This is a tested, working project.

PCF1306, ZX8401, Amstrad 40058 (Different names of the same IC), the RAM multiplexing ASIC of ZX Spectrum computers is long obsolete and very hard (almost impossible) to get as new when it fails. This ASIC is actually a combination of 6 standard logic ICs and was designed by Sinclair Research as a cost reduction. It was reverse engineered by Jose Leandro Novellon in 2007. I re-drew and optimized the PCB design based on his open-source schematic. I moved all 6 ICs on top side and kept all the data lines sandwiched between two ground planes by using a 4-layer design, also reduced the PCB size to almost the same size as the original IC, it can be installed on a Spectrum 48K issue 6, Spectrum+ 128K (Toastrack), and Amstrad made Spectrum +2 128K (Grey) without disturbing any other parts. It is totally plug & play, no mods needed on the computers.

All the parts required is clearly indicated on the PCB silkscreen, the passives (6x 100n and 1x 47p) are SMD size 0805, and SOIC logics can either be of 74LS (recommended) or 74HCT/HC type. If HCT/HC ones are used, you need to install a 47p capacitor (pads provided and indicated on PCB) to get the timing right. Although the board is tested with this timing correction needed for faster HCT/HC CMOS logics, I strongly recommend the use of more period correct 74LS TTL logics. When 74LS type is used this 47p timing correction capacitor is NOT needed and its place should be left blank. DO NOT use a mix of LS, HC and HCT parts, all the logics should be of the same type. Use rounded turned pin (sometimes called "precision") type headers, regular square pin headers are too thick to go into the socket on the ZX Spectrum board.

First batch of prototypes in the photos has the name ZX-MUX on them and the board size is bigger, but after the initial testing, the name "ZX2401" was suggested by some members of the Sinclair society, as a homage to the ASICs first part number ZX8401 (because it was designed in January 1984 and I designed this board in January 2024), so it was changed to ZX2401 and reduced the board size to minimum on this final revision you can order now.