Sinclair ZX81 Capacitor Replacement Guide
Replacing electrolytic capacitors (“recapping”) in your Sinclair ZX81 is a key preventative maintenance step. These capacitors stabilize power rails, reduce video noise, and prevent random crashes or boot failures. Original ZX81 capacitors are now over 40 years old—well past their intended lifespan—so replacement is highly recommended for reliability.
Visual Inspection & Failure Signs
[edit | edit source]- Bulging or leaning cans – Most visible on the large 1000 µF filter capacitor (C5).
- Leakage residue – Brown or green crust at the base of capacitors, especially near C5 or C3.
- Video instability – Flickering, rolling, or loss of sync can be caused by dried-out C3 or C5.
- Unstable booting or resets – High ESR in C8 (reset timing) can cause the ZX81 to fail to start or reset unexpectedly.
If any capacitor is suspect, it’s best to replace all electrolytics at once.
Sinclair ZX81 Capacitor List
[edit | edit source]The ZX81 is a simple design with few electrolytic capacitors. Always cross-check the silkscreen and your board revision before ordering replacements.
| Ref | Capacitance | Voltage | Purpose / Notes |
|---|---|---|---|
| C3 | 22 µF | 16 V | Video circuit smoothing |
| C4 | 1 µF | 16 V | Video sync coupling |
| C5 | 1000 µF | 16 V (or 25 V) | Main +5 V input filter (from external PSU) |
| C8 | 1 µF | 16 V | Reset timing (RC circuit) |
Some early boards may use 10 V or 25 V rated capacitors; always use equal or higher voltage when replacing.
ZX81 Recapping Procedure
[edit | edit source]- Disassemble – Remove the two screws underneath and gently separate the case halves. Detach the keyboard membrane carefully.
- Locate capacitors – Identify C3, C4, C5, and C8 on the PCB.
- Desolder old capacitors – Use solder-wick and a temperature-controlled iron (set to ~350 °C). Take care not to lift delicate PCB pads.
- Install new capacitors – Observe polarity (long lead = positive; match board markings). Use 5 mm lead spacing for small caps, 7.5 mm for C5.
- Trim leads and inspect – Ensure no solder bridges or cold joints.
- Clean up – Remove any flux residue with isopropyl alcohol and a soft brush.
- Reassemble – Refit the keyboard membrane and close the case.
Post-Recap Voltage & Video Checks
[edit | edit source]After recapping:
- Measure +5 V at the RAM chip (IC3, pin 14): should be 4.90–5.10 V.
- Confirm stable video output: clear, steady black-and-white image with no rolling or flicker.
- Test reset: pressing the reset button should reliably restart the ZX81.
Recommended Tools & Parts
[edit | edit source]- 30–60 W temperature-controlled soldering iron (fine tip)
- Solder-wick and/or spring pump
- Leaded 63/37 or high-quality lead-free solder
- 105 °C, low-ESR radial electrolytic capacitors (Nichicon, Panasonic, Rubycon)
- Isopropyl alcohol (99%) and antistatic brush
- Small side-cutters
Extra Tips
[edit | edit source]- Use quality capacitors – Cheap caps can fail quickly; use reputable brands.
- Check the PSU – A faulty external power supply can damage new capacitors and chips.
- Upgrade C5 – If using modern peripherals (RAM packs, composite mods), consider a 2200 µF 16 V for C5 to improve filtering.
- Composite video mod – If you have installed a composite mod, C3 and C4 may be omitted or replaced with different values; check your mod’s instructions.
- Keyboard membrane care – Handle the ribbon gently; replacements are available if brittle.