Sinclair ZX Spectrum +2
 | |
| Sinclair ZX Spectrum +2 (1986), the grey Amstrad-built model | |
| Specifications | |
|---|---|
| Manufacturer | Amstrad (under the Sinclair brand) |
| Type | Home computer |
| Released | 1986 |
| Discontinued | 1988 (superseded by the +2A) |
| Intro price | £149 (1986) |
| CPU | Zilog Z80A @ 3.5469 MHz |
| Memory | 128 KB RAM (8 × 16 KB banks) |
| Storage | Integral "Datacorder" cassette deck |
| Display | 256×192 pixels, 32×24 characters, 15 colours |
| Sound | AY-3-8912A (3 channels) plus ULA beeper |
| Dimensions | 46 cm × 18 cm × 5 cm |
| Weight | 1.8 kg |
| OS / Firmware | Sinclair BASIC (128 BASIC and 48 BASIC) in 32 KB ROM |
| Predecessor | Sinclair ZX Spectrum 128 |
| Successor | Sinclair ZX Spectrum +3 |
| Codename | — |
| Model no. | Z70500 Issue 3, Z70700 Issue 1 |
The Sinclair ZX Spectrum +2 is a 128K home computer released in 1986. It was the first Spectrum produced by Amstrad, following Amstrad's purchase of Sinclair's computer product lines in April 1986, and is distinguished from its successors by its grey case and integral cassette deck.[1][2]
Functionally the +2 is a repackaged Spectrum 128. Amstrad engineers reverse-engineered a Sinclair 128 in considerable haste in order to have a machine ready for Christmas 1986 — work that began before the sale of Sinclair to Amstrad had formally completed.[2]
What Amstrad changed
Compared with the Spectrum 128 "Toastrack", the +2 adds:[2][3]
- An integral "Datacorder" cassette deck, replacing the external tape recorder.
- Two joystick ports using the Sinclair (not Atari) pinout, driven by the Amstrad 40057 — essentially the MT62001 used in the Interface 2.
- A conventional grey case with a proper keyboard, in place of the Toastrack's separate heatsink and rubber-key styling.
Everything else — the Z80A, the ULA, the 128 KB banked RAM, the AY-3-8912A sound chip and the 32 KB ROM — is carried over from the 128.[2]
Board revisions
Two boards were used in the grey +2:[2]
| Board | Notes |
|---|---|
| Z70500 Issue 3 | The Taiwanese-built board, manufactured by Amstrad's contract manufacturer Orion. Uses the Amstrad 40058. A slightly later revision is marked "0500 Issue 3" with only minor layout changes and no functional difference. |
| Z70700 Issue 1 | The UK-built board. Functionally identical to the Z70500 but uses mostly axial capacitors and other UK-sourced components, and carries the PCF1306P where the Z70500 has the Amstrad 40058. It is thought this board existed to use up components Amstrad inherited from ex-Sinclair suppliers. |
Board quality is a genuine concern. The PCB quality on the Amstrad-produced Sinclair boards is notably poor, and great care is needed when replacing components to avoid lifting pads or tracks.[2] This is covered in the soldering section of the Sinclair ZX Spectrum +2 General Maintenance guide.
Architecture
| Device | Function |
|---|---|
| Zilog Z80A @ 3.5469 MHz | CPU |
| ULA | Video, keyboard scan, tape I/O and the beeper. Late machines may use a soldered Amstrad 40056 instead of the usual Ferranti part |
| AY-3-8912A | Three channels of sound in addition to the ULA beeper; also provides bit-banged serial I/O |
| TEA2000 | Colour encoder |
| Amstrad 40057 | Dual joystick port interface |
| 128 KB DRAM | Eight 16 KB banks, paged into the Z80's address space |
Sources: Spectrum for Everyone and Tynemouth Software.[2][3]
RAM numbering. This catches people out: many diagnostic tools report faults using 48K Spectrum IC numbers. On the +2, the upper set IC17–IC24 corresponds to IC15–IC22 on a 48K Spectrum, and the lower set IC25–IC32 corresponds to IC13–IC6 — note the reverse order.[3]
Sound balance. As with all the 128K range, the mixing of the AY's three channels against the ULA beeper is not well balanced, and ULA sounds are usually much louder.[3]
RAM contention. On the 128 and the grey +2, banks 1, 3, 5 and 7 are contended and 0, 2, 4 and 6 are uncontended — the opposite of what the official 128 specification stated. Software written against the real behaviour of these machines can therefore misbehave on the later +2A/+3, whose architecture matches the original specification.[2]
Power supply
Like the 128, the +2 takes an external unregulated supply and generates its own rails on board: a 7805 produces +5 V, and an inverter around transistors of the ZTX650/ZTX213 family — the same arrangement used on the 48K Spectrum — produces ±12 V, here used for the video output and the pseudo-RS232 drivers rather than for RAM.[3][4]
The 128's on-board supply derives:[4]
- +5 V from the 7805.
