Sinclair ZX Spectrum+

The Sinclair ZX Spectrum+ (usually written Spectrum Plus) is a restyled version of the 48K ZX Spectrum, released by Sinclair Research in October 1984 at £179.95.

Sinclair ZX Spectrum+

Sinclair ZX Spectrum+ (1984)
Specifications
Manufacturer	Sinclair Research Ltd
Type	Home computer
Released	October 1984
Discontinued	1986
Intro price	£179.95 (1984)
CPU	Zilog Z80A @ 3.5 MHz
Memory	48 KB RAM (16 KB lower + 32 KB upper)
Storage	External cassette tape (EAR / MIC)
Display	256×192 pixels, 32×24 characters, 15 colours
Sound	Beeper (ULA driven, single channel)
Dimensions	32 cm × 15 cm × 3 cm
Weight	0.55 kg
OS / Firmware	Sinclair BASIC in 16 KB ROM
Predecessor	Sinclair ZX Spectrum
Successor	Sinclair ZX Spectrum +2
Codename	—
Model no.	Issue 4A, 4B, 4S, 5, 6A boards

Electronically it is the same machine as the late 48K rubber-key Spectrum. The changes are to the case and keyboard: the Spectrum+ adopts a larger enclosure styled after the Sinclair QL, with an injection-moulded keyboard of 58 keys in place of the rubber mat, and — for the first time on a Spectrum — a reset button.

Because the electronics are unchanged, almost everything written about servicing the 48K Spectrum applies directly to the Spectrum+, and vice versa. The differences that matter in practice are the keyboard assembly and the reset switch.

Which board is inside?

The Spectrum+ case was used with several different 48K board issues, and knowing which one you have matters for fault-finding:

48K board issues found in the Spectrum+ case

Issue	Notes
4A / 4B	The first issue widely available in the Spectrum+ case. Uses the 6C001E-7 ULA, incorporates earlier DC-DC converter service modifications directly on the PCB, and routes /RAS through spare inverters to delay the signal — which is why Issue 4A and 4B boards will not work with IC24 removed, unlike earlier issues. Some were made on yellow SRBP laminate. Includes provision for the aborted US FCC certification (modulator channel switch, EM suppression coils, case grounding points).
4S	Samsung-manufactured, almost identical to Issue 3B (it does not route /RAS through IC24). Green solder mask both sides, extra solder pads for alternative components. Serial numbers begin S01. The keyboard membranes supplied with these machines are of noticeably better quality and many still work today.
5	Only about 1,000 made, and none were known to survive until recently. Has a layout error requiring a PCB modification and patch wiring. First issue to use the ZX8401/PCF1306P, which replaced six 74-series logic ICs but introduced timing problems needing an extra 74LS04.
6A	The final 48K PCB issue — effectively a bug-fixed Issue 5. Supports the AMI SAGA-1A ULA as an alternative to the Ferranti part, though only one surviving machine with an AMI ULA is known. Has dedicated provision on the PCB for the reset switch, instead of the switch being wired across C27 as on earlier issues fitted in Spectrum+ cases.

A Spectrum+ Issue 6A board in its case. The Astec modulator is top left, the 7805 regulator and its heatsink centre, with the ULA, Z80A, ROM and the two rows of RAM below. The white wires are the reset switch.

Architecture

The Spectrum+ is a 48K Spectrum, so:

• CPU: Zilog Z80A at 3.5 MHz (IC2).
• ULA: the Ferranti Uncommitted Logic Array (IC1) generates video, handles the keyboard scan, tape I/O and RAM multiplexing. On Issue 4A/4B this is the 6C001E-7.
• ROM: 16 KB containing Sinclair BASIC (IC5).
• Lower RAM: 16 KB of 4116 DRAM (IC6–IC13). These need three rails — +5 V, −5 V and +12 V — which is why the on-board DC-DC converter exists and why it is such a common source of faults.
• Upper RAM: 32 KB (IC15–IC22), using 4164, TI 4532 or OKI 3732 devices selected by links on the board.
• Colour encoder: LM1889 (IC14).
• Clock: 14 MHz crystal X1, divided in the ULA; the Z80 receives 3.5 MHz.

