Sinclair ZX80 Capacitor Replacement Guide
Replacing the electrolytic capacitors (“recapping”) in your Sinclair ZX80 is a crucial preventative maintenance step. Original capacitors from 1980 are now far beyond their expected lifespan, and failure can cause instability, video issues, or even prevent startup. Fresh, quality capacitors restore reliable operation and protect delicate logic chips.
🔍 Visual Inspection & Failure Signs
- Leaking or crusty capacitors – Look for brown, green, or white residue at the base of any can.
- Bulging or tilted cans – Even small axial capacitors may bulge or lean if failing.
- Unstable video output – Flickering, rolling, or missing sync often traces to dried-out C5 or C6.
- Boot failures or random resets – Power supply filter caps (C1, C2) with high ESR can cause unreliable startup.
If any capacitor shows trouble, replace all electrolytics on the board.
📋 Sinclair ZX80 Capacitor List
The ZX80 uses a small number of electrolytic capacitors, all easily accessible on the main PCB. Always confirm values against your board’s silkscreen and schematic, as some early kits may differ.
| Ref | Capacitance | Voltage | Function / Notes |
|---|---|---|---|
| C1 | 22 µF | 16 V | +5V input smoothing (main power filter) |
| C2 | 1 µF | 16 V | +5V decoupling (logic rail) |
| C3 | 1 µF | 16 V | Video modulator coupling |
| C4 | 1 µF | 16 V | Video sync coupling |
| C5 | 10 µF | 16 V | Video composite output smoothing |
| C6 | 22 µF | 16 V | Reset circuit (power-on reset timing) |
All original capacitors are axial-lead types. Modern radial types may be used if carefully fitted and insulated.
🛠️ ZX80 Recapping Procedure
- Unplug and disassemble – Remove the case screws and carefully lift the top shell.
- Remove the PCB – Disconnect the keyboard membrane and gently free the main board.
- Desolder old capacitors – Use a temperature-controlled iron (300–350 °C) and desoldering braid or pump.
- Clean pads – Remove old solder and residue; inspect for lifted traces.
- Fit new capacitors – Match polarity (long lead = +). Axial types fit the original holes; radial types can be bent to fit, but insulate leads with heatshrink or sleeving.
- Solder and trim leads – Ensure solid joints and no solder bridges.
- Clean flux residue – Use IPA and a soft brush.
- Reassemble and test – Power up and confirm stable video and reliable boot.
⚙️ Post-Recap Voltage Checks
After recapping, measure the +5V rail at the 7805 regulator output and at the RAM/CPU pins:
| Test Point | Expected Voltage | Max Ripple (p-p) |
|---|---|---|
| 7805 output tab | 4.95 – 5.10 V | < 50 mV |
| RAM/CPU Vcc pin | 4.90 – 5.05 V | < 50 mV |
Excessive ripple or voltage drop may indicate a faulty regulator or incorrect capacitor installation.
🧰 Recommended Tools & Parts
- Temperature-controlled soldering iron (fine tip, 1–2 mm)
- Desoldering braid and/or pump
- Leaded 63/37 or quality lead-free solder
- 105 °C, low-ESR capacitors (Nichicon, Panasonic, Rubycon)
- Isopropyl alcohol (IPA) and antistatic brush
- Heatshrink tubing (for radial cap lead insulation)
💡 Extra Tips
- Double-check polarity! The ZX80 board is single-sided and traces lift easily.
- Use axial capacitors for easiest fit, but radial types are fine if insulated and laid flat.
- Check the 7805 regulator – If it runs hot or output is low, consider replacing it along with the capacitors.
- Clean the edge connector while the board is out for best RAM pack reliability.
- Store the ZX80 in a dry place – moisture accelerates capacitor aging.