Jump to content

Commodore 64 SID Replacement

From RetroTechCollection
Commodore 64 250407 board U18 SID 6581
U18, a MOS 6581 SID, on a 250407 Rev. B board, here soldered straight to the board. C10, one of its two filter capacitors, is above the 2114 colour RAM at U6.

The Commodore 64 long boards carry the 6581 SID sound chip at U18, Commodore part 906112-01.[1] It runs from +5 V on pin 25 and +12 V on pin 28 (VDD), supplied by the board's 7812 regulator VR1; its audio output is pin 27, its external audio input pin 26, and it reads the paddles and mouse through its POT inputs on pins 23 and 24.[2][3][4] Ray Carlsen notes that the 6581 runs hot in normal use and fails often because of it.[5] The Commodore 64C's E board uses the 8580, which needs a different supply and filter capacitors; see Commodore 64C SID Replacement.

Symptoms

[edit | edit source]
  • A normal screen, but no sound or garbled sound.[6]
  • A mouse or graphics-tablet pointer that sticks or jitters, or paddles that misbehave.[6][7]
  • A blank or garbage screen, if the SID has shorted.[6]
  • In Commodore's diagnostic cartridge, a sound test with missing voices, volume levels or noise bursts, or a control port test reporting both paddle inputs bad.[8][9]

Rule out the parts around the SID before replacing it. No +12 V at pin 28 points to VR1 and its feed (Commodore 64 Voltage Regulator Replacement). A paddle fault on one input only usually means the 4066 at U28 or the CIA at U1.[9] On some early boards with the 5-pin video connector, Commodore's cartridge reports the SID bad in its control port test every time because C48 and C93 on pins 23 and 24 were fitted at a value other than the specified 1800 pF; paddles still work and Commodore says the capacitors need not be changed.[9]

Safety

[edit | edit source]
  • Switch off before removing or fitting the chip.[6]
  • The chips are static sensitive.[10]

Tools and parts

[edit | edit source]
  • A replacement for U18. Carlsen's survey of the options:[11]
    • Another 6581. The C64-Wiki lists the 6581, 6581R3, 6581R4 and 6581R4AR as the NMOS parts that take 12 V on pin 28 with 470 pF filter capacitors.[7] Carlsen found one second-hand 6581 that garbled two voices in his test board yet worked in the owner's, and suspects timing differences between boards make some chips marginal.[5]
    • SwinSID and Nano SwinSID: the cheapest, but they lack the POT inputs, so a proportional mouse, paddles or a graphics tablet will not work; joysticks are unaffected.
    • SwinSID Ultimate: programmable for 6581 or 8580 sound from a BASIC program; Carlsen found its output a little quieter than an original SID.
    • ARMSID: selects 6581 or 8580 and PAL or NTSC automatically and supports the POT inputs; Carlsen found it incompatible with Koala Paint.
    • FPGASID: a dual-SID module.
  • The SIDKick pico is another drop-in replacement for the 6581 and 8580; its documentation asks for it to be oriented by the markings on its board and describes solder jumpers that route its audio through the SID socket's pin 27.[12]
  • An 8580 is not a drop-in part for a long board: it needs 9 V on pin 28 and 22 nF filter capacitors.[7][13]
  • A good-quality 28-pin socket if U18 is soldered in, and, for an original 6581, a heat sink.[14][5]

Procedure

[edit | edit source]

Open the case

[edit | edit source]
  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]

Confirm the fault

[edit | edit source]
  1. Run Carlsen's three-voice test program (below). A missing voice, or voices that sound muddy, point to the SID.[17]
  2. With the machine running, measure pin 28 of U18 against ground: about +12 V. Measure pin 25: +5 V.[2][3]
  3. If the screen is blank or garbled and the SID is socketed, switch off, remove it and try again. The C64 starts without a SID fitted; it then has no sound and no mouse or paddle input.[6][10]
  4. If a known-good SID is available and U18 is socketed, substitute it.[10]
 5 REM C64 SID 3-VOICE TEST PROGRAM
 10 FORL=54272TO54296:POKEL,0:NEXT:POKE54296,15:GOSUB60
 20 POKEW,17:POKEA,9:POKEHF,15:POKELF,35:POKES,128:GOSUB50:GOSUB70
 30 POKEW,17:POKEA,9:POKEHF,20:POKELF,40:POKES,128:GOSUB50:GOSUB80
 40 POKEW,17:POKEA,9:POKEHF,25:POKELF,50:POKES,128:GOSUB50:GOTO10
 50 FORX=1TO2000:NEXTX:RETURN
 60 W=54276:A=54277:HF=54273:LF=54272:S=54278:RETURN
 70 W=54283:A=54284:HF=54280:LF=54279:S=54285:RETURN
 80 W=54290:A=54291:HF=54287:LF=54286:S=54292:RETURN

Line 10 clears the SID and sets full volume; lines 20, 30 and 40 add voices 1, 2 and 3 in turn, each for a few seconds, then the program loops.[17]

Remove the SID

[edit | edit source]
  1. If U18 is socketed, lift it a little at a time from each end with an IC extractor or a small taped screwdriver.[15]
  2. If it is soldered in, remove the seven screws holding the board to the lower case, lift the board out, and desolder the chip intact. Carlsen desolders the SID whole unless he is certain it is bad, because it is an expensive custom part.[18][14]
  3. Heat each pin until the solder is molten on both sides, draw the solder out, and check that the pin moves freely. Free every pin before lifting the chip, and stop if one sticks.[14]
  4. Inspect both sides of the board for lifted pads and torn through-hole plating, repair any with a strand of wire soldered on both sides, then fit a 28-pin socket. Never solder a socket with a chip in it.[14]

