ZX Spectrum Technical Details and Trivia

Connectors

  • an expansion port for increased RAM, communication devices, the ZX Printer, drives (such as the Sinclair Microdrive) or access directly to the Z80A microprocessor - 54-pin male edge connector
  • an RF output socket - standard TV aerial type (UHF frequency 603.25-607.75 MHz/Channel 36)
  • an earphone jack (used for reading programs from cassette) - 3.5mm jack socket - if you use the original cable provided, EAR was light grey, MIC was black. Plug the other ends into the same designated sockets on your cassette recorder (light grey lead to EAR, black lead to MIC)
  • a microphone jack (used for writing programs to cassette) - 3.5mm jack socket
  • a 2.1mm co-axial power supply socket - 9V DC centre-negative @ 1.2A (16K, 48K, +, 128, +2 (grey) models). 6-pin DIN socket for power - +5V @ 2A, +12V @ 700mA and -12V @ 50mA (Spectrum +2A/+2B (both black), +3/+3A)

Additional connectors from 128 only:

  • a socket on the left side (128) or on the back (+2 onwards) for the external keypad (6-pin RJ12, BICC BT Type 603A)
  • an RGB socket (8-pin DIN 45326, female at the computer).
  • a MIDI out port (DIN)

Additional connectors on +2/+2A/+3/+2B/+3B only:

  • two Sinclair Interface 1-compatible joystick ports (9-pin DSUB)

Hover your mouse over the ports for a description

Original 16K and 48K ZX Spectrum

ZX Spectrum +

 

ZX Spectrum +2

 

Technical Facts

Spectrum 16K/48K

  • The RAM chips on many ZX Spectrum boards are 64Kx1 DRAMs, even though the Spectrum addresses them as 32Kx1. It is thought that perhaps Sinclair bought a batch of faulty DRAM chips at a good price, where only half the RAM chip was accessible.
  • Issue 1 boards had 16K soldered onto the main PCB, and if it was the 48K variant, this additional 32K was on a daughterboard that plugged into two sockets at the top/rear of the main PCB. By Issue 2, this daughterboard had been eliminated, with 16K models having 8 DRAM IC sockets on the main PCB to the right of the base 16K DRAM chips, and 48K models typically having these additional DRAM chips soldered directly onto the main PCB. This means you can usually tell if a model was originally sold as a 16K Spectrum and upgraded to 48K: if you have 32K of socketed DRAM rather than 48K soldered directly, it was originally a 16K model.
  • Issue 1 boards have the BASIC ROM chip socketed and the ULA soldered directly onto the PCB. All subsequent board revisions had the ULA socketed and the ROM soldered in.
  • Board issues 3-6A are all 48K models (only Issues 1 and 2 were used in 16K models).
  • All Spectrum models from the original 16K and 48K up to the grey +2 have an onboard DC-DC converter to generate all required voltage rails on the PCB. From the +2A and +3, these were supplied from a new multi-voltage PSU.

Spectrum+

  • The Spectrum+ was designed and released in just 4 months, although it only involved cosmetic enhancements. You could buy a kit to convert your old 16K or 48K ZX Spectrum to a '+', and since the keyboard was much better it was something a good number of owners went for.
  • It's identical internally to the 48K ZX Spectrum but comes with a reset switch that is basically a short across the CPU reset capacitor.
  • You can tell if your Spectrum + is a home-upgrade or purchased as an original Spectrum + by checking for a serial number on the underside. The £50 upgrade pack has a casing with no serial number on it (obviously), or the word 'UPGRADED' is embossed in the location where an original + has a serial number. Furthermore, if it has an Issue 4B or older board you know its an old rubber keyed Spectrum in a + case, because these boards were released before the +.

