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Commodore 64 Voltage Regulator Replacement

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Commodore 64 250407 board power section VR1 VR2 CR4 F1
The power section of a 250407 Rev. B board: fuse F1 at the top, bridge rectifier CR4 below it, C88 and C90 (470 µF 50 V) and C19 (2200 µF 16 V), diodes CR5 and CR6, VR1 (7812) on the left and VR2 (7805) bolted to its heat sink.

The Commodore 64 long boards and the 250466 (B-3) board make two supplies of their own from the 9 V AC that the external power supply provides. VR2, an MC7805CT, gives "+5 V CAN" for the video and clock circuits; VR1, an MC7812CT, gives +12 V for the VIC-II, the SID and the audio stages. Both are fed from bridge rectifier CR4, with diodes CR5 and CR6 raising VR1's input.[1][2] The logic's +5 V comes straight from the external supply and does not pass through either regulator.[1] When VR1, VR2 or their feed fails, the VIC-II loses its supplies and the machine shows a blank screen or no picture at all.[3][4] The 250469 (E) board has neither regulator.[5]

Symptoms

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  • A blank screen at power-up. Commodore's troubleshooting table lists VR2, CR4 and VR1 after the external supply and the chips.[3]
  • A white band scrolling down the screen (60 Hz hum): the external supply or VR2.[3]
  • A black band scrolling through the picture as the machine warms up: the external supply, C90, C88, CR4 or VR2.[3]
  • No video signal at all. On one 250407 board Retro Repairs and Refurbs measured about 1.5 V on both the VIC-II's 5 V and 12 V supplies.[4]

The external supply's 9 V AC feeds both regulators, so a supply with no 9 V AC, or a blown F1 in the computer, gives the same blank screen with the red LED lit.[6][7] Test the external supply first: Commodore 64 Power Supply Testing and Repair. Failed chips also cause blank screens; see the Commodore 64 Troubleshooting Guide.

Safety

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Tools and parts

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  • A multimeter for the DC and AC readings. Commodore specifies an oscilloscope for all its readings, and one is needed to see AC riding on DC at CR6.[8][9]
  • Replacement regulators: a 7805 for VR2 and a 7812 for VR1, in TO-220 packages. Commodore lists MC7805CT (901527-02) and MC7812CT (901527-01).[2] Retroleum, a UK parts seller, stocks both for the breadbin C64.[10]
  • For the feed: a bridge rectifier for CR4 (Commodore lists S2VB10, part 251026-01, on the 250407 and 250425, and a Varo VM08 on the 326298) and 1N4001 diodes for CR5 and CR6 (900750-01).[11][12] Retroleum notes that a faulty 1N4001 leaves the 12 V rail bad.[10]
  • Thermal grease for VR2's heat sink. Retroleum sells it for renewing the dried-up grease found on some C64s.[13]
  • A soldering iron, solder sucker and braid, and a small spanner and screwdriver for the heat-sink bolt.[14]
The regulators and their feed (long boards and B-3)[1][11][2]
Part Type Input Output Feeds
VR2 MC7805CT CR4 rectified 9 V AC, reservoir C19 (2200 µF 16 V) +5 V CAN, filtered by C102 and C103 Vvid (video) and Vc (clock circuits)
VR1 MC7812CT 9 V AC added to the rectified DC through CR6, rectified by CR5, filtered by C88 and C89 +12 V, filtered by C57 and C59 VIC-II, SID, audio amplifiers
CR4 Bridge rectifier 9 V AC through F1 and SW1 Unregulated +9 V VR2, VR1 via CR5/CR6, cassette motor

Procedure

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C64 power input and regulator circuit (Service Manual C64-C64C p. 4)
Commodore's drawing of the power input and the two regulators (service manual p. 4).

