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Commodore 64 Power Supply Testing and Repair

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Commodore 64 power supply, vented wedge type
A Commodore 64 power supply of the vented wedge type.

The Commodore 64 takes its power from an external supply that provides regulated +5 V DC and 9 V AC on a DIN plug.[1] Ray Carlsen gives the ratings as 1.5 A on the +5 V output and 1 A on the 9 V AC.[2] Nothing on the C64's board regulates the +5 V: it passes a filter and the power switch and feeds the logic directly.[1] A supply whose regulator fails short-circuit can put almost 12 V on that line and destroy chips in the computer, the RAM first; Carlsen calls it a "silent killer".[2] Carlsen describes the black and white "brick" supplies as potted in epoxy and unrepairable; other Commodore supplies are held together with screws and can be repaired, and the vented one pictured opens to show an ordinary circuit board.[3][4][5] The same supply powers the Commodore 64C.[6]

Symptoms

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  • The computer will not switch on and the red power LED stays off: the +5 V is low or missing. Carlsen gives this as the commonest failure of the original brick.[2]
  • A blank screen, lock-ups, a garbage screen, hum bars on the screen, hum in the audio, damaged RAM chips, and faults that appear only after warm-up.[7] Commodore's troubleshooting table lists the external supply first for a blank screen at power-up, a white band scrolling down the screen, a wavy screen after warm-up and a black band scrolling through the screen when warm.[8]
  • Several RAM chips failing at once. Commodore's Dead Test manual and Carlsen both treat this as a sign of a failed supply.[9][3]
  • On a long-board C64, a missing 9 V AC gives a blank screen with the red LED on and a normal +5 V. On the 64C's E board the computer still starts, but has no sound, no cassette motor and no time-of-day clock for the CIAs.[2]

A blown fuse inside the computer also removes the 9 V AC; see Commodore 64 Troubleshooting Guide.[10]

Safety

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  • The transformer's primary winding runs at mains voltage; Buchan's schematic of the US 902503-02 shows 117 V AC. Unplug the supply from the wall before opening it.[11]
  • Carlsen notes that the transformer has a thermal fuse that cuts the mains when it overheats.[4]
  • Never test a suspect supply on a working computer. A failing supply may only produce over-voltage when warm, and it will damage every computer connected to it.[2]

Tools and parts

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  • A digital multimeter.
  • A dummy load. Carlsen's tester is a 7-pin female DIN socket (for example the power socket from a scrap C64 board) wired to two 12 V lamps, one on the +5 V output and one on the 9 V AC; he uses type 1003 lamps, each drawing under 1 A. Resistors can replace the lamps: 5 to 8 Ω 20 W on the +5 V line and 10 to 15 Ω 20 W on the 9 V AC. Both lamps and resistors get hot.[2]
  • For a repairable supply: fuses, regulator and capacitors as described below, a soldering iron and desoldering tools.
  • Optionally, a Commodore 64 Power-Supply Protector (C64 Saver).
Power connector pins[1][2][11]
Pin Supply output C64 board (CN7)
1 Not used Ground
2 Ground Ground
3 Not used Ground
4 Not used Not connected (long boards); +5 V (E board)
5 +5 V DC +5 V DC
6 9 V AC 9 V AC
7 9 V AC 9 V AC

Carlsen tests the supply between pin 5 (+5 V) and pin 2 (ground), and pins 6 and 7 (9 V AC).[2] On the E board, pin 4 is wired to +5 V alongside pin 5.[6]

C64 power input and regulator circuit (Service Manual C64-C64C p. 4)
Commodore's power input drawing for the 326298 board (service manual p. 4): CN7, the power switch SW1, F1 and the board's own 7805 and 7812 regulators, which run from the 9 V AC.

Procedure

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Identify the supply

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  1. Read the label for the mains input and the output ratings. Carlsen gives the C64 supply's ratings as 1.5 A on the +5 V output and 1 A on the 9 V AC.[2]
  2. Check whether the case is held by screws. Retroleum, a UK parts seller, describes an early C64 supply with four screws underneath that can be opened, and a common grey "wedge" supply with a mains fuse holder at the rear.[12] P. David Buchan documents the repairable US supply, part 902503-02.[11]
  3. A supply with no screws and a solid, heavy case is potted in epoxy. Carlsen and the C64-Wiki both class these as unrepairable.[4][5]

Test the supply on a dummy load

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  1. Plug the supply's DIN plug into the dummy load, not the computer, and plug it into the mains.[2]
  2. Run it for about half an hour, or until the case is warm, measuring the +5 V between pins 5 and 2 with the meter.[2]
  3. Watch for a reading that creeps upwards as the supply warms. A leaky regulator lets the voltage rise slowly; Carlsen has seen supplies that always worked cold and failed when warm.[2]
  4. Measure the 9 V AC between pins 6 and 7. It is either present or absent; it does not drift.[2]
  5. Watch the lamps. Both should be steady. A lamp that flickers, goes out or brightens means a failing supply; the +5 V lamp gets very bright if the regulator shorts, because the output then rises to about 11 V.[2]
Reading the +5 V output
Reading Carlsen (loaded)[2] C64-Wiki[5]
4.95-5.10 V "Almost exactly 5 volts" is correct Normal
Below 4.9 V Usually failing capacitors
4.7 V or less Blank screen and crashes on the computer
5.10-5.20 V Minor concern; measure more often
Above about 5.2 V The regulator is failing 5.2-5.5 V: failing; replace or repair
5.5-6.0 V The computer is in danger
Above 6 V The level at which most supply-killed C64s died, usually RAM first

Carlsen warns that a failing supply may still read 5 V when it is not loaded.[2] The C64-Wiki gives about 8.1 V AC as the lowest 9 V AC on which the C64 works, and notes that a transformer reads higher unloaded than loaded.[5]

