This is not a cycle-perfect zx spectrum compatible computer and, to be honest, it is not perfect at all. But it works and was able to run all games I've already tried. The intention was to create a computer using mostly technologies of years 1980-1990 so all components are THT except USB-C connector and jack connectors. And, of course, WeACT rp2350b Core board, but it is also kind of THT :) 

No SD cards, no other speed-loading methods. You can only use your cassette recorder or smartphone to load/save programs via audio cable.

However, the video output is VGA and HDMI. VGA output uses 720x576p on 50Hz, horizontal lines are doubled to emulate ZX Spectrum video resolution. 576 lines is less than 625 PAL lines so I had to reduce bottom border to 48 lines instead of original 56.

HDMI is still in progress.

Note that almost everything Spectrum-specific is implemented programmatically in rp2350b firmware. In general you have classic Z80 computer with 16K ROM out of 64K 27c512 chip switchable by DIP-switches, 32K static RAM dedicated to Z80 and 16K static RAM shared between rp2350b and Z80. You can write your own ROM and your own rp2350b firmware to implement any kind of a video controller and input/output functionality. For example it may be a good challenge to use all 16K of shared RAM as a video memory and emulate CGA video modes if you still have CGA monitor. Also there is an expansion connector having all Z80 signals. I used it to connect kempston-like interface for old-style GamePort joystick but it can be used for any external device.  

Software sources and KiCAD projects can be found in my git repository https://github.com/kysilv/z80-rp2350b-zx-compatible

The project is in progress. Initially I made several mistakes in schematic and even in PCB so first version of working device doesn't look very nice. KiCAD projects have been fixed but fixed version is not implemented "in metal" yet.

I will add more project details and keep this page alive.