Jump to content

Commodore 64 General Maintenance

From RetroTechCollection
Revision as of 09:51, 1 October 2026 by Josh (talk | contribs) (Add Commodore diagnostic manual supply figures (9-12 V AC at CN7, VIC-II supply pins, reset timing))
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. 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

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 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.[9]

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:[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

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

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

  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

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 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.
Retrieved from "https://wiki.retrotechcollection.com/index.php?title=Commodore_64_General_Maintenance&oldid=17410"