Spectrum +2A/+3 RGB Differences

The Spectrum +2A and +3 also provides a female 8 pin DIN for its RGB socket and the pin-out configuration is shown below.

Spectrum +2A/+3 RGB Socket Pinout
Pin Signal Level
1- +12V
20 Volts DC 0V
3Audio Out 200mV pk-pk
4Composite Sync 1.2V
5- +12V
6Green 5V
7Red 5V
8Blue 5V

The voltages quoted are for when there is no load resistance connected to the composite sync and colour signals.

The internal circuitry of the RGB socket on the Spectrum +2A/+3 is shown below.

Spectrum +2A/+3 Video Circuitry

The resistors on the red, green and blue lines are 150 ohms instead of 68 ohms as on the Spectrum 128. The composite sync (CSYNC) signal has an inline 470 ohms resistor with 150 ohms resistor to ground, where as the Spectrum 128 just has an inline 68 ohms resistor. The BRIGHT signal is combined with the colour signals internally via 150 ohm resistors and signal diodes. The BRIGHT, composite video (CVBS) and vertical sync (VSYNC) signals are no longer output at the RGB socket, but are replaced with AUDIO OUT and two +12V lines. These +12V lines are delivered through a 1k resistor. This arrangement is very similar to that available from the Spectrum +2 when links LK2, LK3 and LK8 are fitted instead of links LK1, LK4 and LK7.

The Spectrum +2A/+3 RGB colour outputs require series resistors connected else the bright circuitry is not effective and results in the voltage levels at the SCART socket being too high. The circuitry required is shown below. Note that the Spectrum +2A and +3 outputs do not drop with low value loads connected as happens on the Spectrum 128 and +2.

Spectrum +2A/+3 SCART Cable Wiring

The calculation of the resistor value for each colour signal line is as follows:

75 x 5= 0.7hence R=311 ohms

The nearest standard resistor value is 330 ohms and this yields a voltage of 0.68V for the bright shade of colour, and 0.46V for the normal intensity shade.

The calculation of the VIDEO IN signal line is as follows:

75 x 150= 50 ohms
50 x 5= 0.48V

This is directly suitable and so no inline resistor is required.

The monitor socket does not provide signals suitable for driving the BLANKING and FUNCTION SWITCHIING lines. These must instead be derived from an external power supply using a voltage regulator circuit such as that shown above.