Open the case

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  1. Switch off, unplug the supply and remove all cables and cartridges.[15]
  2. Remove the three screws along the front edge of the underside, lift the top case, and unplug the keyboard and power LED connectors.[15][16]
  3. Remove any cardboard-and-foil or metal shield over the board.[15]

Find the failed part

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Commodore's checks, made with the power on and readings within 10 %:[8][9]

  1. Measure +5 V CAN at pin 40 of the VIC-II (U19) and +12 V at its pin 13. Correct readings here mean the regulators are working.[8]
  2. If +5 V CAN is wrong, measure VR2's output. If it reads +5 V, the fault is an open trace between VR2 and U19.
  3. Measure VR2's input. +9 to +12 V DC with a wrong output means VR2 has failed.
  4. If VR2's input is wrong, measure the AC at CR4's pins 4 and 6. 9 to 12 V AC on both means CR4 has failed.
  5. If CR4's pin 4 (or both pins) has no AC, measure CN7 pin 6: 9 to 12 V AC there points to L4 or the power switch SW1, none to the external supply. If CR4's pin 6 has no AC, measure CN7 pin 7: AC there points to L4 or the fuse F1, none to the external supply.
  6. If +12 V is wrong, measure VR1's output. If it reads +12 V, the fault is an open trace.
  7. Measure VR1's input. +15 to +18 V DC with a wrong output means VR1 has failed.
  8. If VR1's input is wrong, look at the anode of CR6 with an oscilloscope. 9 to 12 V AC riding on +9 to +12 V DC means CR5 has failed; AC only means CR6; DC only means the +5 V CAN supply is at fault, so go back to step 2.

Commodore's manual numbers each regulator's input pin 1 and its output pin 2.[9] The 250407 board prints the same numbering beside VR1 and VR2, with pin 3, the ground, in the centre.[17] The TI datasheet for the 7805 and 7812 numbers the same TO-220 legs 1 (input), 2 (ground) and 3 (output), so go by function, not by number, when reading from a datasheet.[18]

Retro Repairs and Refurbs found a fault that these checks lead to indirectly: on a 250407 with about 1.5 V on both VIC-II supplies, CR4 measured about 11 V AC in and about 18 V DC out, but VR2 had no input because CR4's DC ground pin was floating. An earlier repair had damaged the via under the bridge; a short wire from that pin to a ground point under the power switch cured it.[4]

Replace a regulator

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  1. Remove the seven screws holding the board to the lower case and lift the board out.[19]
  2. For VR2, undo the nut and bolt that hold it to its heat sink.[17][20]
  3. Desolder the three legs, clearing each hole with the solder sucker, and lift the regulator out.[14]
  4. Clean the old thermal grease from the heat sink. Retroleum and the Linux Jedi blog both found it dried out on old boards.[13][20]
  5. Fit the new regulator the same way round as the old one, with its metal tab against the heat sink for VR2. Apply fresh thermal grease, refit the bolt and nut, then solder the legs.[13][17]
  6. Replace CR4, CR5 or CR6 the same way if the checks pointed to them, matching the polarity marks on the board.[17]

Reassemble

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Refit the board with its seven screws, then the shield, the keyboard and LED plugs, the top case and its three screws.[19][15]

Testing

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Expected readings
Point Expected
VR2 input +9 to +12 V DC[9]
VR2 output (+5 V CAN), VIC-II pin 40 +5 V; a 7805 holds 4.8-5.2 V[8][18]
VR1 input +15 to +18 V DC[9]
VR1 output (+12 V), VIC-II pin 13 +12 V; a 7812 holds 11.5-12.5 V[8][18]
CR4 AC pins 4 and 6 9 to 12 V AC[9]

After a few minutes of running, check the picture for the scrolling bands Commodore traces to the supply circuit.[3]