Fit the replacement

[edit | edit source]
  1. Fit the new SID, or the replacement board, with its pin 1 notch the same way round as the original.[15][12]
  2. For a replacement board, set any audio-routing jumpers as its maker describes. On the SIDKick pico, the "L" solder jumper sends the sound out through pin 27 and the board's own audio circuit.[12]
  3. For an original 6581, consider a heat sink. Carlsen fits sinks to the SID, the PLA and the CPU on every C64 he repairs, and notes that Commodore fitted a metal shield with fingers that touch the hot chips on later boards.[5][19]

Reassemble

[edit | edit source]

Refit the board, the shield, the keyboard and LED plugs and the top case, then the three case screws.[18][15]

Testing

[edit | edit source]
  1. Run the three-voice program again; all three voices should be clear.[17]
  2. Run Commodore's diagnostic cartridge. Its sound test, the last it runs, plays three voices at three volume levels and three bursts of noise, and its control port test reads the POT inputs.[8][9][5]
  3. Carlsen's final test before returning a machine is the disk program Music Construction Set, which he finds sometimes reveals faults the other tests miss.[5]
  4. Test a mouse or paddles if the replacement is meant to support them.[11]

Board revisions

[edit | edit source]
  • 326298, 250407, 250425: U18 6581, 906112-01; filter capacitors C10 and C11, 470 pF on the 326298 and 250407 lists.[1]
  • 250466 (B-3): U18 "SID 6581 R3", 906112-01; C10 and C11 470 pF.[20]
  • 250469 (E): SID at U9, built with an 8580 or a 6581 and a matching zener and capacitors; see Commodore 64C SID Replacement.[21]
[edit | edit source]

References

[edit | edit source]
  1. ↑ 1.0 1.1 Commodore Business Machines, Service Manual C64/C64C, March 1992, PN-314001-03, hosted at Commodore 64 / 64C Service Manual. Parts lists for 326298 (pp. 19-20), 250407-04 (pp. 24-25) and 250425-01 (pp. 29-30): U18 6581 SID, 906112-01; C10, C11 470 pF ceramic on the 326298 and 250407 lists.
  2. ↑ 2.0 2.1 Service Manual C64/C64C (1992), pin assignments, p. 27: 6581 pin 25 VCC, pin 28 VDD, pin 27 audio out, pin 26 audio in, pins 23 and 24 POT inputs.
  3. ↑ 3.0 3.1 Service Manual C64/C64C (1992), Circuit Theory, "The Power Supply", p. 4: +12 V from VR1 for the VIC, the SID and the audio amplifiers.
  4. ↑ Service Manual C64/C64C (1992), Circuit Theory, "8 Pin Video and Audio Output Circuits", p. 10.
  5. ↑ 5.0 5.1 5.2 5.3 5.4 5.5 Ray Carlsen, "Evaluating the 6581 SID (sound) chip", updated 15 July 2018. Repair notes.
  6. ↑ 6.0 6.1 6.2 6.3 6.4 Ray Carlsen, "C64 chips and common symptoms", updated 7 January 2020, entry for U18. Repair notes.
  7. ↑ 7.0 7.1 7.2 C64-Wiki, "SID", sections "Chip variations" and "Troubleshooting". Retrieved 2026-10-25.
  8. ↑ 8.0 8.1 C64 Diagnostic Instruction and Troubleshooting Manual, Section 1, 5.16 "Sound Test", p. 1-7.
  9. ↑ 9.0 9.1 9.2 9.3 Commodore Business Machines, C64 Diagnostic Instruction and Troubleshooting Manual, Assembly #326070-01, hosted at Commodore 64 Diagnostic Instruction and Troubleshooting Manual (326070-01). Section 1, 5.11 "Control Port Test", p. 1-6.
  10. ↑ 10.0 10.1 10.2 Ray Carlsen, "C64 blank screen", updated 27 November 2021. Repair notes.
  11. ↑ 11.0 11.1 Ray Carlsen, "SID replacements", 2021. Repair notes on SwinSID, Nano SwinSID, SwinSID Ultimate, FPGASID and ARMSID.
  12. ↑ 12.0 12.1 12.2 frntc, "SIDKick pico", README: installation, orientation and audio routing jumpers. Retrieved 2026-10-25.
  13. ↑ Ray Carlsen, "8580 to 6581", 22 February 2021. Repair notes: the two SIDs' supply voltages and filter capacitors.
  14. ↑ 14.0 14.1 14.2 14.3 Ray Carlsen, "Replacing IC chips in Commodore equipment", updated 19 December 2012. Repair notes.
  15. ↑ 15.0 15.1 15.2 15.3 15.4 15.5 Creative Micro Designs, JiffyDOS C-64 Installation Guide, pp. 1-4: opening the C-64 and lifting and fitting a socketed chip. PDF.
  16. ↑ Ray Carlsen, "Commodore keyboards", updated 17 November 2015. Repair notes.
  17. ↑ 17.0 17.1 17.2 Ray Carlsen, "SID 3-voice test program", 13 March 2012.
  18. ↑ 18.0 18.1 Retroleum, "Commodore 64 fixings": seven screws hold the breadbin motherboard to the lower case. Parts seller. Retrieved 2026-10-25.
  19. ↑ Ray Carlsen, "Heat sinking ICs in the C-64", updated 10 May 2014. Repair notes.
  20. ↑ Service Manual C64/C64C (1992), parts list for 250466-01 (B-3), pp. 34-35: U18 "LSI SID 6581 R3", 906112-01; C10, C11 470 pF.
  21. ↑ Service Manual C64/C64C (1992), parts list for 250469-01 (E), p. 40, and schematic 252312, p. 42.
Retrieved from "https://retrotechwiki.com/index.php?title=Commodore_64_SID_Replacement&oldid=29852"