Did some work on ZX Spectrum 128 +3 I have. One thing was I wanted to make it work with small “TFT Color monitor” (for a car rear view). This monitor has a composite video input.
First I tried using CSYNK and GND signals for a composite video input. You get the video but it was terrible quality. It seems that “The 128 ZX Spectrums including the +3 (and +2A, +2B) have audio sound modulated onto the video, which reduces the clarity of the composite signal that is available at the modulator input.”(1).
Then I found this document – “Video fixes and video cables
for all ZX Spectrum 128K models”. There is a chapter on Creating composite video-out on a +2A / +2B / +3:
“Although the ZX Spectrum 128K heat sink model and the grey ZX Spectrum +2 have a composite video-out signal on pin 1 of the RGB connector, the ZX Spectrum +2A, +2B and +3 are missing this feature.
Instead of the composite video signal they have a dangerous 12V on pin 1 that will seriously damage your TV or monitor!
But you can add the composite video signal yourself, with the same fairly simple circuit as that is integrated on the heat sink model and +2.
The steps:
1. First remove the 12V from pin 1 by removing resistor R44 (should be the same for +2A, +2B and +3, and very close to the RGB connector).
2. Add this circuit:”
And here is the result:
This circuit does not work at all with my +3. “No signal” reported using my AV2HDMI converter. Every so often the picture will flash on the screen, I have modified the above circuit to the standards needed for all my other timex computers that work well with the AV2Hdmi converter. Alas, I have given up because I believe these converters have problems converting PAL to HDMI.. I have found a similar problem using my SCART to HDMI with any of the Spectrum computers, +2, +2A, and +3. I have two converters I purchased a year ago that work great with all of them( using my LG 49″ TV, and my HDMI input monitor.) However three identical units I just purchased fail to work but for a miscolored fuzzy screen or very dark video that is almost non existent. The version numbers on the internal PCBs has changed from V2 to V1.1, and no one has been able to find an answer, even the guy that makes special peritel/SCART cables for these computers. Here again, I suspect the new SCART to HDMI converters cannot work with a PAL input signal, but the manufacturer failed to own up to it, and are issuing a refund. Very disappointing.