- +12 V from the TR4/TR5 inverter, rectified and smoothed by D15/C44.
- −12 V from a charge pump (C111/C112, D28/D29) off the same square wave.
- −5 V from the −12 V rail via zener D19, for the expansion port.
A working +2 draws roughly 800 mA to 1.2 A depending on the type of RAM fitted, which makes a current-limited bench supply a useful diagnostic aid.[3]
Circuit diagrams
Amstrad did not publish a service manual for the grey +2. Because it is functionally a repackaged Spectrum 128, the ZX Spectrum 128 Service Manual is the closest official documentation, and its circuit diagrams describe the same architecture.
| Circuit diagram (sheet 1) | Circuit diagram (sheet 2) |
|---|---|
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| Circuit diagram (sheet 3) | The machine |
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How the +2A and +2B differ
The black +2A and +2B that followed are not minor restylings of the grey +2 — they are different machines built on the +3 platform, and repair information does not carry across. The key differences:[2]
- They use the Z70830 board (shared with the +3, with the floppy components unpopulated) or the Z70833, a cut-down board with the floppy area physically absent.
- Most of the ULA, discrete logic, PCF and 40057 functions are consolidated into a single Amstrad 40077 gate array.
- The DC-DC converter is deleted. The rails come directly from a new power supply using a 6-pin DIN connector instead of the 2.1 mm barrel jack — so grey +2 and +2A power supplies are not interchangeable.
- RAM changes from sixteen 64 Kbit × 1 devices to four 64 Kbit × 4 devices.
- The split-bus arrangement moves inside the gate array, so external peripherals can no longer drive the keyboard.
- Contention matches the original 128 specification, which is why some software written on real 128/grey +2 hardware sounds wrong on a +2A.
- The Z70830 has a known audio design error causing heavily distorted output, corrected on the later Z70833.
General maintenance
The Datacorder belts, the fragile Amstrad PCB, the keyboard membrane and the grounding of the tape deck are the main jobs. See the Sinclair ZX Spectrum +2 General Maintenance guide.
Troubleshooting
Most real-world +2 faults are the Datacorder, RAM, and the audio hum caused by poor grounding on Taiwanese-built boards. See the Sinclair ZX Spectrum +2 Troubleshooting Guide.
Capacitor replacement
See the Sinclair ZX Spectrum +2 Capacitor Replacement Guide.
See also
- Sinclair ZX Spectrum+ — the 1984 48K machine
- Sinclair ZX Spectrum — the original
- Sinclair ZX Spectrum Keyboard Membrane Repair
References
- ↑ "ZX Spectrum", Wikipedia.
- ↑ 2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 "ZX Spectrum Models", Spectrum for Everyone. Source for the PCB history, the Z70500/Z70700 board details, the reverse-engineering origin, the Amstrad 40057 joystick IC, RAM contention behaviour and the poor PCB quality note.
- ↑ 3.0 3.1 3.2 3.3 3.4 3.5 Tynemouth Software, "Spectrum 128K +2 Grey Repairs". Source for the RAM IC mapping between 48K and +2 numbering, the AY/TEA2000/40057 chip roles, typical current draw, and repair observations.
- ↑ 4.0 4.1 ZX Spectrum 128 Service Manual, Sinclair Research Ltd. Source for the on-board power supply description, the rails and their loads, the expansion-connector supplies, the clock frequencies and the basic-checks procedure. The grey +2 is functionally a repackaged Spectrum 128.