Sinclair ZX81 Capacitor Replacement Guide
The Sinclair ZX81 has twelve capacitors, and two of them are electrolytic: C3, the 22 µF reservoir ahead of the 7805 regulator, and C5, the 1 µF capacitor in the power-on reset circuit. The other ten are ceramic discs. Sinclair's kit instructions list every one by designator, value, marking and type.[1]
Visual inspection and failure signs
[edit | edit source]- Residue, bulging or a split can on C3 or C5.
- A dead machine with a cold regulator. Sinclair's check is the supply and the plug being in the power socket; C3 is the only part between that socket and the regulator.[1][2]
- A clear screen with no cursor. Sinclair's remedy is to disconnect the supply, wait a few seconds and try again; the reset network is R15 and C5 on the Z80's /RESET pin.[1][2]
- Horizontal black and white stripes passing through the picture. Sinclair puts these down to the supply: it needs better smoothing or a slightly higher output.[1]
See Capacitor Failure Symptoms for what failing electrolytics look like in general.
ZX81 capacitor list
[edit | edit source]Values, markings and types are from Sinclair's component list; circuit positions are read from the circuit diagram.[1][2]
| Ref | Value | Marking | Type (Sinclair) | Circuit position |
|---|---|---|---|---|
| C1 | 47 pF | 47 | Ceramic disc | With R1 (10 kΩ) at the base of TR1, which drives the Z80's /WAIT line |
| C2 | 47 nF | 473 Z | Ceramic disc | +5 V rail at the regulator output |
| C3 | 22 µF | 22µ | Electrolytic, 16 V min. | Across the DC input, ahead of the 7805 |
| C4 | 47 nF | 473 Z | Ceramic disc | +5 V rail |
| C5 | 1 µF | 1µ | Electrolytic, 5 V min. | With R15 (220 kΩ) on IC3 (Z80A) pin 26, /RESET |
| C6 | 100 pF | 100, 101, n10 | Ceramic disc | With R4 (18 kΩ) and X1 at ULA pin 35, the ceramic oscillator |
| C7 | 47 pF | 47 | Ceramic disc | Across R6 (2.2 kΩ) between ULA pin 14 and TR2, the CPU clock buffer |
| C8 | 47 nF | 473 Z | Ceramic disc | +5 V rail |
| C9 | 47 nF | 473 Z | Ceramic disc | +5 V rail |
| C10 | 10 nF | 10n, 103 | Ceramic disc | EAR input, in series with R33 (4.7 kΩ) to ULA pin 20; R34 (220 Ω) across the socket |
| C11 | 47 nF | 473 Z | Ceramic disc | MIC output, across the socket with R27 (1 kΩ) |
| C12 | 47 pF | 47 | Ceramic disc | MIC output, in series with R29 (1 MΩ) from the ULA's TV/tape line |
The same capacitor list appears in a clean colour scan of the kit instructions in the Ferranti Archives 2026, which is easier to read than the hosted copy.[3] The kit list is for the Issue One board. Television found that later issues keep the same component numbers.[1][4] Check the board in front of you before ordering.
Replacement parts
[edit | edit source]- C3: 22 µF electrolytic. Sinclair's minimum is 16 V. The standard adaptor gives about 14 V off load, so a 25 V part has more margin.[1][5]
- C5: 1 µF electrolytic, 5 V minimum per Sinclair; it sits on the +5 V side of the reset network.[1][2]
- Polarity: both electrolytics are marked + or − on the can, and the + lead is usually the longer. The board shows each capacitor as a capacitor symbol, not an outline box.[1]
Replacement procedure
[edit | edit source]- Unplug the power, TV and cassette leads, and remove any RAM pack.
- Turn the machine over and remove the base screws; some are under the rubber feet. Lift off the base and remove the two screws that hold the board to the top half.[1][4]
- Keep the screws apart. The kit has three long black screws and four short yellow ones, and Sinclair warns that a long screw in the wrong hole causes serious damage.[1]
- Turn the board over towards the front and unplug the keyboard tails from KB1 and KB2. Handle them with great care; they are easily damaged.[4]
- Note the polarity of C3 and C5, desolder them, and fit the new parts the same way round.
- Clean off the flux and check for solder bridges and splashes.
- Refit the tails: with the case face down and the board component side up beside it, edge connector at the front left, loop the tails over and push them in with a slight rocking movement. Pushing too hard buckles and splits them.[4]
- Refit the board with the two short screws, then the base.[4]
Checks after the work
[edit | edit source]| Point | Expected |
|---|---|
| Positive lead of C3 | Adaptor voltage: about 11 V on load at 400 mA, about 14 V off load[4][5] |
| 7805 output | +5 V at every IC[4] |
| IC3 (Z80A) pin 26, /RESET | Low for a moment at switch-on, then high[2] |
| Screen | White-on-black (inverse) K cursor at the bottom left[4] |
A clear screen with no cursor after the work points at C5 and R15. Horizontal stripes through the picture point at the supply and C3.[1]
A Sinclair ZX Printer has four electrolytics of its own (C2 100 µF 16 V, C3 and C11 10 µF, C4 100 µF 63 V on its +50 V rail); they are listed on that page.
Tools and parts
[edit | edit source]See Recommended Tools. For this job: a Pozidriv No. 1 screwdriver (Sinclair's specified tool for the case screws), a temperature-controlled iron, desoldering braid or a pump, one 22 µF 25 V and one 1 µF electrolytic, and a multimeter.[1]
Related pages
[edit | edit source]- Sinclair ZX81
- Sinclair ZX81 General Maintenance
- Sinclair ZX81 Troubleshooting Guide
- Sinclair ZX81 Assembly Instructions
- Sinclair ZX80 Capacitor Replacement Guide
- Sinclair ZX Printer
- Capacitor Failure Symptoms
- Battery Explosion, Capacitor or Corrosion Damage
References
[edit | edit source]- ↑ 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 Sinclair Research Ltd, ZX81 Assembly Instructions (1981): section 3 (component identification), section 6 (fault finding, p. 4), section 7 (case assembly), section 8 (component list, pp. 5–6) and section 9 (power supply, p. 7). Hosted in File:Sinclair ZX81 Service and Assembly Manual.pdf, PDF pages 2–8; see Sinclair ZX81 Assembly Instructions.
- ↑ 2.0 2.1 2.2 2.3 2.4 Sinclair, ZX81 circuit diagram, drawing SRC 049. Hosted as File:Sinclair ZX81 circuit diagram.png.
- ↑ Sinclair Research, ZX81 Assembly Instructions (1981), Ferranti Archives 2026 (ZX81-SIN-LIT-131): a clean colour scan of the same edition as the copy at Sinclair ZX81 Assembly Instructions, with section 8 (the capacitor list) and the circuit diagram on single sheets.
- ↑ 4.0 4.1 4.2 4.3 4.4 4.5 4.6 4.7 "Servicing Sinclair Computers, Part 2", Television magazine, 1986. Transcribed at https://k1.spdns.de/Vintage/Sinclair/82/Sinclair%20ZX%20Spectrum/Repair/rep2%20-%20the%20ZX81.html (retrieved 2026-10-01).
- ↑ 5.0 5.1 "Sinclair ZX81 Connectors", The Sinclair ZX Archive, https://k1.spdns.de/Vintage/Sinclair/80/Sinclair%20ZX81/Tech%20specs/Connectors.html (retrieved 2026-10-01).