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Commodore PET 4000 Series Capacitor Guide

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The universal dynamic PET logic board

Electrolytic capacitors in the Commodore PET 4000 Series: the logic board, the supply chassis, the mains filter and the display board. Commodore's own drawings give the values for the logic board, the supply chassis and two of the display boards.

Warning: the supply chassis holds a 23,000 µF filter capacitor and the display board runs the CRT. Unplug the machine, discharge the CRT (see CRT Discharge Procedure) and bleed the large filter capacitor through a resistor before working inside.

Logic board

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The 4000 series uses the universal dynamic PET board, drawing 8032090. Commodore's parts list gives these electrolytics and tantalums; the rest of the C1-C97 range is ceramic and mica.[1]

Universal dynamic PET board, drawing 8032090[1]
Ref Value Type Commodore part no.
C1, C16, C17, C23, C24 10 µF 25 V Low-leakage electrolytic 900109-03
C2 470 µF 16 V Electrolytic 900100-44
C5 4700 µF 25 V Electrolytic 900100-45
C7, C11 47 µF 16 V Electrolytic 900101-37
C6, C51 1 µF 35 V Tantalum 900402-13

Fit low-leakage parts at C1, C16, C17, C23 and C24, which Commodore marks as low-leakage.[1] Tantalum capacitors on PET logic boards fail short and pull a rail down: Hugo Holden found two shorted on one PET board, and a VCFed member tracing a short on a 4032's 12 V rail was advised to pull the RAM chips or lift tantalum leads to find it.[2][3] Tynemouth Software found several electrolytics on an 8032 board of the same family out of tolerance with high ESR and replaced all but the large one.[4]

Supply chassis

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The supply chassis, drawing 8032051, has one capacitor: C1, the 23,000 µF main filter, Commodore part 900102-01.[1] Console5 lists it as a screw-terminal 23,000 µF 15 V part,[5] and the label on one original reads "TYPE CGS 23000MFD 15VDC".[6]

Some originals are still good. Tynemouth measured one in an 8032 at 29,830 µF against a marked value of 22,000 µF with a tolerance of 0 to +50 %, and left it in.[7] Holden, on the other hand, found the main filter defective in his PET.[2] If it has to be replaced, Console5 sells a 22,000 µF 25 V screw-terminal capacitor as a replacement for the original 23,000 µF part.[8] A capacitor that has stood unused for decades may need re-forming before full voltage is applied.[6]

Mains filter

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PETs use a separate mains filter in a metal can. Tynemouth's example is rated "0.1uF + 2x2500pF XY + 2x100uH 250V ~ 2A".[9] Its suppression capacitors fail: Adam's Vintage Computer Restorations notes that they sit before the power switch, so they can fail whenever the machine is plugged in, even when it is switched off.[10] Replace the filter, or its capacitors with X and Y rated parts, on any PET that has not had it done.

Display boards

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4000-series machines were built with 9-inch and 12-inch displays, and the board differs.[1] Commodore's lists for two boards are hosted at Commodore PET 4000 Series Display Board Documentation.

12-inch display, PCB assembly 8032033

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12-inch display board 8032033 Rev. M, electrolytics (Commodore)[11]
Ref Value Commodore part no. Notes
C2 4.7 µF 250 V 900100-47 3.3 µF 200 V (900100-02) listed as a substitute
C7, C17 470 µF 25 V 900100-49
C8 10 µF 63 V 900100-08
C14, C21 1.0 µF 25 V 900100-32
C15 10 µF 25 V, non-polarised 900103-01
C25 1000 µF 25 V 900100-05
C27 4700 µF 35 V 900111-03 900100-50 listed as a substitute

The same parts list gives the board's +18 V regulator as VR2, a 7818.[11] Holden advises that the large non-polarised electrolytic on the 12-inch board be replaced with a film capacitor; an owner on the same thread replaced six electrolytics, two of which had mild leakage, including that one.[12] Holden also names R752 and R753 as a very common failure in the 12-inch monitor. Those designators belong to the 12-inch board version with three-digit numbering, not the 8032033 board above; they are resistors, but check them during a recap if your board has them.[12]

9-inch display, PCB 320084-02

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Display assembly on PCB 320084-02, electrolytics (Commodore cross reference)[13]
Ref Value Commodore part no.
C1 4700 µF 25 V 900100-45
C3, C12, C13, C17 47 µF 16 V 900100-33
C9 1.0 µF 25 V 900100-32
C15 470 µF 16 V 900100-44
C22, C27 3.3 µF 200 V or 4.7 µF 250 V 900100-02
C23 220 µF 25 V 900100-03
C28 10 µF 25 V, non-polarised 900103-01

