Commodore 64 General Maintenance: Difference between revisions
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 |
Tolerance note: cite diagnostic manual 10 % |
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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" /> | 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" /> | ||
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)]]. | 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)]]. | ||
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| +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 | | +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" /> || | | 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 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> | ||
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| Phase 2 clock out || 6510 pin 39<ref name="smpin" /> || Same frequency | | Phase 2 clock out || 6510 pin 39<ref name="smpin" /> || Same frequency | ||
|- | |- | ||
| +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> | | +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 || 4.8-5.2 V (7805 limits)<ref name="ti78" /> | | +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 | | 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 | ||
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Commodore fitted Motorola MC7812CT and MC7805CT regulators | 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" /> | 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" /> | ||
Latest revision as of 09:52, 1 October 2026
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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]
| 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]
| 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]
| 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]| 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]| 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
[edit | edit source]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
[edit | edit source]- Disconnect the power supply before opening the case.
- Remove dust with dry air and a soft anti-static brush.
- Clean the cartridge and user-port edge connectors and the keyboard contacts with isopropyl alcohol.
- Inspect the joints on VR1, VR2 and the bridge rectifier CR4, the power parts Commodore names for a blank screen.[13]
- 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]
- Measure the supply's +5 V output before reconnecting it.
Tools
[edit | edit source]- Digital multimeter; oscilloscope or logic probe
- Temperature-controlled soldering iron, flux, desoldering braid and pump
- Isopropyl alcohol, lint-free wipes, soft brush
- A diagnostic cartridge: see Commodore Test Cartridge Index and Commodore 64 Diagnostic Instruction and Troubleshooting Manual (326070-01)
See also Recommended Tools.
Related pages
[edit | edit source]- Commodore 64 · Commodore 64C · Commodore 64 Troubleshooting Guide · Commodore 64 Capacitor Replacement Guide
- Commodore 64 / 64C Service Manual
- Commodore 64 Power-Supply Protector (C64 Saver) · Capacitor Failure Symptoms · Recommended Tools
- Floppy Drive Repair
References
[edit | edit source]- ↑ Commodore Business Machines, Service Manual C64/C64C, March 1992, PN-314001-03, hosted at Commodore 64 / 64C Service Manual.
- ↑ 2.0 2.1 Service Manual C64/C64C (1992), "C64 Board Identification" and "C64C Board Identification", p. 17.
- ↑ 3.0 3.1 3.2 3.3 3.4 3.5 Service Manual C64/C64C (1992), Circuit Theory, "The Power Supply", p. 4.
- ↑ 4.0 4.1 4.2 4.3 C64-Wiki, "Power Supply", sections "Power Supplies of the Commodore 64" and "Failure". Retrieved 2026-10-01.
- ↑ 5.0 5.1 Service Manual C64/C64C (1992), E schematic 252312, p. 42.
- ↑ C64-Wiki, "User Port", pin assignment table. Retrieved 2026-10-01.
- ↑ 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).
- ↑ 8.0 8.1 8.2 8.3 Service Manual C64/C64C (1992), pin assignments for U7 6510 and U17 PLA, p. 27.
- ↑ 9.0 9.1 Service Manual C64/C64C (1992), Circuit Theory, "Reset Logic Circuits", p. 5.
- ↑ Service Manual C64/C64C (1992), Circuit Theory, "The C64 Clock Circuits", p. 6; dot clock frequencies in the notes to schematic 252312, p. 42.
- ↑ 11.0 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.
- ↑ 12.0 12.1 Service Manual C64/C64C (1992), Circuit Theory, "8 Pin Video and Audio Output Circuits", p. 10.
- ↑ 13.0 13.1 13.2 Service Manual C64/C64C (1992), "64 Troubleshooting Guide", pp. 14-16.
- ↑ 14.0 14.1 14.2 14.3 Service Manual C64/C64C (1992), "Parts List - PCB Assembly #326298", pp. 19-20.
- ↑ 15.0 15.1 Service Manual C64/C64C (1992), "Parts List - PCB Assembly #250407-04", pp. 24-25.