Sinclair ZX81 Capacitor Replacement Guide

Sinclair's ZX81 circuit diagram, drawing SRC 049. The regulator and its reservoir C3 are at the bottom left; the R15/C5 reset network is beside the Z80A.

The Sinclair ZX81 has twelve capacitors and only two of them are electrolytic. Sinclair published the complete list — value, marking and type for every one — in the ZX81 Service and Assembly Manual supplied with the kit, so there is no guesswork involved.

Because there are only two electrolytics, the ZX81 is not a machine that needs a wholesale recap. Replace those two if they are old and the machine is misbehaving; leave the ceramics alone.

Visual inspection and failure signs

• Leaking or crusty electrolytics — brown, green or white residue at the base of C3 or C5.
• Machine dead, regulator cold. C3 is the only reservoir between the incoming unregulated supply and the 7805. Check the supply, the jack and C3.
• No inverse-K cursor at switch-on. The Z80A is held in reset by R15 (220 k) charging C5 (1 µF) at IC3 pin 26. Sinclair's own first response to a clear screen with no cursor is to disconnect the supply, wait a few seconds and try again — that gives C5 time to discharge.
• Horizontal black-and-white stripes drifting through the picture. Sinclair attributes this to the supply: inadequate smoothing, or output too low on load.

Capacitor list

Sinclair's own component list, cross-referenced to the circuit diagram for what each part does.

Sinclair ZX81 capacitors

Ref	Value	Marking	Type	Where it sits
C1	47 pF	47	Ceramic disc	Video output stage with TR1 (ZTX313), R1 (10 k) and R17 (1 k), feeding the modulator
C2	47 nF	473 Z	Ceramic disc	Decoupling on the regulated +5 V rail
C3	22 µF	22u	Electrolytic, 16 V minimum	Across the incoming unregulated supply, ahead of the 7805 regulator. The board's only reservoir capacitor
C4	47 nF	473 Z	Ceramic disc	Decoupling on the regulated +5 V rail
C5	1 µF	1u	Electrolytic, 5 V minimum	Power-on reset network — with R15 (220 k) on IC3 (Z80A) pin 26, /RESET
C6	100 pF	100, 101, n10	Ceramic disc	Master oscillator round the 6.5 MHz ceramic resonator X1, at IC1 pin 35
C7	47 pF	47	Ceramic disc	Same oscillator circuit, with R6 (2k2) and TR2
C8	47 nF	473 Z	Ceramic disc	Decoupling on the regulated +5 V rail
C9	47 nF	473 Z	Ceramic disc	Decoupling on the regulated +5 V rail
C10	10 nF	10n, 103	Ceramic disc	EAR (tape input) network with R33 (4k7) and R34 (220 R)
C11	47 nF	473 Z	Ceramic disc	MIC (tape output) network with R27 (1 k) and R29 (1 M)
C12	47 pF	47	Ceramic disc	Same MIC output network

Three points worth stating plainly, because they are commonly got wrong:

• There is no 1000 µF capacitor anywhere on a ZX81. C5 is 1 µF. Fitting a 1000 µF part in the C5 position puts a very long time constant on the CPU reset line.
• C4 and C8 are ceramic discs, not electrolytics. They have no polarity and are not a service item.
• C3 is not a video capacitor. It is the supply reservoir. The video coupling is C1.

Choosing replacements

• C3 — 22 µF, 16 V or higher. It sits on the raw input, which is nominally 9 V DC and rises to about 14 V open-circuit, so 16 V is the floor, not a suggestion.
• C5 — 1 µF. Sinclair specified 5 V minimum; it sits on the 5 V rail, and any modern 16 V or 25 V part is fine.
• Use 105 °C aluminium electrolytics from a recognised manufacturer. Equal capacitance, equal or higher voltage.
• Sinclair's originals are axial. Radial parts fit if you form and sleeve the leads so nothing can short to a neighbouring track.
• No lead-pitch figures are given here because Sinclair did not publish any; measure your own board.

Replacement procedure

1. Unplug the supply and remove the case screws — four on the base, three of them under rubber feet — then the two screws holding the board.
2. Free the board and withdraw the keyboard tails from KB1 (5-way) and KB2 (8-way). Pull them straight out. They crack where they bend, and a folded tail goes open circuit sooner or later.
3. Note the polarity of C3 and C5. Sinclair's assembly manual points out that both are marked with a + or − symbol on the can; the board is marked with a capacitor symbol rather than an outline.
4. Add fresh flux and solder to the old joints and let it mix before wicking. Do not work the pads hot and dry.
5. Fit the replacements the right way round, solder and trim.
6. Clean the flux with isopropyl alcohol and inspect under magnification for splashes as well as bridges.
7. Reassemble and test.

Checks after the work

The ZX81 has one internal rail. There is no −5 V and no +12 V — those belong to the ZX Spectrum, whose 4116 DRAM needs them. The ZX81's RAM is static.

ZX81 checkpoints

Point	Expect	Source
Power jack, 3.5 mm, tip positive	DC only, need not be regulated but should be well smoothed; 8–12 V on load, about 14 V open-circuit and roughly 11 V on load at 400 mA; not less than 600 mA capability, or 1.2 A if the ZX Printer shares the supply. Below about 7 V the regulator will not work correctly
REG (7805) output	+5 V, reaching every IC on the board
IC3 (Z80A) pin 11 Vcc / pin 29 ground	+5 V / ground
IC4 (4118 RAM) pin 24 Vcc / pin 12 ground	+5 V / ground
IC4a, IC4b (2114 RAM) pin 18 Vcc / pin 9 ground	+5 V / ground
IC3 (Z80A) pin 26	/RESET low briefly at switch-on, then high
IC1 (ULA) pin 16, to the modulator	About 2 V peak from picture peak to the bottom of the sync pulses

The ZX81 has no reset switch. The only reset is the R15/C5 network acting at switch-on, which is why the remedy for a hung machine is to unplug it and wait a few seconds.

A clear, steady black-and-white picture with no rolling or flicker, and the inverse-K cursor at the bottom left, is the sign of a good result.

Tools and materials

See Recommended Tools. For this job:

• Temperature-controlled iron with a fine tip
• Desoldering braid and/or a pump
• 63/37 leaded solder and fresh flux
• Two 105 °C electrolytics: 22 µF 16 V (or higher) and 1 µF 16 V (or higher)
• Isopropyl alcohol and a soft antistatic brush

Notes

• Check the supply first. A supply re-terminated with reversed polarity is a recognised way to destroy the 5 V regulator on this family — which, as the trade press noted at the time, usually saves the rest of the board.
• Board issues. Sinclair's component list is for the Issue 1 board. At least three PCB versions exist, and the component reference numbers are the same on all of them; the notable difference on the Issue 3 board is that the resistor packs RP1 and RP3 are replaced by individual resistors R35–R42 and R43–R47. Always confirm against your own board's silkscreen.
• If a composite video modification is fitted, the video output stage round C1 and TR1 may have been altered. Work from the modification's own documentation, not from this table.
• See Capacitor Failure Symptoms for general failure modes, and Battery Explosion, Capacitor or Corrosion Damage if there is corrosion to clean up.

Related pages

• Sinclair ZX81
• Sinclair ZX81 General Maintenance
• Sinclair ZX81 Troubleshooting Guide
• Sinclair ZX81 Service and Assembly Manual
• Sinclair ZX80 Capacitor Replacement Guide
• Capacitor Failure Symptoms