Spectrum 128

  • The 32K ROM in the Spectrum 128 consists of 16K for the original Spectrum 16K/48K BASIC, and a further 16K that holds the Spectrum 128K BASIC, so it appears as two seperate ROMs to the rest of the system.
  • The tape speed on the Spectrum 128K is 1200 baud.
  • The Spectrum 128K BASIC has a full-screen editor (no more line by line edits), and the user can enter commands letter by letter rather than using key combinations. It also has new commands to support the use of a RAM disk (part of the RAM emulates a disk drive), MIDI out and new sound chip support.
  • Most Spectrum 128s have board revision 6K (c) 1985, but apparently a 9G revision exists that has a 9-pin DSUB for the RS-232 port.
  • Romantic Robot's Multiface 1 became incompatible with the 128 and beyond, so they introduced the Multiface 128 specifically for the Spectrum 128.
  • The Spectrum 128 improved the display quality over the 16K/48K/+ by providing a dedicated socket that outputs both composite video and RGB signals. This RGB output is at 5V TTL level (0V=no colour, +5V=colour on), which means it can only generate one shade of each colour, i.e. no 'BRIGHT' colours. To overcome this, the Spectrum 128 outputs a further TTL signal that indicates whether it's outputting a normal or bright intensity of the colour. There are ways to combine these signals so you can output these signals directly to a SCART socket on a TV.
  • The video cable designed for the Spectrum 128 is not compatible with the Spectrum +2A or +3, as these later models have different signals output by each pin (which ARE directly compatible with a SCART socket). The Spanish Spectrum 128's RGB socket is still an 8-pin DIN but again has different signals to each pin.
  • Unfortunately the Monitor socket on the Spectrum 128 does not provide a sound output. This was introduced with the +2A and +3. Instead you can obtain the mono sound output from the MIC cassette port (3.5mm jack).
  • The Spectrum 128 has a built-in test screen to check colours and shades when tuning into a TV set. To activate this test screen, hold down BREAK while resetting the Spectrum 128. It also outputs a 440 Hz tone every other second to assist in tuning.

Spectrum +2, +2A, +2B

  • The +2 was the first Spectrum to no longer use the one-touch, one-keyword BASIC - all words had to typed in full. This was the default setting in 128 BASIC, but if you chose 48 BASIC, the old one-touch one-keyword was back. This however, was made rather difficult for most users, as the keyboard no longer showed which key corresponded to which keywords.
  • Most +2A boards don't have have the Floppy controller logic. Some of the later +2A boards were in fact complete +3 boards complete with the Zilog FDC (Floppy disk controller) chip, a clone of the NEC µPD765, and some supporting logic installed on the motherboard. This was completely removed in the +2B boards, most likely to save costs. You can upgrade your +2A/B to a +3 on these boards by simply populating the board with the missing chips and adding a floppy drive. The FDC chip is very hard to find although apparently replacement FDCs can be sourced from a WD1772/3 - apparently Atari ST's are a good source for these chips. The upside of the later boards that DO have the FDC logic is that they also come with the +3 ROM, which means you can easily add an 8-bit IDE interface using just a handful of cheap components and a +3e ROM!
  • The Spectrum +2A and +3 support true RGB output so they can be plugged directly into the SCART socket of a TV. These computers don't have the composite video output signal.

Spectrum +3

  • The Spectrum +3 came fitted with a 3" floppy disk drive manufactured by Hitachi, identical to that used in the Amstrad CPC-6128. The specs for this were as follows: single-sided, 40 track, 9 sectors/track, 512 bytes/sector). It stored files on floppy disk in a CP/M compatible file/directory structure.
  • The operating system used in the Spectrum +3 was called "+3 DOS", developed by Locomotive Software who also wrote the BASIC for the Amstrad CPC computers.
  • The 64K ROM in the Spectrum +3 is made up of 4 x 16K pages. This contained the 16K ROM for 48K BASIC, another 16K ROM for 128K BASIC, and a third 16K ROM to hold the +3 DOS firmware.
  • The +3 power supply can be used interchangeably with the +2A/B models, and vice versa. Pins 1 and 2 are +5V @ 2A, pins 3 and 4 are +12V @ 700mA (used to power the video, floppy drive and RS232), pin 5 is -12V @ 50mA (for RS232), and pin 6 is GND
  • The floppy disk controller chip used in the +3 was the same one Amstrad used in their CPC6128 and DDI-1 peripheral. This was the 40-pin NEC µPD765. Other "765"-compatible chips were sometimes used, including Zilog Z0765A08PSC, NEC D765AC-2, NEC D765AC, and the UMC UM8272A. All operate identically.

