STEREO SOUND FOR SPECTRUM +3

Many of us who at some point have connected our +3 to an amplifier have realized the terrible sound quality of this machine. In fact, if we compare it with the +2 or 128k sound output, the difference in quality is remarkable. The problem in an error in the implementation of the sound circuitry of the first +3 models.

The solution is based on replacing the original sound circuit with a new one that will provide an ACB stereo output that will substantially improve the sound quality of the equipment.

IDENTIFIYNG +3 VERSION

The error is found in the first version of the motherboard (+ 3A). This has a very bad and distorted sound quality. The second version (+ 3B) has the bug fixed but the output is still mono. The corresponding version will be printed on the top of the plate:

Spectrum +3A: AMSTRAD Z70830 ISSUE 2 (c)1987
Spectrum +3B: AMSTRAD Z70835 ISSUE 1 (c)1988

The screen printing of the board components varies between diferent versions. Throughout the entire article the screenprint of + 3A will be used. The only differences in names found in this article are the following:

+3A	+3B
C38	C201
R42	R208
R43	R207
R72	R200

If you make this modification in a + 3B replace the names used by its corresponding in the table. Its located inside the board on a similar location, except R200 which is next to one of the keyboard ribbon connections. All other operations will be identical on both revisions.

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 and 4 of IC11.
Cut track between pin 4 and 5 of IC11.

IC11 track cuts

Pins 1,4 and 5 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 other components:

Remove capacitor C38

Capacitor C38

Remove resistor R72.

Resistor R72

With these modifications we managed to isolate the original sound circuit from the outputs of the AY chip.

CONSTRUCTION OF NEW SOUND CIRCUIT

Circuit Diagram


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

Circuit ubication inside +3

CONNECTION OF THE CIRCUIT TO THE BOARD

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

We will connect pins 1,4 and 5 to the corresponding inputs of our circuit. The best solution is to solder directly over the chip pins on the components side. We will use pin 6 of the integrated as the 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 our circuit directly, but we still have to introduce the BEEP in our circuit. To do this, we will connect the input marked BEEP in the connection diagram with C37 capacitor.

C37 capacitor connection

RGB/PERITEL CONNECTOR MODIFICATIONS

Originally there is a unique audio output signal for a mono output. We need another pin to divide the output into two channels and fortunately the 12v signal is repeated so we can perform the output without losing any of the original connector signals. The operations to modify the RGB / PERITEL are the following: