Commodore PET 2001 Capacitor Replacement Guide

The first-generation PET 2001 (1977 – 1979)—assemblies 320008 / 320081 / 320132—uses a simple linear supply together with an internal 9" monochrome CRT. After forty-plus years, the original electrolytic capacitors are at (or past) their design life. Re-capping:

• stabilises the +5 V / +12 V rails used by the logic and VIC video section,
• removes vertical “jail-bars”, shimmering, or ticking noises caused by dried-out CRT filter caps, and
• protects irreplaceable ICs such as the 6502, the twin 6520/6522 PIA/VIA I/O chips, and the early mask ROM/SRAM devices.

Tip ► The PET will usually still boot with tired capacitors—but voltages sag as it warms up, causing random lock-ups and screen collapse. Fix the caps before chasing “mystery” digital faults.

Visual Inspection

• Bulging or domed tops – particularly the two large 4 700 µF smoothing cans (C9 & C10).
• Brown crust or green verdigris at the rubber bungs.
• ‘Fishy’ smell when the board warms up – electrolyte seepage.
• White salt deposits around the CRT board’s high-voltage electrolytics.

If any of the above are present, _replace the whole set_ – piecemeal swaps often lead to repeated strip-downs.

Capacitor Reference Tables

Values are taken from Commodore parts sheets parts-3.gif (system board) and parts-6.gif (video board) for the PET 2001 series. Board lettering differs slightly between early 6540/6550 “static” boards (320008) and later 2316B/2114 “dynamic” boards (320132); positions are therefore given as silkscreen “Cx” + functional description.

🔌 Main Power / Logic Board (all 2001 revisions)

Power & Regulator Section

Designator	Location / Function	Capacitance	Voltage	Notes
C9	5 V rail smoothing (after bridge)	4 700 µF	25 V	Axial can, glue-mounted
C10	5 V rail secondary filter	4 700 µF	25 V	Identical to C9
C11	12 V booster reservoir	2 200 µF	25 V	Generates +12 V for VIC/audio
C12	5 V pre-reg filter	1 000 µF	25 V	Near 7805 regulator heatsink
C13 / C14	Logic decoupling bulk caps (left & right board edges)	220 µF	16 V	2 pcs
C16 – C27	Local bypass (scattered across DRAM/ROM rows)	10 µF	16 V	12 pcs

Use 105 °C low-ESR parts for C9–C13—they run hot next to the transformer.

CRT (Video/Deflection) Board

9″ Monitor / Deflection Section

Designator	Function	Capacitance	Working V	Qty
C101	B+ rail reservoir	220 µF	200 V	1
C102	Horizontal output stage	100 µF	200 V	1
C103	Vertical sweep supply	47 µF	160 V	1
C108	Video amplifier decouple	10 µF	50 V	1
C110 / C111	Sync & oscillator timing	1 µF	160 V	2
C115	Heater/filament filter	100 µF	25 V	1
C120 – C123	Frame-store / blanking filters	22 µF	50 V	4

Safety! C101 & C102 sit at ~160 V DC. Discharge them _with a 100 kΩ 2 W resistor_ before desoldering.

Optional Dynamic-RAM Board (PET 2001-8 upgrade)

Early 2001-8 machines add a small plug-in board carrying four 2114 SRAMs and four 10 µF / 16 V electrolytics (C201–C204). Replace these at the same time.

🛠️ Replacement Procedure

1. Isolate power & remove the 2 A slo-blo fuse.
2. Disassemble – take out the motherboard (4 screws) and pivot the monitor/keyboard assembly.
3. Document with photos; note capacitor polarity (square pad = positive on PET PCBs).
4. Desolder using braid or a spring pump. PET boards are single-sided—lifted pads are common; be gentle.
5. Clean pads with isopropyl alcohol; scrape away corroded lacquer if necessary.
6. Fit new capacitors; keep lead length minimal and apply fresh thermal compound to the 7805 if removed.
7. Re-assemble, checking that C9/C10 do not short against the case bottom.
8. Power-on with series-bulb limiter (or bench-supply) and verify rails:
   1. +5 V = 4.95 – 5.10 V at the 6502 (pin 8 = GND, pin 40 = Vcc)
   2. +12 V ≈ 11.6 – 12.6 V at VIC pin 18
9. Run the PET for 30 min burn-in; touch-test that the 7805 < 65 °C and C101/C102 remain cool.

Tooling Checklist

• Temperature-controlled soldering iron (fine 1 mm conical tip)
• Desoldering wick 2 mm & spring pump
• ESR meter or capacitance tester (optional but recommended for pre-check)
• 100 kΩ / 2 W resistor lead for safe high-voltage discharge
• Non-acidic IPA (99 %) & ESD brush
• Kevlar gloves / eye protection for CRT section work

Post-Recap Adjustments

The PET’s PSU has no trimmer; voltage is set by the transformer and 7805. If +5 V is low after recapping, check:

• bridge rectifier D1–D4 for heat damage,
• continuity of the power-switch contacts, and
• correct mains-tap wiring (120 V vs 240 V).

The monitor focus & vertical-height pots (_on the video board_) may drift after fresh caps—re-adjust for crisp text with a plastic tool.

Additional Notes

• C9/C10 sometimes leak _inside_ the aluminium can and look fine externally—always meter for ESR.
• Early _“blue label”_ PETs (with 6540 ROMs) omit C11; later logic boards include it—verify before ordering.
• If you plan a modern switch-mode 5 V module in place of the 7805, you may down-rate C9/C10 to 2 200 µF 16 V low-ESR; ripple is dramatically lower.

🔗 Related Pages

• Commodore PET 2001 Troubleshooting Guide
• Commodore VIC-20 Capacitor Replacement Guide