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Commodore 64C SID Replacement

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Commodore 64C 250469 board U9 SID 8580 and SID supply
Part of a 250469 Rev. B board: the SID at U9 (a CSG 8580R5 here), the filter capacitor positions C70, C80 and C81, the R8 position, and the SID's supply parts Q5, CR3, CR4, CR7, R100 and C66 at the right.

The 250469 (E) board used in most Commodore 64C machines carries its SID at U9, and Commodore built it with either an 8580 (318013-01) or a 6581 (906112-01).[1] The two chips are pin-compatible but need different supplies on pin 28 and different filter capacitors, so the board around U9 is built to match the chip fitted.[2][3] Fitting the other type means changing those parts to match. The 250466 (B-3) board, the other 64C board, keeps the long boards' 6581 arrangement with 12 V from VR1; for it, follow Commodore 64 SID Replacement.[4]

Symptoms

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  • A normal screen but no sound or garbled sound; a mouse or graphics-tablet pointer that sticks or jitters.[3]
  • A blank or garbage screen, if the SID has shorted. The computer starts with the SID removed, which is a quick test.[3]

On the E board a blown fuse F1, or a supply that has lost its 9 V AC, also silences the SID: the computer still works, but without sound, cassette motor or 9 V AC at the user port.[3][5] The C64-Wiki likewise points to a missing supply when there is no audio at the A/V socket.[6] Check F1 and the supply before the chip; see Commodore 64 Power Supply Testing and Repair and the Commodore 64C Troubleshooting Guide.

Safety

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  • Switch off before removing or fitting the chip.[3]
  • The chips are static sensitive.[7]

Tools and parts

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  • A replacement SID: an 8580 or 6581, or a modern replacement board such as the SIDKick pico, which emulates either type.[8][9]
  • To change the board over from one SID type to the other: a zener diode for CR7, two capacitors for C70 and C71, two for C80 and C81, and for a 6581 the resistor R8.[10][2]
  • A 28-pin socket if U9 is soldered in; Commodore's E-board parts list includes 16-, 28- and 40-pin sockets.[1]
  • A multimeter, a soldering iron, a solder sucker and braid.[11]
Parts that depend on the SID, schematic 252312[10][2]
Part For a 6581 For an 8580
CR7 (sets the SID's VDD) 12.6 V zener 9.6 V zener
C70, C71 (filter capacitors, pins 1-2 and 3-4) 470 pF 22000 pF
C80, C81 (POT X and POT Y) 1800 pF 2200 pF
R8 (audio output, pin 27) Fitted Removed

Commodore's E-board parts list gives four zener types for CR7 (RD10ESB2, RD13ESB2, HZ11A1, HZ12B2) without saying which goes with which SID; the schematic notes give the voltages above.[1][10] The same parts list gives C80 and C81 at both 1800 pF (251069-11) and 2200 pF (251069-12), but C70 and C71 only at 22000 pF (251074-09); read the parts on your own board.[1] Carlsen gives R8 as 1 kΩ.[12]

Procedure

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Open the case

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Follow CMD's 64C procedure: remove the three screws underneath, lift the front of the upper case, unplug the keyboard cable, unscrew the keyboard from the lower case and slide it out from the front tabs, unplug the LED cable, and remove any shield.[13]

Identify the build

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Commodore 64C E board SID supply Q5 CR7 (schematic 252312)
The E board's power input and SID supply on schematic 252312: F1, the bridge CR2, and the circuit C65, CR3, CR4, C66, R100 and Q5, with zener CR7 setting the "+9/+12" output.
  1. Read the marking on U9: 8580 or 6581.[1]
  2. With the machine running, measure U9 pin 28 to ground. Carlsen gives about +9 V for an 8580 build and +12 V for a 6581 build.[12]
  3. Read the markings on C70, C71, C80 and C81 and check whether R8 is fitted, and compare them with the table.[10]
  4. If the SID is silent, also check pin 25 (+5 V) and the supply circuit: the +9/+12 supply comes from transistor Q5, whose base is held by CR7, fed from C66, CR3, CR4 and C65 off the 9 V AC after F1 and the bridge CR2.[2][14]

Replace the SID with the same type

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  1. If U9 is socketed, lift it a little at a time from each end and fit the replacement the same way round.[13]
  2. If it is soldered in, remove the board, desolder the chip intact, inspect the holes and fit a 28-pin socket. Never solder a socket with a chip in it.[11][15]

Change the board for the other SID type

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Commodore 64C E board SID circuit (schematic 252312)
U9 on schematic 252312, with the filter capacitors C70 and C71, the POT capacitors C80 and C81, R8 on the audio output and Commodore's note of the values for each SID.

