Table of Contents

Introduction

This is the Bill of Material for a replica ZX Spectrum 48K Issue 3B. It is optimized for PABB’s ZX Spectrum 48 Issue 3B Redrawn boards.

Read Me First!

If you want to build your own ZX Spectrum, be aware that the machine was designed in the early 1980s.

While almost all of the standard components are still available, some components are rare by now. You will need all of the listed components (except of those marked optional). We recommend that you try to get the components marked as Rare first, so you won’t waste your money on standard components if you fail to get all the rare ones.

This bill of material only comprises of the components required for the mainboard itself. For a complete ZX Spectrum replica, you will need:

• This board, fully assembled and tested.
• A ZX Spectrum or ZX Spectrum+ case with keyboard. You can use an original case, but there are also replica cases, faceplates, and rubber keymats in all kind of colors, as well as new keyboard membranes.
• A power supply with correct polarity: 9 V, 1.5 A, 5.5mm/2.1mm barrel plug, center negative. Most wall power supplies on the market are center positive, and would instantly kill your ZX Spectrum.

If you are new to electronics and soldering, there are easier ways to build your own ZX Spectrum. For example, there are kits for the Harlequin 48K and Harlequin 128K clone that include all components and detailed assembly instructions.

Rare Parts

The easiest way to get these parts is to buy an old or broken ZX Spectrum, and salvage them.

• ULA: Instead of the original ULA, you can use modern replicas like Retroleum Nebula or vRetro vLA82.
• Z80A: After 48 years in production, its end-of-life was announced in 2024. It will probably still be on stock at many retailers for a while, and can also be found as NOS part. If you cannot find one in a DIP-40 package, you can use a Z80 adapter board for other case types. The original clock frequency is 4MHz. A CPU with a higher clock rate can be used as well, but won’t make your ZX Spectrum run faster.
• 4116: This DRAM type is long out of production, but NOS parts are still available. Alternatively there is an SRAM module that replaces all eight 4116 with a single board. With a small hardware modification, you can replace them with 4164 DRAM chips, which will also solve the problem with the coil (see below).
• 4532: This DRAM is actually a 4164 where one half of the memory turned out to be defective on production. If you use 4532 chips, make sure they are of the same brand and type (e.g. taken from the same ZX Spectrum). It’s easier to use 4164 chips as drop-in replacement though, they are still available as NOS parts. You can mix 4532 and 4164 chips.
• ROM: The easiest way is to salvage a ROM from a broken ZX Spectrum. A 27C128 or 27C256 EPROM can be used as replacement, but requires a modification to the hardware.
• LM1889N: This part is long out of production, but can still be found as NOS part. You can use a TI SN94459N as replacement, but this type is even harder to find and the image quality is inferior (align the SN94459N to the top and leave the bottom pins open).
• Transistors: The original ZTX transistors are out of production, but can be replaced by standard types. The replacement types in this list are known to be good, but are not necessarily the only or the best choice. If you use replacement types, check the datasheets for correct orientation! For example, if you use a BC557 as replacement for TR5, it must be reverse mounted (i.e. the flat side must face in the opposite direction).
• Coil: The coil was custom made for Sinclair. There are new coils available on auction platforms sometimes, but they are expensive. The best way is to salvage a coil from a broken ZX Spectrum. If it is defective, it can be repaired easily. Another alternative is to use the SRAM module or 4164 chips like mentioned above, as they do not need -5V and +12V. You could then use a 5V to 12V DC/DC converter to generate the 12V for the LM1889, and leave out the coil. Note that a few expansions might need -5V or -12V, and won’t work properly with this modification.
• Keyboard Connectors: They can be found as NOS part from time to time, and otherwise can be salvaged from a broken ZX Spectrum. There are replacement parts in this Bill of Material list that are still in production, however they are not optimal.
• TV Modulator: The modulator needs to be reused from a broken ZX Spectrum. However, as most modern TVs don’t have a matching input anymore, it can easily be replaced with a simple RCA connector to get a composite output (“composite mod”). You could also use an s-video module, which will enhance the image quality even more (“s-video mod”). Do not place C65 if you use the s-video module.
• Speaker: The speaker should have an impedance of 40Ω, but similar speakers will do as well.
• 7805 and Heatsink: They should be replaced with a modern DC/DC converter, as it stays cool and does not need a heatsink. Since the original ULA gets pretty warm itself, and cannot be cooled with a heatsink in the original ZX Spectrum case for space reasons, any additional heat source should be avoided.

Sockets

This Bill of Material includes sockets for all chips. All sockets are optional, of course. In an original ZX Spectrum, sockets were used as deemed necessary by Sinclair at that time, but usually the ULA was always socketed, while the lower RAM and the LM1889N were never socketed.

Generally we recommend to use precision sockets. However, if you are going to use replica parts (e.g. a replica ULA or a memory board), you should check if they fit into precision sockets, and maybe resort to standard sockets.

If you are going to use the Retroleum Nebula as ULA replacement, all nearby chips (IC3, IC4, IC24, IC25) should be soldered directly to the board, as they would collide with the Nebula board otherwise.

If you want to heatsink the original ULA, you might have to solder it directly to the board for space reasons.

Configuration

Four wire jumpers need to be set depending on the components that were used.

The first set of jumpers can be found to the right of the MIC connector. They need to be configured depending on the type of the upper RAM chips.

• TI TMS4532-NL3: Set the TI and 3 jumper.
• TI TMS4532-NL4: Set the TI and 4 jumper.
• OKI MSM3732H: Set the OKI and H jumper.
• OKI MSM3732L: Set the OKI and L jumper.
• 4164: If you use only 4164 chips, you should set the TI jumper, and one of the jumpers 3 or 4.

Again, note that all eight chips must be of the type that is configured by the jumpers. However, you can mix them with any number of 4164 RAMs.

The other set of jumpers can be found next to the speaker. They configure the manufacturer of the ROM chip.

• Hitachi: Set both H jumpers.
• NEC: Set both N jumpers.

Disclaimer

This is not an official list! It was collected and reviewed by Sinclair enthusiasts, and at least one working board has been built from the components.

Although we strive to make the information in this project as helpful and accurate as possible, it is provided “as is” and without warranties of any kind either expressed or implied.

In other words: You might spend a lot of money, and end up with a non-functioning board or a cardboard box full of useless components.

Use at your own risk!

Contribute

This list is meant to be a community work. Our goal is to have a canonical list that people can rely on when ordering parts for building an own ZX Spectrum mainboard.

However, this list may not be free of errors. If you have found one, please open an issue or send a patch.

License

This project is distributed under the terms of GNU General Public License (GPLv3).