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.

Pin Signal Level 1 - +12V 2 0 Volts DC 0V 3 Audio Out 200mV pk-pk 4 Composite Sync 1.2V 5 - +12V 6 Green 5V 7 Red 5V 8 Blue 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.

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 there 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.

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

75 x 5	= 0.7		hence R=311 ohms
(R+75+150)

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
(75+150)

50 x 5	= 0.48V
(470+50)

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

The calculation of the BLANKING signal line is as follows:

75 x 12	= 0.84V
(75+1000)

This is below the value required by the SCART specification to select RGB mode but may be enough for some TV sets.

The +12V from the AUX or RS232 socket can be used to drive the BLANKING input at 2.5V using an inline 180 ohm resistor. Alternatively, the +12V from these sockets is sufficient to drive both the FUNCTION SWITCHING line at 10.5V and the BLANKING line at 1.125V by using an inline resistance of 620 ohms, which can be constructed from a 470 ohm resistor and a 150 ohm resistor. A voltage regulator circuit can also be used as shown for the lead circuitry of the Spectrum 128.