 

Hover your mouse over the circuit board for a description of the components

Note: The above PCB is an Issue 3 Spectrum 48K board

Component details reproduced with friendly permission from Sothius' Home, www.sothius.com

 

Note: The above PCB is an Issue 2 Spectrum 128 board

 

 

An Issue 2 Spectrum +2A board (note the empty locations for the +3's floppy disk controller chip and logic ICs)

 

Trivia

  • The ZX Spectrum's ROM contains both the BASIC interpreter and the operating system. The BASIC was much enhanced over the ZX81's, including a true ASCII character set (the ZX81's was non-standard), new commands for the enhanced capabilities of the Spectrum, and some new options for existing commands.
  • The ZX Spectrum was called the ZX82 during development.
  • In order to tell an original 16K ZX Spectrum from a 48K ZX Spectrum is almost impossible. The RAM chips on the 48K were in sockets, so a 48K unit can feasibly be 'downgraded' to a 16K unit. A yellow sticker on the underside of the ZX Spectrum should read "16K" on original 16K units. Early models (both 16K and 48K Issue 1 boards) had light grey keys, where later Issue models had light blue keys (as in the main picture on the Summary page) - this change was apparently to improve clarity of the keys under electric lighting.
  • Sir Clive was known to have bought 'regrade' RAM chips where only have the chip worked. These were used in the 'upper' 32K sockets. RAM chips need to be frequently 'refreshed' if they are to retain their contents, even while power is continuously supplied. The 'lower' 16K of RAM is automatically refreshed by the video ULA, since the mere act of reading the display buffer refreshes the the whole 16K. The upper 32K is refreshed by the Z80 CPU.
  • Just as the Spectrum +2 is a mixture of the Spectrum 128 and Amstrad CPC-464, the Spectrum +3 is a mix of the Spectrum 128 and Amstrad CPC-6128.
  • After the launch of the first Amstrad “Sinclair computer”, the Spectrum +2, Amstrad redesigned the motherboard and the content of the ROMs to produce the Spectrum +2A/B and +3. They also made some internal changes on the bus and removed the keypad scanning routines of the Spectrum 128 and +2 (remember the keypad sold with the Spanish Spectrum 128k).
  • As on the Spectrum +2, two BASIC versions are implemented: the 48k BASIC to remain compatible with the original Sinclair Spectrum, and the 128k BASIC which was already introduced with the Spectrum 128. These were held on two separate 16K ROM chips on the PCB.
  • Due to limitations of the Z80 CPU which can only address 65536 bytes, the full 128K RAM in the 128, +2 and +3 is not directly usable (unless using bank-switching routines - a method of moving blocks of memory into and out of the main accessible area of memory), but can be used as a RAM disk (drive M :).
  • You can use the Amstrad FD-1 external floppy drive on the Spectrum +3 as a second disk by connecting it directly to the "DISK B PORT" on the back of the +3!
  • The +2B model was actually launched after the +3, but this was just a minor revision on the already-released +2A.
  • The Spectrum +3e is an aftermarket (non-Amstrad endorsed) enhanced version of the +3 with firmware written by Garry Lancaster, consisting of updated Operating System ROM with many bug fixes, and lots of extra commands added to the +3 BASIC. One of the most important enhancements though, is the support for hard disks! The +3e ROM also allows the loading of .Z80 snapshot files and access +3 disk files through the standard streams and channels interface. Upgrading a black +2A or +3 to a +3e is simple - just install the +3e ROM chips and optionally add the hard disk interface.
  • Because there were still large unsold stocks of Spectrum+ in the UK, the Spectrum 128K was first launched in Spain where the Spectrum models were already very popular. It was sold there with an external numeric keypad to connect to the main unit. This “Spanish model” doesn't boot to a “start menu” as the UK version does (thus Spanish 128K models are slightly different than UK models). After being introduced in UK it had a short market life as it was soon replaced with the Spectrum +2.
  • On the Spectrum +2, the "Tape Tester" option on the startup menu was removed, as the tape recorder was now built-in.
  • Spectrum+ sales outstripped rubber keyboard Spectrums 2 to 1 as soon as it was launched, although some retailers reported failure rates of up to 30% compared to 5-6% for the old model, because the new tactile keyboard still sat on the existing membrane used by the outgoing rubber keyboard.
  • The Spectrum +2B and +3B were functionally similar to the +2A and +3, but electronically the 'B' models improved the generation of audio output signals to resolve problems with clipping, and also removed the external cassette loading input, 'EAR'. Also, unlike the +2A, the +2B motherboard (Amstrad part Z70833) does not have provision for floppy disk controller circuitry, so cannot be assembled as a +3B. The +3B motherboard is Amstrad part Z70835.
  • Zilog Z80 CPUs were made by a number of manufacturers including NEC, Hitachi, SGS, and more. A number of these CPUs were substandard, having a non-working "M1" line on pin 50. Spectrums with these 'faulty' Z80s will function just fine but don't support NMI instructions which are used by many peripherals, including the Interface 1, Multiface, and the DivIDE interface. All SGS Z80s came without M1 support at all, whereas others may just have a very weak M1 signal. It is known that a good number of NEC 780C-1 CPUs are like this and Zilogs are renowned for being the worst - so it seems none of the manufacturers have completely 100% known-good Z80s.
  • Spectrum + and Spectrum 128 (toastrack) keyboards and membranes are 100% interchangeable - the only difference is the Spectrum 128 keyboard has "128K" embossed on it. If you have a potential fault with the keyboard or membrane, you can confirm it's the keyboard/membrane assembly what temporarily swapping it out with one from another computer (either a + or 128).

