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Sinclair ZX80 Capacitor Replacement Guide

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Sinclair ZX80 circuit diagram
Sinclair's ZX80 circuit diagram. The power input, C8 and the regulator IC22 are at the top centre and right.

The Sinclair ZX80 board has fifteen capacitor positions, of which thirteen are fitted, and only two of those are electrolytic: C8, the 22 µF reservoir on the DC input, and C10, the 1 µF capacitor in the power-on reset circuit. The rest are ceramic.[1]

Visual inspection and failure signs

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  • Residue, bulging or a split can on C8 or C10.
  • A machine that is dead with a cold regulator. C8 is the only capacitor between the input socket and the regulator IC22, so check it after the adaptor and the socket.[2]
  • A clear screen with no cursor. Sinclair's check for this is the C10 and R21 reset network.[1]
  • Horizontal lines passing through the picture. Sinclair attributes these to too much ripple on the supply, which points at the adaptor first and C8 second.[1]

See Capacitor Failure Symptoms for what failing electrolytics look like in general.

ZX80 capacitor list

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Values and markings are from Sinclair's component list; circuit positions are read from the circuit diagram.[1][2] The component location drawing that goes with the list is of the Issue 2 board. The Sinclair ZX Archive has photographs of Issue 3 and Issue 3A boards, which the kit instructions do not cover, so check the positions on a later board before ordering.[1][3]

Sinclair ZX80 capacitors
Ref Value Markings (Sinclair) Type Circuit position
C1, C2, C4, C5, C6 47 nF 473M, 473Z Ceramic Across the +5 V rail at the output of IC22
C3, C7 Not used Pads on the board, no part fitted
C8 22 µF 22µ 16v Electrolytic, 16 V Across the DC input, ahead of IC22
C9 47 pF 47, 47p, 470J Ceramic With R2 (470 Ω) on the 2Φ clock line
C10 1 µF 1µ 63v Electrolytic With R21 (220 kΩ) on IC1 pin 26, /RESET
C11 47 pF 47, 47p, 470J Ceramic With R25 (470 Ω) at IC11 pin 2
C12 100 nF (diagram: 47 nF) 104, 0.1 Ceramic In series with R29 (180 Ω) from the EAR socket
C13 47 nF 473M Ceramic With R34 (1 kΩ) at the MIC socket
C14 47 pF 47, 47p, 470J Ceramic With R35 (1 MΩ) from SYNC to the MIC socket
C15 220 pF (33 pF for 60 Hz US machines) 220, 220p, 221, n22 Ceramic In the IC20 oscillator with X1; the diagram marks it "0–330P"

Sinclair's component list and circuit diagram disagree on C12: 100 nF in the list, 47 nF on the diagram. Either works in a DC-blocking position on the tape input; check what is fitted before ordering.[1][2] The circuit diagram printed with an earlier printing of Sinclair's assembly instructions shows C12 as 100n, agreeing with the list, and that printing gives C15 as "0 - 330p (see text)", the text being its remedy for a jittering picture: experiment with the value of C15. Its C8 is 22 µF 16 V and its C10 1 µF 63 V, as in the hosted edition.[4]

Replacement parts

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  • C8: 22 µF electrolytic. Sinclair fitted a 16 V part. The standard adaptor gives about 14 V off load, so a 25 V part has more margin.[1][5]
  • C10: 1 µF electrolytic. It sits on the 5 V side of the reset network; Sinclair's part was marked 63 V.[1]
  • Polarity: the circuit diagram and the component location drawing both mark the positive lead of C8 and C10.[1][2]
  • Mounting: C8 stands upright on the board. C10 lies flat, and Sinclair's construction order notes that the top case will not fit if it is not flat.[1]

Replacement procedure

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  1. Unplug the power, TV and cassette leads.
  2. Open the case. The two case halves are held together by five plastic rivets and the board is held to the lower moulding by two more; the case has no screws.[1]
  3. Lift the board out. The keyboard is part of the board, so there are no ribbon cables to disconnect.[1]
  4. Note the polarity of C8 and C10 against the board markings.
  5. Desolder each part. The board has plated-through holes, so clear the solder from the hole fully before pulling the lead.[1]
  6. Fit the new part with the positive lead in the marked hole. Keep C10 flat on the board.
  7. Clean off the flux and check for solder bridges before refitting the board.

Checks after the work

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ZX80 supply checks
Point Expected
Positive lead of C8 The adaptor voltage: rated 9 V, about 14 V off load[6][5]
IC22 output +5 V[2]
IC1 pin 26 (/RESET) Low for a moment at switch-on, then high[2]
Supply current 350 to 500 mA, typically 400 mA[1]

A clear screen with no cursor after the work points at C10 or R21. Horizontal lines through the picture point at the supply and C8.[1]

Tools and parts

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See Recommended Tools. For this job: a temperature-controlled iron, desoldering braid or a pump, one 22 µF 25 V and one 1 µF electrolytic, and a multimeter for the checks above.

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References

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  1. ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 1.12 1.13 1.14 Sinclair Research Ltd, ZX80 Assembly Instructions (1980). Hosted as File:Sinclair ZX80 Assembly Instructions.pdf. Section 3 (electrolytics C8 and C10), section 4 (construction order), section 6 (power supply), section 9 (component list) and the Issue 2 component location drawing on the last page.
  2. ↑ 2.0 2.1 2.2 2.3 2.4 2.5 Sinclair, ZX80 Circuit Diagram. Hosted as File:Sinclair ZX80 circuit diagram.png.
  3. ↑ "Sinclair ZX80 PCB" image directory, The Sinclair ZX Archive, https://k1.spdns.de/Vintage/Sinclair/80/Sinclair%20ZX80/PCB/ (accessed 2026-10-10): photographs of Issue 3 and Issue 3A boards.
  4. ↑ Sinclair ZX80 assembly instructions, an earlier 1980 printing with the ZX80 circuit diagram on the back of the component location sheet, Ferranti Archives 2026 (ZX80-SIN-LIT-130); the differences from the hosted edition are listed at Sinclair ZX80 Assembly Instructions. Source for the 7 to 11 V supply range, the C15 range and jitter remedy, IC2 as a 2332, and the circuit diagram details cited here.
  5. ↑ 5.0 5.1 "Sinclair ZX80 Connectors", The Sinclair ZX Archive, https://k1.spdns.de/Vintage/Sinclair/80/Sinclair%20ZX80/Tech%20specs/Connectors.html (retrieved 2026-10-01).
  6. ↑ Science of Cambridge Ltd, ZX80 Operating Manual (1980), chapter 2: "9 volts DC @ 600 mA unregulated". Transcription by Tony Barnett (2003), https://k1.spdns.de/Vintage/Sinclair/80/Sinclair%20ZX80/ZX80%20Operating%20Manual.pdf (retrieved 2026-10-01).


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