Commodore 64 General Maintenance

Commodore 64 "bread bin".

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.

The electrical figures on this page are taken from Commodore's own C64 / C64C Service Manual (March 1992, PN-314001-03), hosted here.

Identify your board

The assembly number is silkscreened on the board. Commodore's own identification tables:

C64 and C64C mainboard assemblies

Version	Identifying factor	PCB assembly	Schematic
Original	5-pin video connector (CN5)	326298-01	326106
A (CR)	8-pin video connector	250407-01	251138
B	8-pin, reduced oscillator circuit	250425	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
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–CR105 are no longer piggybacked but fitted at CR9 and CR12–CR16.

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 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 V DC and a 9 V AC output; the 5 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.

Everything else is made on the board from the 9 V AC:

• +12 V DC — 9 V AC is added to 9 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 V CAN — 9 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 V unregulated — rectified from the 9 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 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.

Power connector CN7 (7-pin DIN), from Commodore's schematics

Pin	Function
1, 2, 3	Ground
4	Not connected on long boards; +5 V on the E board
5	+5 V DC, regulated, from the brick
6, 7	9 V AC

Checking the supply

Measure the brick before you plug it into the computer. Its 5 V output is what is going to reach the CPU, the RAM, the PLA and the SID unchanged.

The user port is the easiest place to measure on a running machine: +5 V is on pin 2, ground on pins 1 and 12, and the 9 V AC is brought out on pins 10 and 11. Measure the AC between pins 10 and 11, not from one of them to ground.

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.

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.

Test points

Go/no-go checks, from Commodore's schematics

Signal	Where	Expected
+5 V	6510 / 8500 pin 6 (VCC). Pin 21 is GND	4.9 – 5.1 V
RESET	6510 / 8500 pin 40	Low for roughly 0.5 s at power-up, then +5 V
Φ0 in	6510 / 8500 pin 1	1.023 MHz NTSC / 0.985 MHz PAL
Φ2 out	6510 / 8500 pin 39	Same frequency
+12 V (long boards)	VR1 output, or VIC-II pin 13 (Vdd) and SID pin 28 (Vdd)	11.5 – 12.5 V
+5 V (user port)	CN2 pin 2; ground on pins 1 and 12	4.9 – 5.1 V
9 V AC	CN2 pins 10 to 11	9 – 11 V rms
SID audio out	SID pin 27; audio in is pin 26	Signal present when a voice is running
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 V at pin 40, Φ2 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.

The reset itself comes from U20, an LM556 timer configured as a one-shot; Commodore gives the pulse width as 1.1 × R34 × C24, about half a second, with the active-high output on pin 9.

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:

• C88 — "intermittent display", listed as a possible bad connection rather than a failed part. Reflow its joints before replacing it.
• C90, C88, CR4, VR2 — a black band scrolling through the screen once the machine warms up, after checking the external supply.
• C70 — "wrong frequency".

On the long boards the main reservoir is C19, 2200 µF 16 V; C88 and C90 are the supply-filter cans; C91 is 100 µF 16 V. There are no 220 µF parts in the 9 V AC rectifier path and there are no SMD tantalums on any C64 board.

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

Long-board positions, from Commodore's parts lists

Function	Position	Part	Typical symptom
PLA	U17	82S100, Commodore 906114-01	Black screen, random colours, crashes with a cartridge fitted
CPU	U7	6510, 906107-01	Blank screen; cursor locks up; cassette motor faults
VIC-II	U19	6567, Commodore 906109-04 (the B-3 list marks it "VIC NTSC", 906109-01)	Blank screen, graphic characters instead of letters
SID	U18	6581, 906112-01	No sound
CIA 1 / CIA 2	U1, U2	6526, 906108-01	Dead keyboard or joystick; DEVICE NOT PRESENT; cursor vanishes when warm
Main RAM	U9–U12, U21–U24	4164, 901505-01	OUT OF MEMORY on power-up, random characters
Colour RAM	U6	2114, 901453-01	Abnormal colours in the letters
ROMs	U3 BASIC, U4 KERNAL, U5 character	2364 / 2332 mask	Various; see the fault table below
Reset timer	U20	LM556, 901523-03	No reset, RESTORE key dead, cursor locks on power-up

Commodore's own fault table

The manual's troubleshooting guide is a symptom-to-component list of about thirty entries. A representative selection:

Symptom	Commodore's check
Blank screen on power-up	External power supply; U4, U17, U7, U3, U8, U19, the RAM, VR2, CR4, VR1
Out of memory error on power-up	U9–U12, U21–U24
Powers up with a graphic display and a blinking cursor	U14 (74LS258)
All characters displayed as blocks	U26 (74LS373)
Abnormal colours in the letters	U6 (2114 colour RAM), U16 (4066)
White band scrolling down the screen (60 Hz hum)	External power supply; VR2
Wavy screen after warm-up	External power supply; U31, U30
Cursor disappears after the system warms up	U1 (CIA)
Cassette motor keeps running, TIP29 gets 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

Note how often the first item is the external supply. Commodore's own ordering is the right ordering.

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 V and an 8580 wants 9 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

1. Test the brick's +5 V output before connecting it, every session with an original supply.
2. Check the fuse rating if you have replaced it: Commodore specifies F1, 1.5 A, 250 V, normal blow.
3. Inspect and reflow the regulator and rectifier joints.
4. Reseat socketed chips annually.
5. Clean the edge connectors.
6. Replace the supply electrolytics if the machine shows a scrolling band once warm — after clearing the brick.

Tools

• Anti-static strap and mat; digital multimeter; oscilloscope or logic probe
• Temperature-controlled soldering station, flux, braid and a pump
• Isopropyl alcohol, lint-free wipes, soft brush, contact cleaner
• 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.

Related pages

• Commodore 64 · Commodore 64C · Commodore 64 Troubleshooting Guide · Commodore 64 Capacitor Replacement Guide
• Commodore 64 / 64C Service Manual — the source for the figures on this page
• Commodore 64 Power-Supply Protector (C64 Saver) · Capacitor Failure Symptoms · Battery Explosion, Capacitor or Corrosion Damage · Recommended Tools · Retrobrite
• Floppy Drive Repair for 1541 and 1581 mechanisms