Introduction
This PCB replaces the RF modulatorbox on a Sinclair ZX Spectrum computer. It's main purpose is to improve the video output of the ZX Spectrum by seperating the luma/sync (Y) and chroma (C) signals.
The ZX Spectrum does have a composite signal that goes into the modulatorbox, a simple modification is to cut off the +5 volt to the modulator, disconnect the composite signal from the RF modulator and feed the signal to the RCA jack of the modulator. This results in a signal that is slightly better than RF. This is a common mod and is easy to perform. However, the image has a lot of dot crawl, because the pixel clock is not synchronised with the PAL colour subcarrier.
Composite output works, why designing this board?
Why not? I use a RetroTink 2X (https://www.retrotink.com/product-page/retrotink-2x), this little device converts an analog signal to HDMI output. The Spectrum's composite output is absolutely horrible on this. The RetroTink provides an s-video input, that gave me the idea to seperate the Y/C signals to convert the composite output to s-video, improving the video signal. Some googling later I found this thread on the World of Spectrum forums:
https://www.worldofspectrum.org/forums/discussion/47120/new-video-about-s-video-modification-on-issue-two-48k-spectrum/p1.
Credits to MAK1024 and RGA24 for providing the basics for this design.
I first built it up on a piece of stripboard and did some testing and it worked quite well with my RetroTink. A nice clear picture without dot crawl. It also gives a good picture on a Commodore 1701 CRT.
This PCB makes the modification a bit easier to carry out and it is reversible.