Sinclair ZX Spectrum+ Reset Switch Repair
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The Sinclair ZX Spectrum+ added a reset button to the 48K Spectrum.[1] It is a push switch on a lead, pressed into a recess in the left-hand side wall of the case bottom, and its two wires are soldered across capacitor C27 on the main board.[2][3] On the Issue 3 diagram C27 is the 1 µF capacitor on the Z80's RESET line (IC2 pin 26), so shorting it with the switch pulls RESET low.[4] The position of C27 varies between board issues, and later boards have pads set aside for the switch; Sinclair's Issue 6A diagram shows an "optional reset switch" connection at C27.[2][5]
Symptoms
[edit | edit source]| Symptom | Cause in the sources |
|---|---|
| A random pattern on the screen in place of the copyright message | The reset switch fitted wrongly or shorting out[2] |
| The button does nothing | A lead that has come loose from C27; Sinclair warns that leads wound round C27's legs without solder can come loose and damage the computer[2] |
| Keys or the reset lead trapped when the case closes | Leads caught between the case halves or behind the support post[2] |
A machine that resets by itself with the switch disconnected has a different fault; Sinclair's first check for self-resetting is TR6.[6] Crashes and resets when the machine is moved are usually the power plug or socket.[7]
Tools and parts
[edit | edit source]- A No. 1 cross-head screwdriver.[2]
- A fine-tipped soldering iron of about 15 W to 25 W.[2]
- A replacement push switch with its leads, if the original has failed. No part number is given in the sources.
Procedure
[edit | edit source]Open the case
[edit | edit source]- Turn the Spectrum+ face down and remove the eight screws in the base: seven 8 mm pan-head screws round the overhang of the top, and a longer 13 mm countersunk screw at the back next to the expansion slot, which may be under a warranty sticker.[2][8]
- Turn the machine over and lift the top from front to back. Pull the two keyboard ribbons straight out of their sockets, holding them close to the board.[8]
- The reset switch is pressed into its recess in the left side wall of the case bottom; ease it out with its leads.[2]
Check the switch and its wiring
[edit | edit source]- Look at the two leads where they are soldered to C27 or to the pads for the switch. Check that both joints are properly soldered, since a lead wound round C27 without solder can work loose, and that no solder has spread across neighbouring tracks.[2]
- With the power off, check the switch with a meter: it should read open circuit when released and closed when pressed. A switch that reads closed when released holds the Z80 in reset.[4]
- Check the leads for damage where they pass the support post and the case edges.[2]
Refit or replace the switch
[edit | edit source]- Find C27 for your board issue from Sinclair's drawings; on later boards use the pads provided for the switch.[2][5]
- Solder the bare ends of the switch leads to the wire ends of C27 or to the pads. Do not let solder spread across the board.[2]
- Press the switch into the recess in the left-hand side wall of the case bottom, making sure the leads are not trapped behind the support post.[2][3]
Close the case
[edit | edit source]- Reconnect the keyboard ribbons, then lay the top on the base and check that neither the ribbons nor the reset lead are trapped between the two halves.[2]
- Refit the seven 8 mm screws round the overhang and the 13 mm countersunk screw at the back. Do not overtighten them; the plastic of some cases strips easily.[2][8]
Testing
[edit | edit source]- Power up: the copyright message should appear normally.[2]
- Press the reset button: the machine should restart to the copyright message, as at power-on.[4][2]
Related pages
[edit | edit source]- Sinclair ZX Spectrum+
- Sinclair ZX Spectrum+ Keyboard Membrane Replacement
- Sinclair ZX Spectrum+ Troubleshooting Guide
- Sinclair ZX Spectrum+ Upgrade Instructions
- Sinclair ZX Spectrum Z80 CPU Replacement
References
[edit | edit source]- ↑ ZX Spectrum Service Manual, Sinclair Research Ltd, Supplement No. 2, section 10.1 (p. 57). Hosted as Sinclair ZX Spectrum Service Manual (PDF).
- ↑ 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 2.10 2.11 2.12 2.13 2.14 2.15 2.16 2.17 Sinclair Research Ltd, ZX Spectrum + Upgrade Instructions, undated leaflet, 8 pages: Step 1 (dismantling), Step 3 "Fitting the reset switch" with board drawings, Step 6 (case-bottom assembly and final screws) and the problem page. Hosted as Sinclair ZX Spectrum+ Upgrade Instructions (PDF).
- ↑ 3.0 3.1 ZX Spectrum Service Manual, Sinclair Research Ltd, Supplement No. 2, section 10.2 "Conversion Procedure – Spectrum to Spectrum+" (p. 57): the reset switch across C27 and pressed into the side-wall recess. Hosted as Sinclair ZX Spectrum Service Manual (PDF).
- ↑ 4.0 4.1 4.2 ZX Spectrum - Issue 3 circuit diagram, Sinclair Research Ltd: C27 (1 µF) on the RESET line at IC2 pin 26. Hosted as File:Sinclair ZX Spectrum Issue 3 circuit diagram.png.
- ↑ 5.0 5.1 ZX Spectrum (Issue 6A) Circuit Diagram, Sinclair Research Ltd: the optional reset switch connection at C27. Hosted as File:Sinclair ZX Spectrum Issue 6A circuit diagram.png.
- ↑ ZX Spectrum Service Manual, Sinclair Research Ltd, section 6.4.4 "Self-resetting" (p. 32). Hosted as Sinclair ZX Spectrum Service Manual (PDF).
- ↑ Ian Worsley, "Repairing Spectrum & Spectrum Plus", from the Lil Old Sinclair Technical Information Repository, republished by World of Spectrum (retrieved 2026-10-25).
- ↑ 8.0 8.1 8.2 "Replacing the Keyboard Membrane – Sinclair Spectrum+ and 128", DataServe Retro (retrieved 2026-10-25): the eight base screws, lifting the top and removing the ribbons, and soft case posts.
