Editing Commodore PET 2001 Capacitor Replacement Guide

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= 🧰 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 ==
{| class="plainlinks" style="margin:0 0 1em 1em; float:right;"
 | [[File:PET2001_PS_caps.jpg|180px|thumb|Typical PSU caps (C9, C10)]]
 | [[File:PET2001_Video_caps.jpg|180px|thumb|Leaking 220 µF/200 V on video board]]
|}
* '''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 :contentReference[oaicite:0]{index=0}.  
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) ===
{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+'''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 ===
{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+'''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 ==
# '''Isolate power''' & remove the 2 A slo-blo fuse.  
# '''Disassemble''' – take out the motherboard (4 screws) and pivot the monitor/keyboard assembly.  
# '''Document''' with photos; note capacitor polarity (square pad = positive on PET PCBs).  
# '''Desolder''' using braid or a spring pump.  PET boards are single-sided—lifted pads are common; be gentle.  
# '''Clean pads''' with isopropyl alcohol; scrape away corroded lacquer if necessary.  
# '''Fit new capacitors'''; keep lead length minimal and apply fresh thermal compound to the 7805 if removed.  
# '''Re-assemble''', checking that C9/C10 do not short against the case bottom.  
# '''Power-on with series-bulb limiter''' (or bench-supply) and verify rails:  
## +5 V = 4.95 – 5.10 V at the 6502 (pin 8 = GND, pin 40 = Vcc)  
## +12 V ≈ 11.6 – 12.6 V at VIC pin 18  
# 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]]

[[Category:Commodore Systems]]
[[Category:Commodore Maintenance Guides]]
[[Category:Capacitor Replacement Guides]]