Carlsen's procedure for fitting a 6581 to an 8580 board:[12]

  1. Replace zener CR7 with one for the higher supply, so that pin 28 receives about +12 V. The schematic note calls for 12.6 V.[10]
  2. Fit R8 (1 kΩ). Commodore's note says R8 is removed for the 8580.[12][10]
  3. Change C70 and C71 to 470 pF.
  4. Commodore's notes also change C80 and C81 to 1800 pF for the 6581.[2]

For an 8580 in a board built for a 6581, reverse each change: a 9.6 V zener at CR7, 22000 pF at C70 and C71, 2200 pF at C80 and C81, and R8 removed.[10][2] Carlsen adds that a 330 kΩ resistor from pin 26 (audio input) to ground lets an 8580 play the digitised samples that rely on a quirk of the 6581.[12]

Commodore 64C E board schematic 252312 notes
Notes 3 to 8 on schematic 252312: R8 removed for the 8580, CR7 12.6 V for the 6581 and 9.6 V for the 8580, and the French build's R2 and C63.

Reassemble

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Refit the board, the keyboard under its front tabs with its retaining screws, the cables, the shield and the upper case, then the three case screws.[13]

Testing

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  1. Measure pin 28 again with the SID fitted: about +12 V for a 6581, about +9 V for an 8580.[12][6]
  2. Run Carlsen's three-voice BASIC test, given on Commodore 64 SID Replacement; all three voices should play cleanly.[16]
  3. Test a mouse or paddles; the POT inputs go through U18, a 4066.[1][3]
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References

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  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 Commodore Business Machines, Service Manual C64/C64C, March 1992, PN-314001-03, hosted at Commodore 64 / 64C Service Manual. "Parts List, PCB Assembly #250469-01 C64-E", p. 40: U9 8580 (318013-01) or 6581 (906112-01); U18 and U21 4066; CR7 zener types; C70, C71 and C80, C81; IC sockets.
  2. ↑ 2.0 2.1 2.2 2.3 2.4 2.5 Service Manual C64/C64C (1992), E schematic 252312, p. 42: U9 with C70, C71, C80, C81 and R8, the note "C70,71: 22000pF for 8580, 470pF for 6581; C80,81: 2200pF for 8580, 1800pF for 6581", and the SID supply Q5, CR7, R100, C66, CR3, CR4 and C65.
  3. ↑ 3.0 3.1 3.2 3.3 3.4 3.5 Ray Carlsen, "C64C (true C short board) - chips vs common symptoms", updated 6 January 2020, entries for U9 and F1 and the closing notes. Repair notes.
  4. ↑ Service Manual C64/C64C (1992), parts list for 250466-01 (B-3), pp. 34-35: U18 SID 6581 R3; VR1 MC7812CT.
  5. ↑ Ray Carlsen, "A tester for the C64 power supply", updated 7 July 2005. Repair notes: loss of the 9 V AC on the C64C.
  6. ↑ 6.0 6.1 C64-Wiki, "SID", sections "Chip variations" and "Troubleshooting". Retrieved 2026-10-25.
  7. ↑ Ray Carlsen, "C64 blank screen", updated 27 November 2021. Repair notes.
  8. ↑ Ray Carlsen, "SID replacements", 2021. Repair notes.
  9. ↑ frntc, "SIDKick pico", README. Retrieved 2026-10-25.
  10. ↑ 10.0 10.1 10.2 10.3 10.4 10.5 10.6 Service Manual C64/C64C (1992), E schematic 252312, p. 42, notes 6 and 8: "6581 SID version: CR7 = 12.6V should be used. 8580 SID version: CR7 = 9.6V should be used"; "R8 resistor should be remove for 8580 SID".
  11. ↑ 11.0 11.1 Ray Carlsen, "Replacing IC chips in Commodore equipment", updated 19 December 2012. Repair notes.
  12. ↑ 12.0 12.1 12.2 12.3 12.4 12.5 Ray Carlsen, "8580 to 6581", 22 February 2021. Repair notes: fitting a 6581 to a 250469 board (CR7, R8, C70 and C71) and the 330 kΩ resistor for an 8580.
  13. ↑ 13.0 13.1 13.2 Creative Micro Designs, JiffyDOS C-64 Installation Guide, pp. 1-4: 64C disassembly and lifting a socketed chip. PDF.
  14. ↑ Service Manual C64/C64C (1992), pin assignments, p. 27: SID pin 25 VCC, pin 28 VDD.
  15. ↑ Retroleum, "Commodore 64 fixings": C64C motherboard screws. Parts seller. Retrieved 2026-10-25.
  16. ↑ Ray Carlsen, "SID 3-voice test program", 13 March 2012.
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