The Spectrum's split-bus arrangement means external peripherals can drive the keyboard lines, a behaviour later lost on the +2A/+3 machines.

The DC-DC converter

The 4116 lower RAM requires −5 V and +12 V, which the Spectrum generates on-board from the single 9 V input using an inverter built around TR4 (ZTX650), TR5 (ZTX213) and a small coil.

This circuit is the single most failure-prone part of the machine, and it is the reason so many Spectrums present with a white or patterned screen. It was progressively improved across board issues — Issue 3B was a minor revision specifically to improve DC-DC converter reliability, and Issue 4A incorporated the earlier field modifications directly onto the PCB.

Ports

Spectrum+ rear panel: expansion edge connector, MIC and EAR sockets, RF output and the 9 V power input.

• Expansion edge connector — the full Spectrum bus, carrying +5 V, +9 V unregulated, +12 V and −5 V as well as the data, address and control lines.
• EAR and MIC 3.5 mm jacks for cassette load and save.
• RF output to a television via the internal Astec modulator.
• Power: 2.1 mm barrel jack, centre negative, fed from the external 9 V DC supply.

Keyboard

The Spectrum+ keyboard is a moulded 58-key assembly over a membrane, held down by a backing plate with nine screws. The matrix is unchanged from the rubber-key machine: eight blocks of five keys horizontally and five blocks of eight keys vertically. If a block of five fails, the fault is in the KB2 circuitry or the 8-way membrane tail; if a block of eight fails, it is KB1 or the 5-way tail.

Two faults are specific to the Spectrum+ keyboard assembly and are covered in the Sinclair ZX Spectrum+ Troubleshooting Guide: dead keys on the bottom row caused by the clamped ribbon, and a key that repeats continuously because the backing plate is over-tightened.

Power supply

The Spectrum+ uses the same external supply as the 48K Spectrum: a simple mains transformer, rectifier and smoothing capacitor delivering 9 V DC (unregulated) through a 2.1 mm centre-negative plug. On the board, a 7805 regulator on a heatsink produces +5 V, and the TR4/TR5 inverter derives −5 V and +12 V for the lower RAM.

The manual's expected readings are +9 V DC ± 2.0 V at the regulator input (below +7 V the regulator may not operate correctly) and +5 V DC ± 0.25 V with no discernible ripple at its output.

Circuit diagram

Main board	Circuit diagram (48K)

The circuit diagram is from the ZX Spectrum Service Manual and applies to the Spectrum+, which uses the same 48K board.

Successor

The Spectrum+ was followed by the Sinclair ZX Spectrum 128 in 1985 and, after Amstrad bought Sinclair's computer lines in 1986, by the Sinclair ZX Spectrum +2.

General maintenance

Main article: Sinclair ZX Spectrum+ General Maintenance

Priorities are the keyboard membrane and its clamped ribbons, the 7805 and its heatsink, and the health of the DC-DC converter. See the Sinclair ZX Spectrum+ General Maintenance guide.

Troubleshooting

Main article: Sinclair ZX Spectrum+ Troubleshooting Guide

The screen pattern at power-on identifies the failing subsystem, and there is a documented POKE-based procedure that names the exact failed RAM chip. Most real-world faults trace to the DC-DC converter, the lower RAM, the ULA or edge-connector damage. See the Sinclair ZX Spectrum+ Troubleshooting Guide.

Capacitor replacement

Main article: Sinclair ZX Spectrum+ Capacitor Replacement Guide

Sinclair published a component-level parts list for the Spectrum, so the capacitor designators and values are documented. See the Sinclair ZX Spectrum+ Capacitor Replacement Guide.

See also

• Sinclair ZX Spectrum — the original rubber-key machine
• Sinclair ZX Spectrum +2 — the Amstrad-built 128K successor
• Sinclair ZX Spectrum Keyboard Membrane Repair
• Sinclair QL — the machine whose styling the Spectrum+ case borrowed