Repair a supply that opens

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  1. Unplug the supply from the mains and remove the screws.[12]
  2. Check the fuses. Retroleum lists two 20 mm fuses inside the early four-screw supply, 1.6 A on the 9 V AC output and 2.5 A on the +5 V output, and a 160 mA 250 V mains fuse in the rear holder of the wedge supply.[12] Buchan's schematic of the US 902503-02 shows a 2 A fuse in the 9 V AC output and a 5 A fuse in the +5 V circuit.[11] Fit the rating marked on the fuse or the supply.
  3. On the 902503-02, the regulator is a 3052P, a +5 V 2 A part; Buchan gives the NTE1934 as an equivalent. Its pin 3 returns to ground through a 30 Ω 2 % ½ W resistor (30.1 Ω on his schematic), which he replaced with a 3 W 30 Ω part. The rectifiers are 1N5400 diodes, for which he used 1N5408s.[11]
  4. Carlsen attributes a low output and ripple to dried-out filter capacitors, and an output that rises to the regulator.[4][2] Buchan's schematic of the 902503-02 shows 4700 µF 16 V on the regulator input and 47 µF 10 V on its output.[11] Read the values on your own supply's board before ordering parts.
  5. Reassemble and repeat the dummy-load test for at least half an hour before connecting the computer.[2]

Replace the supply

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Carlsen's advice is that any failure of a brick means replacing it, preferably with something better.[4] The C64-Wiki's notes for choosing or building one:[5]

  • The original supplies give 1.5-1.7 A at +5 V DC and 1.0-1.11 A at 9 V AC. Most replacements match that, or are uprated to about 2-5 A at +5 V and 1.5-2 A at 9 V AC, which helps with modern cartridges and with REUs.
  • Galvanic separation between the +5 V DC and the 9 V AC is recommended.
  • A 9 V transformer rated about 10-15 VA suits the 9 V AC; a much heavier one runs the C64 hotter.

Carlsen describes Commodore's later REU supply as a switching type rated 2.5 A at +5 V and 1 A at 9 V AC with a plug that fits the C64, and notes that a C128 supply can be given a C64 plug.[2] For a home-built supply he would use a commercial +5 V switching module and a separate 9 V AC transformer.[4]

Fit a Commodore 64 Power-Supply Protector (C64 Saver) between any supply and the computer if you keep using an original brick.[4]

Testing

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  1. On the dummy load: +5 V steady near 5.0 V for half an hour, 9 V AC present, lamps steady.[2]
  2. Connected to the C64: the red LED lights and the machine starts.[2] With the machine running, 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.[13][1]
  3. Keep the supply where cool air reaches it. Carlsen runs his on the floor, with nothing on or around it to block the airflow.[4]
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References

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  1. ↑ 1.0 1.1 1.2 1.3 Commodore Business Machines, Service Manual C64/C64C, March 1992, PN-314001-03, hosted at Commodore 64 / 64C Service Manual. Circuit Theory, "The Power Supply", p. 4, and the power input drawing on the same page: CN7 pins 1-3 ground, 4 not connected, 5 +5 V, 6 and 7 9 V AC; user port pins 10 and 11 carry the 9 V AC.
  2. ↑ 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 2.10 2.11 2.12 2.13 2.14 2.15 2.16 2.17 2.18 2.19 2.20 Ray Carlsen, "A tester for the C64 power supply", updated 7 July 2005. Repair notes: the dummy-load tester, readings, the connector pinout, and additional notes on the black brick.
  3. ↑ 3.0 3.1 Ray Carlsen, "RAM (Random Access Memory) problems in the C64/128 computer", updated 9 June 2011. Repair notes: several failed RAM chips point to a failing supply; the black or white bricks are potted with epoxy.
  4. ↑ 4.0 4.1 4.2 4.3 4.4 4.5 4.6 4.7 Ray Carlsen, "All about the Commodore power supply", updated 5 February 2014. Repair notes: design, failure modes, potting, screwed later packs, thermal fuse, REU supply, replacement advice.
  5. ↑ 5.0 5.1 5.2 5.3 5.4 C64-Wiki, "Power Supply", sections "Failure", "Repair" and "Designing a Home Built Power Supply". Retrieved 2026-10-25.
  6. ↑ 6.0 6.1 Service Manual C64/C64C (1992), E schematic 252312, p. 42: CN7 pins 4 and 5 +5 V on the 250469 board.
  7. ↑ Ray Carlsen, "C64C (true C short board) - chips vs common symptoms", updated 6 January 2020, entry "Power pack". Repair notes.
  8. ↑ Service Manual C64/C64C (1992), "64 Troubleshooting Guide", pp. 14-16.
  9. ↑ Commodore Business Machines, Diagnostic Manual, C-64 Dead Test, PN-314139-02, January 1988, p. 2-2, hosted as File:C64 Dead Test Diagnostic Manual (314139-02, 1988 Jan).pdf.
  10. ↑ Ray Carlsen, "C64 blank screen", updated 27 November 2021. Repair notes: loss of the 9 V AC from a bad supply or a blown fuse in the computer.
  11. ↑ 11.0 11.1 11.2 11.3 11.4 11.5 P. David Buchan, "Commodore 64 External Power Supply, Part No. 902503-02", with his redrawn schematic and repair notes. Retrieved 2026-10-25.
  12. ↑ 12.0 12.1 12.2 Retroleum, "Misc. components for Commodore 64", product notes on fuses for the early four-screw C64 supply and the wedge supply. Parts seller. Retrieved 2026-10-25.
  13. ↑ C64-Wiki, "User Port", pin assignment table. Retrieved 2026-10-01.
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