MACHINE REFERENCE

Sinclair ZX Spectrum 128

Name: Sinclair ZX Spectrum 128
Brand: Sinclair Research, developed with its Spanish distributor Investrónica (part of El Corte Inglés). Amstrad bought the Sinclair brand in 1986 and made the +2 afterwards

The Sinclair ZX Spectrum 128 carries its power regulator on the outside of the case: a ribbed aluminium heatsink along the right-hand end, which is where the ‘toastrack’ nickname comes from. Sinclair Research built it with its Spanish distributor Investrónica, showed it at SIMO 85 in September 1985 at 44,250 pesetas, and released it in the UK in January 1986.

Why it matters

The 128 took the Spectrum from 48 KB to 128 KB of RAM on a Z80 that can only address 64 KB. The trick was bank switching: 80 KB paged in as eight 16 KB banks, with two 16 KB ROMs swapped in and out. It also added the AY-3-8912 sound chip for three-channel audio, an RS-232 and MIDI-capable port and an RGB output. At £179.95 it was the last Spectrum Sinclair Research launched before Amstrad bought the brand in 1986, and the +2 followed at £149.

What to check before you buy

Ask whether the 7805 regulator on the heatsink has been replaced or reflowed. It is rated at 1 A and the board asks a lot of it.
Test the sound on the AY chip against the beeper. A large volume gap is common and fixable with one resistor.
Check the RGB and RF pictures for jailbars, ghosting or a very faint image.
Confirm the power supply is the correct 9V DC, negative-centre unit. A wrong-polarity supply is a known way to kill the regulator.
Load a 128-only game to prove the extra RAM banks work, and run the menu options (128 BASIC, Calculator, 48 BASIC, Tape Loader).
Press every key on the rubber-and-plastic keyboard, and look for lifted or worn membrane tracks.

Photo: Bill Bertram, Wikimedia Commons (CC BY-SA 2.5)

Released

1985

RAM

128 KB: the original 48 KB plus 80 KB paged in as eight 16 KB banks at the top of the address space. Also provides RAM disc commands

CPU

Zilog Z80A at about 3.5469 MHz (Centre for Computing History; Wikipedia says only 3.5 MHz)

Launch price

£179.95 in the UK, January 1986 (Wikipedia gives £179). 44,250 pesetas in Spain, 1985

Manufacturer

Sinclair Research, developed with its Spanish distributor Investrónica (part of El Corte Inglés). Amstrad bought the Sinclair brand in 1986 and made the +2 afterwards

Form factor

Home computer in a single case with an integrated keyboard. Spectrum+ styling (black, rubber-and-plastic QL-style keyboard, red “128” lettering) with a large ribbed aluminium heatsink on the right-hand end, which gave it the “toastrack” nickname. About 320 x 150 x 45 mm (Centre for Computing History)

CPU

Zilog Z80A at about 3.5469 MHz (Centre for Computing History; Wikipedia says only 3.5 MHz)

Chipset

A Ferranti ULA (video, memory, keyboard and beeper logic, revised for the 128) plus the General Instrument AY-3-8912 sound chip. The exact ULA part number was not confirmed

RAM

128 KB: the original 48 KB plus 80 KB paged in as eight 16 KB banks at the top of the address space. Also provides RAM disc commands

ROM

32 KB: the original 16 KB 48K BASIC ROM plus a new 16 KB ROM holding the 128K BASIC and editor, bank-switched

Graphics

256×192 pixels with attribute colour: 8 colours in normal and bright tones (15 distinct colours in all, since bright black is still black). RF output as before, plus a new RGB monitor port. Attribute clash is unchanged

Audio

AY-3-8912 three-channel sound chip (through the TV or monitor rather than an internal speaker) as well as the ULA beeper. MIDI compatibility through the AY output. BASIC gained the PLAY command

Storage

Cassette tape via the usual EAR/MIC sockets. No built-in tape or disc drive. The 128 also has a volatile RAM disc

Ports

RF TV output, 7-pin DIN RGB monitor output, RS-232 serial/MIDI port, external keypad connector, expansion edge connector (some lines changed from the 48K), EAR and MIC tape sockets and DC power input

Sinclair BASIC. 128K BASIC has a menu (128 BASIC, Calculator, 48 BASIC, Tape Loader) and an improved editor. A SPECTRUM command drops into 48K mode, and there is no command to switch back short of a reset

PSU

External 9V DC supply, negative centre pin. Sources agree it is the higher-current unit (about 1.85 A, sold as the UK1850) rather than the 1.4 A supply of the 16K/48K/Plus. Inside, a 1 A 7805 regulator mounted on the external heatsink produces the 5V rail, and the board also has a 12V rail used by the RGB output. The exact current comes from retailer and enthusiast pages, not the original manual

Units sold

Unknown. The only figure found is “around 30,000” from the Centre for Computing History, with no source given and no clarity on whether it covers Spain, the UK or both. In Spain, Investrónica kept selling versions until 1991, after Amstrad brought a lawsuit

Successors

Sinclair Spectrum +2 (Amstrad, 1986, grey case with built-in Datacorder, £149, technically almost identical), then the +3 (1987, built-in 3-inch disc drive) and the +2A/+2B (1988 onward, using the +3 board)

1 A regulator running near its limit

The 7805 is bolted to the external heatsink and is rated at 1 A, which is close to what the board draws. A common fix is a 2 A 78S05 switching regulator. Cracked solder joints on the regulator are also worth reflowing.
AY sound much quieter than the beeper

A balance error means the AY chip's output is far quieter than the ULA beeper. Repair guides swap R115 (near the ULA) for a 1.65 kilohm resistor.
Jailbars on RGB and a weak RF picture

Audio leaks into the video chain and puts vertical jailbars on the RGB picture. One documented fix reroutes capacitor C126 via R130 and TR11. Extra smoothing on the 12V rail also helps, and a failed modulator is another cause of a weak RF picture.

1982 The original Spectrum launches with 16 KB or 48 KB of RAM and a rubber keyboard
1984 The Spectrum+ arrives in October with a QL-style keyboard
1985 The 128 is unveiled at SIMO 85 in Spain in September, priced at 44,250 pesetas
1986 The 128 reaches the UK in January at £179.95. Amstrad buys the Sinclair brand later that year and launches the +2 at £149
1987 The +3, with a built-in 3-inch disc drive, appears at £249. In Spain, Amstrad sues Investrónica to stop 128 sales