A 4032 owner on VCFed found PCB 320084-02 in his machine.[14] Console5's list for its "9VATP4" board has the same values but gives C2 where Commodore's cross reference has C3.[5]

9-inch display with three-digit designators

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Some 9-inch PET displays use a board numbered in the C600-C900 range. Console5's list for this "CDU-B79" type: C901 and C903 2200 µF 35 V; C902 47 µF 16 V; C621 100 µF 10 V; C602, C651 and C661 33 µF 16 V; C652 470 µF 10 V; C716 470 µF 16 V; C751 47 µF 50 V; C752 4.7 µF 50 V; C701 1 µF 160 V; C715 6.8 µF 25 V non-polarised low-ESR.[5] Console5's kit replaces C715 with a metallised polypropylene film capacitor.[15] Commodore's documents for this board are not hosted here; check the designators on your board before ordering.

Symptoms

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  • A hum bar rolling through the picture usually means mains ripple on a supply rail. VCFed's daver2 advises checking the sync signals from the logic board first, then the display board's low-voltage regulated rail for noise.[14]
  • A rail that has collapsed on the logic board points at a shorted tantalum.[2]

See also the Commodore PET 4000 Series Troubleshooting Guide.

Procedure

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  1. Unplug the machine, discharge the CRT, and bleed the 23,000 µF supply capacitor.
  2. Note which display board is fitted and use its table. Photograph the board.
  3. Mark the polarity of each electrolytic and tantalum. C15 on the 12-inch board and C28 on the 320084-02 board are non-polarised.
  4. Replace parts one at a time, at the listed capacitance and at least the listed voltage.
  5. Power up with the CRT connected and check the logic board's +5 V, +12 V and −5 V rails before reassembling; see the Commodore PET 4000 Series Maintenance Guide.
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References

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  1. ↑ 1.0 1.1 1.2 1.3 1.4 Commodore Business Machines, PET 4000 Series Technical Manual, P/N 990438, for models 4016 and 4032, hosted at Commodore PET 4000 Series Technical Manual. Drawing 8032090 Rev. H, parts list, PDF pp. 3-7; power supply assembly 8032051, parts list, PDF p. 39.
  2. ↑ 2.0 2.1 2.2 Commodore PET 8032 monitor problem(s)?, page 3—link(accessed 2026-10-01) Hugo Holden: "The usual cause for the power rails crashing is the blue Tant capacitors shorting out."
  3. ↑ Commodore Pet 4032 problems—link(accessed 2026-10-01)
  4. ↑ Tynemouth Software, Commodore 8032 mainboard repair, June 2015—link(accessed 2026-10-01)
  5. ↑ 5.0 5.1 5.2 Console5 TechWiki, Commodore PET—link(accessed 2026-10-01) Capacitor lists for the PET system boards, 9-inch displays and power supply (secondary source).
  6. ↑ 6.0 6.1 Is this capacitor still good? Commodore PET 2001—link(accessed 2026-10-01)
  7. ↑ Tynemouth Software, Commodore PET 8032 repair and restoration, December 2014—link(accessed 2026-10-01)
  8. ↑ Console5, 22000uF 25v Kemet screw terminal electrolytic capacitor—link(accessed 2026-10-01)
  9. ↑ Tynemouth Software, Exploding Commodore PET mains filters, August 2015—link(accessed 2026-10-01)
  10. ↑ Adam's Vintage Computer Restorations, 1982 Commodore PET 8032 repair and restoration, 26 January 2026—link(accessed 2026-10-01)
  11. ↑ 11.0 11.1 Commodore Business Machines, "P.C.B. Assy, Display 12"", drawing 8032033 Rev. M (1980), parts list sheets 2, 3 and 5, hosted at File:Commodore PET 12-inch Display PCB Assembly 8032033 Parts List and Schematic.pdf.
  12. ↑ 12.0 12.1 Commodore PET 8032 monitor problem(s)?—link(accessed 2026-10-01)
  13. ↑ Commodore Business Machines, "Display Assembly Component Cross Reference" (PCB 320084-02), hosted at File:Commodore PET Display Assembly Component Cross Reference (PCB 320084-02).pdf.
  14. ↑ 14.0 14.1 Repairing a beautiful 4032, screen flickering, February 2015—link(accessed 2026-10-01)
  15. ↑ Console5, Commodore PET 2001N 9" monitor cap kit, CBM-CDU-B79—link(accessed 2026-10-01)
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