />
 
{| 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:Commodore Maintenance Guides]]
[[Category:Maintenance Guides]]
[[Category:Commodore 64]]
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