 

Sinclair Board Revisions

Issue 1 = 16K or 48K. These had 16K soldered onto the main PCB, and if it was the 48K variant, the additional 32K was on a daughterboard plugged into two sockets at the top/rear of the main PCB. Easily identified due to light grey keys. All later issues had blue keys. ~ 60,000 sold.
Issue 2 = 16K or 48K. 32K daughterboard eliminated. 16K models have 8 DRAM IC sockets on the main PCB to the right of the base 16K DRAM chips, and 48K models with more RAM soldered directly onto the main PCB. Introduced late 1982. Still some problems with the ULA but improved due to adding a transistor on top of the CPU. ~ 500,000 sold.
Issue 3 = 48K only. Complete redesign. Production switched to Taiwan. Lower power ULA with self-adjusting colour to eliminate the problems with the poor background colours and difficulties with tuning on Issue 1 and 2 boards. Uprated speaker to make BEEPs louder. Over 3,000,000 sold.
Issue 3B = 48K only. Minor redesign. This board is most commonly found in Spectrum + units.
Issue 4A/4B = 48K only. ULA changed to use the 6C001-7, giving a much cleaner and brighter output to the screen. This board issue is also the most common found to have the faulty CPU M1 line, meaning that the use of Interface 1, Multiface or divIDE interface fails to work (the NMI instruction just resets the computer). Replacing the CPU with one that has a functional M1 line will resolve this.
Issue 4S = Thought to be a 4A/4B manufactured by Samsung, hence the 'S'.
Issue 5 = 48K only. Board design tidied up - Six separate decoder ICs replaced with a single Mullard ULA (ZX8401). A 74LS04 hex inverter chip completed the redesign.
Issue 6 = 48K only. Final issue. Certain capacitors and resistors changed, and some models had a ULA provided by Saga instead of Ferranti.
Issue 6U = 128K only.

Amstrad Board Revisions

Spectrum +2 boards:
Issue 1 = Spectrum +2.
Issue 3 = Spectrum +2. Part number Z70500.
Issue 4 = Spectrum +2B. Part number Z70833. No floppy controller circuitry.

Spectrum +3 boards:
Issue 1 = 1987.
Issue 2 = 1987. Part number Z70830.
Issue ? = +3B board. Part number Z70835.

 

This page was last updated on 6th April 2018.