STEREO SOUND FOR SPECTRUM + 128

The objective of this modification is to provide a quality ACB stereo sound at +128 and integrate its output into the original RGB/PERITEL connector avoiding any modification on the original case. The solution is based on replacing the original sound circuit with a new one that will substantially improve the original sound quality of the equipment.

In this article we will discuss the Spanish version developed by Investronica (board rev. 2 & 2-1). The English version can also be modified with the same circuit but the modifications in the motherboard as well as in the RGB/PERITEL connector are different.

COMPONENTS LIST

2 x Capacitor 33pF
2 x Capacitor 10nF
2 x Resistor 470ohm
2 x Resistor 100K
4 x Resistor 10K
2 x Resistor 1K

BOARD MODIFICATIONS

Remove the computer motherboard to start. First we must cut some tracks:

Cut track between pin 1 to 4 on IC32.
Cut track between pin 4 to 5 on IC32.

IC32 track cuts

Pins 1,4 and 5 on IC32 correspond to the three sound channels that the AY-3-8912 can generate simultaneously. With this we will be able to break the union of the outputs of the three channels that turned the sound into mono. It will be here where we will connect the new sound circuit.

We must also remove some resistors from the board to isolate the original sound circuit:

Remove R114 resistor.

Resistor R114

Remove R115 resistor.

Resistor R115

Remove R134 resistor.

Resistor R134

CONSTRUCTION OF NEW SOUND CIRCUIT

Circuit diagram


*Printed circuit (component side)*	*Printed circuit (solder side)*

Circuit ubication inside +128

CONNECTION OF THE CIRCUIT TO THE BOARD

Now we must connect the sound input of our assembly to the sound output of the AY. We need 3 integrated signals and ground.

We will connect pins 1,4 and 5 to the corresponding inputs of our assembly. The best solution is to solder directly over the chip pins on the components side. We will use pin 6 of the integrated as ground for our circuit. These are the only soldering we will have to do with a little more care.

AY output connections

At this time any sound generated by the AY will enter into our circuit directly. To add the BEEP signal we will connect the input marked as BEEP in the connection diagram with the output of the resistor R115 previously removed.

R115 resistor connection

RGB/PERITEL CONNECTOR MODIFICATIONS

Originally there is no audio signal in the RGB connector. We need two pins to divide the output into two channels. Pin 1 is originally empty so can be used for one of the audio channels. The second pin we will use will be pin 3 that originally provides the Composite Video signal.

We lose the ability to visualize the Spectrum thought Composite Video although it can be easily added removing the RF modulator and using its connector for it. It should be remembered that due to an error in the board reviews of the Spanish model, this output produces a black and white image, which drastically reduces its utillity.

However, we still have the available Composite Synchronism output available to connect the Spectrum by RGB and with this modification, with audio signals integrated in the connector. On this way we can build a SCART cable without the typical hose with the jack connected to the MIC and with excellent image and sound quality.

The operations that we must perform to connect the output to RGB/PERITEL are the following:

Left output to RGB/PERITEL pin 1.
Right output to R132 resistor R132 previously removed.
GND to RGB/PERITEL pin 2.


*Right output connection*	*GND and Left* *output connection*

Here we can see the original pinout of the connector and the result once the modifications have been made.


*RGB/PERITEL Original*	*RGB/PERITEL Modiffied*

Final aspect of the board with the new audio circuitry installed.

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