Board revisions

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  • 326298, 250407, 250425: VR1 and VR2 as above. The 326298's bridge is a Varo VM08; the 250407 and 250425 lists give S2VB10.[12][11][21]
  • 250466 (B-3): keeps VR1 (MC7812CT) and VR2 (MC7805CT).[2]
  • 250469 (E): no 7805 or 7812. The SID's supply comes from transistor Q5 and zener CR7 instead; see Commodore 64C Troubleshooting Guide.[5]
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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", "12VDC Generation", "+5VDC CAN Generation" and "9VDC Unregulated Generation", with the power input drawing, p. 4.
  2. ↑ 2.0 2.1 2.2 2.3 Service Manual C64/C64C (1992), parts list for assembly 250466-01 (B-3), pp. 34-35: VR1 MC7812CT, 901527-01; VR2 MC7805CT, 901527-02.
  3. ↑ 3.0 3.1 3.2 3.3 3.4 Service Manual C64/C64C (1992), "64 Troubleshooting Guide", pp. 14-16.
  4. ↑ 4.0 4.1 4.2 themightymadman, "Commodore 64 (ASSY 250407) Board Repairs", Retro Repairs and Refurbs, 23 July 2021, board 12.
  5. ↑ 5.0 5.1 Service Manual C64/C64C (1992), parts list for 250469-01 (E), p. 40, and schematic 252312, p. 42.
  6. ↑ Ray Carlsen, "A tester for the C64 power supply", updated 7 July 2005. Repair notes: loss of the 9 V AC gives a blank screen on early C64s with the red LED on.
  7. ↑ Ray Carlsen, "C64 blank screen", updated 27 November 2021. Repair notes.
  8. ↑ 8.0 8.1 8.2 8.3 8.4 Commodore Business Machines, C64 Diagnostic Instruction and Troubleshooting Manual, Assembly #326070-01, hosted at Commodore 64 Diagnostic Instruction and Troubleshooting Manual (326070-01). Section 2, "Basic Preliminary Checks", p. 2-1: +5 V CAN at U19 pin 40, +12 V at U19 pin 13, readings within 10 %.
  9. ↑ 9.0 9.1 9.2 9.3 9.4 9.5 C64 Diagnostic Instruction and Troubleshooting Manual, Section 2, "1. System Power Supply", pp. 2-3 and 2-4.
  10. ↑ 10.0 10.1 Retroleum, "Misc. components for Commodore 64": 7805 and 7812 regulators for the breadbin C64; 1N4001 diode. Parts seller. Retrieved 2026-10-25.
  11. ↑ 11.0 11.1 11.2 Service Manual C64/C64C (1992), parts list for assembly 250407-04, pp. 24-25: CR4 bridge S2VB10, 251026-01; CR5 and CR6 1N4001, 900750-01; VR1, VR2 voltage regulators; F1 1.5 A.
  12. ↑ 12.0 12.1 Service Manual C64/C64C (1992), parts list for assembly 326298, pp. 19-20: CR4 "Bridge, Varo VM08".
  13. ↑ 13.0 13.1 13.2 Retroleum, "Heatsinks, modifications": thermal grease for renewing the dried-up grease on some C64s. Parts seller. Retrieved 2026-10-25.
  14. ↑ 14.0 14.1 Ray Carlsen, "Replacing IC chips in Commodore equipment", updated 19 December 2012. Repair notes.
  15. ↑ 15.0 15.1 15.2 15.3 Creative Micro Designs, JiffyDOS C-64 Installation Guide, pp. 1-2: opening the C-64. PDF.
  16. ↑ Ray Carlsen, "Commodore keyboards", updated 17 November 2015. Repair notes.
  17. ↑ 17.0 17.1 17.2 17.3 Photograph of a 250407 Rev. B board by Sven Petersen, Wikimedia Commons, File:0430 - C64 Mainboard ASSY250407 RevB.jpg, read at full size: VR2 bolted to its heat sink, the pin numbers printed beside VR1 and VR2, and the polarity marks at CR4.
  18. ↑ 18.0 18.1 18.2 Texas Instruments, LM340, LM340A and LM7805 Family Wide VIN 1.5-A Fixed Voltage Regulators, SNOSBT0L, pin functions and electrical characteristics tables for the 7805 and 7812.
  19. ↑ 19.0 19.1 Retroleum, "Commodore 64 fixings": seven screws hold the breadbin motherboard to the lower case. Parts seller. Retrieved 2026-10-25.
  20. ↑ 20.0 20.1 LinuxJedi, "The Australian Commodore 64", 19 August 2022: replacing the 5 V regulator on a 250407 board, with the aged thermal paste and the heat-sink screw.
  21. ↑ Service Manual C64/C64C (1992), parts list for assembly 250425-01, pp. 29-30.
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