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Commodore 128 Capacitor Replacement Guide

From RetroTechCollection
A Commodore 128 main board with the shields removed

The tables below list the electrolytic capacitors on the Commodore 128 main board, PCB assembly 310379, as printed in Commodore's Service Manual C-128/C128D Computer (PN-314001-08, November 1987).[1] Both lists open their capacitor section with "All caps are 25v, +80%, -20% unless noted otherwise"; every electrolytic below has its rating printed. Ceramic and Mylar parts are not included.

The C128 board has few electrolytics. Nightfall Crew found two electrolytics on one board, a 220 µF 50 V and a 330 µF 50 V, with their leads gone,[2] and Adam's Vintage Computer Restorations replaced C91 and C92, which set the reset pulse, during a board repair.[3] The external power supply has its own capacitors; see below.

Identify the board

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The flat C128 uses PCB assembly 310379. Commodore's manual has separate parts lists for rev. 6 and rev. 7.[1] The first C128D used the same main board as the C128;[4] the later C128DCR board, assembly 250477, is in the same manual without capacitor values.[1]

Rev. 6

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PCB assembly 310379 rev. 6, electrolytics (Commodore)[1]
Ref Capacitance Rating Notes
C61, C62, C63 10 µF 16 V Aluminium
C69 10 µF 16 V Aluminium
C80 2.2 µF 25 V
C85, C88 10 µF 16 V Aluminium
C91 1 µF 16 V Reset timing
C92 10 µF 16 V Aluminium; reset timing
C101 10 µF 35 V
C104 220 µF 50 V
C105 330 µF 50 V ±20 %
C106 1000 µF 25 V
C107 100 µF 16 V On the +5 V supply; Commodore's +5 V test point
C111 10 µF 16 V Aluminium; on the +12 V supply

Rev. 7

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PCB assembly 310379 rev. 7, electrolytics (Commodore)[1]
Ref Capacitance Rating Notes
C61, C62 10 µF 16 V Aluminium
C63 100 µF 6.3 V Aluminium; 10 µF on rev. 6
C80 2.2 µF 25 V
C85, C88 10 µF 16 V Aluminium
C91 1 µF 50 V 16 V on rev. 6
C92 10 µF 16 V Aluminium
C101 10 µF 35 V ±20 %
C104 220 µF 50 V
C105 330 µF 50 V ±20 %
C106 1000 µF 25 V
C107 100 µF 16 V
C111 10 µF 16 V Aluminium

C69 is not in the rev. 7 list.[1] Console5's kit list agrees with these values and rates C91 at 50 V for both revisions.[5]

The functions in the notes column come from Commodore's C128 Diagnostic Instruction and Troubleshooting Manual: it measures +5 V on the + leg of C107 and +12 V on the + leg of C111, and names C91 and C92 in the reset circuit.[6]

Power supply

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Commodore's service manual prints two power supply schematics "for reference only": a Mitsumi supply, PN-252449-01, and a Dee-Van supply, PN-252449-03. Its C128D parts list names the same two part numbers as the power supply assembly and its substitute.[1] One restorer found a Mitsumi 252449-01 board inside an external C128 supply labelled 310416-05.[7]

Electrolytics on Commodore's supply schematics[1]
Supply Capacitors
Mitsumi 252449-01 C4 2200 µF 35 V (after the bridge rectifier); C9 4700 µF 16 V (on the +5 V output)
Dee-Van 252449-03 C1, C2 3300 µF 25 V; C4 1000 µF 10 V; C5 220 µF 25 V; C6 1 µF 16 V; C9 10 µF 25 V

World of Jani has drawn the schematic of the PAL supplies made by Ismet (310416-01 to -03), which also have a 4700 µF output capacitor at C9, and reports that when C9 fails and the crowbar protection then fails too, the 5 V output can rise to between 12 and 15 V.[8] Replace that capacitor in any old supply and test the output on a load before connecting the computer; see the maintenance guide.

Warning: these supplies contain mains wiring and a transformer primary. Unplug the supply and let the filter capacitors discharge before working inside.

Procedure

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  1. Unplug the computer, remove the case screws and lift off the top. Remove the keyboard connector and the RF shield.
  2. Note whether the board is rev. 6 or rev. 7 and use the matching table. Photograph the board.
  3. Mark the polarity of each electrolytic before removing it.
  4. Desolder each part fully; do not pull leads through the plated holes.
  5. Fit parts of the same capacitance and at least the listed voltage. C91 can take a 50 V part on either revision.
  6. Clean the board and check the solder side. Power up and check the rails at C107 and C111.

Checks after the recap

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Commodore's diagnostic manual takes all readings to a ±10 % tolerance.[6]

Supply checks (Commodore)[6]
Point Expected
+ leg of C107, or pin 25 of the SID (U5) +5 V
+ leg of C111, or pin 28 of the SID +12 V
Anode of CR10 About +18 V unregulated
AC inputs of bridge CR13 9 V AC
Pin 40 of the 8502 (U6), RESET 0 V at power-up, rising to +5 V within a second
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References

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  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 Commodore Business Machines, Service Manual C-128/C128D Computer, November 1987, PN-314001-08, hosted at Commodore 128 128D Service Manual (314001-08). Part list, PCB assembly 310379 rev. 6, capacitors, printed p. 65; rev. 7, printed p. 72; power supply schematics, printed pp. 78-79.
  2. ↑ Nightfall Crew, Commodore 128 repair, 18 July 2017—link(accessed 2026-10-01)
  3. ↑ Adam's Vintage Computer Restorations, Commodore C128 board repairs, 18 May 2021—link(accessed 2026-10-01)
  4. ↑ Commodore 128D—link(accessed 2026-10-01) "The first version of c128d had the same motherboard as the c128."
  5. ↑ Console5 TechWiki, Commodore 128—link(accessed 2026-10-01) Capacitor lists for 310379 rev. 6 and rev. 7, the C128DCR and the power supplies (secondary source).
  6. ↑ 6.0 6.1 6.2 Commodore Business Machines, C128 Diagnostic Instruction and Troubleshooting Manual, Assembly #314060-01/02 (1986), sections 2.2 and 2.3, hosted at Commodore 128 Diagnostic Instruction and Troubleshooting Manual (314060).
  7. ↑ Muldrf's Hobbytronic, Commodore 128 power supply recap, 23 February 2019—link(accessed 2026-10-01)
  8. ↑ C128 PSU schematic (PAL) P/N 310416-01, -02, -03—link(accessed 2026-10-01)
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