Many ZX Spectrum +2A/+2B/+3 users connect their machine to TV with an RGB to SCART cable.
SCART however needs a 1 to 3 volt signal on pin 16 to select RGB input instead of composite video.
This is called the ‘BLANKING’ signal.
Now the problem is that there is no proper way of achieving this voltage from the RGB socket of the ZX Spectrum, as there is no suitable voltage that can be used.
The RGB socket has a 12V output, but there is an internal 1K resistor before that output, making it impossible to get any voltage higher than 0.8V for SCART pin 16 (every TV has an internal 75 Ohm resistor, so the 1K inside the ZX Spectrum and the 75 Ohm inside the TV act as voltage divider).
Some TV’s accept this 0.8V as RGB selection, but many don’t, so the RGB mode is not properly selected on the TV.
Often the result is a picture with a lot of so-called ‘ghosting’, as shown here:
There are two ways of solving this:
1. Modifying the ZX Spectrum +2A/+3 by adding a 1K resistor on top of R44, close to the RGB socket.
This will result in a voltage of about 1.5V, for a clear RGB selection on SCART pin 16, the BLANKING pin.
2. Injecting the proper voltage externally to the SCART cable.
You can use an external power supply to achieve the proper 1 to 3V on SCART pin 16, the blanking pin.
To achieve 1 to 3V with this solution, there is still a resistor needed between the power supply and SCART pin 16.
Use one of these resistor values:
With a 12V power supply: 680 Ohm resistor
With a 5V power supply: 220 Ohm resistor
