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Sinclair ZX Interface 1 Capacitor Replacement Guide

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Sinclair ZX Interface 1
The ZX Interface 1

The Sinclair ZX Interface 1 has three electrolytic capacitors, all 47 µF axial parts. Two serve the negative supply that the interface makes for its RS-232 drivers, and one sits on the +5 V rail. The rest of the board's capacitors are small ceramics and a 47 nF decoupler.[1]

Capacitor list

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Electrolytic capacitors in the ZX Interface 1
Ref 1985 parts list 4 July 1983 drawing Position
C1 47 µF 16 V, axial 47 µF 16 V Charge pump for the RS-232 drivers' negative rail
C2 47 µF 16 V, axial 47 µF 16 V Reservoir on the negative rail ("−12VDC" on the drawing)
C6 47 µF 16 V, axial 47 µF 10 V +5 V rail

Values: the 1985 parts list and Sinclair's 1983 drawing.[1][2] The drawing gives C6 a 10 V rating where the production parts list gives 16 V; fit 16 V parts. These are the only electrolytics on the drawing and in the parts list.[1][2]

Before replacing them

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The negative rail is easy to check in circuit. With the interface on a working Spectrum, the 1985 manual expects about −7 V at R17, the charge pump's output, and +12 V at R11.[1] A low reading at R17 with good rails from the Spectrum points to C1, C2, D1 or D2 (reasoning from the circuit).[2] The +5 V rail at IC1 pin 40 should be within 0.25 V of 5 V with no visible ripple.[1]

Procedure

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  1. Switch the Spectrum off and remove the interface. Never connect or disconnect it with the power on.[1]
  2. Open the case: five screws in the base, then the two 5/8 in screws that fix it to the Spectrum, then the two screws holding the board to the top cover.[1]
  3. Use anti-static precautions; the board carries CMOS chips.[1]
  4. Note the polarity of each capacitor, remove it, and fit a 47 µF part of at least 16 V with the same polarity.
  5. Inspect the board for solder splashes between tracks and connector pins, as the manual asks after any component change.[1]
  6. Refit the board and refit the two self-tapping screws in their recesses before fitting the base.[1]
  7. Repeat the supply checks above and the tests on Sinclair ZX Interface 1 Troubleshooting Guide.
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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 Thorn (EMI) Datatech Ltd for Sinclair Research Ltd, Servicing Manual for ZX Microdrive, Interface 1 and Interface 2, October 1985 (File:Sinclair ZX Microdrive, Interface 1 and Interface 2 Servicing Manual (Thorn EMI Datatech, 1985).pdf). Table 5.1, ZX Interface 1 PCB assembly: C1 and C2 47 µF 16 V electrolytic axial; C3 100 pF and C4 22 pF ceramic; C5 47 nF +80 % −20 % 50 V axial; C6 47 µF 16 V electrolytic axial; C7 and C8 330 pF; L1 22 µH 0.41 A; D1 and D2 BA157 rectifiers, p. 5.1. Section 2 para. 2 (disassembly), p. 2.1; Section 4 para. 3.3 (supply checks: −12 V rail at R17 reading −7 V; +12 V at R11; +5 V at IC1 pin 40 within 0.25 V with no discernible ripple), p. 4.5; para. 4.1 (desoldering and inspecting for solder splashes), p. 4.8.
  2. ↑ 2.0 2.1 2.2 Sinclair Research, circuit diagram "ZX Interface One", 4 July 1983, clean A3 photocopy, Ferranti Archives 2026 (IF1-SIN-SCH-007). C1 and C2 47 µF 16 V on the RS-232 negative supply with D1 and D2; C6 47 µF 10 V on the +5 V rail.


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