ZX hardware - Adding TV-Speaker Audio Output to the ZX Spectrum

Sound on the TV

You can also listen to the sound through the speaker built into the TV set. To do this, you need to make a small modification to the computer.

Although this modification may look very simple, I do not recommend it to anyone without the necessary knowledge and instruments. The oscillator circuit is the most sensitive part. If you do not understand it and have no means of tuning it to the correct frequency, do not attempt this modification. The LM1889 integrated circuit, fitted in most Spectrums (other types of this circuit have also been used as replacements), is responsible for generating the picture.

However, this IC was designed so that sound could also be added to the picture. For reasons known only to them, the computer designers did not use this feature when building the Spectrum 48K. They did use it when building the 128K.

Schematic:

1. Resistor (1 kΩ, 0.25 W)
2. Capacitor (3.9 pF)
3. Capacitor (100 pF)
4. Coil (L = 5.5 MHz)
5. Capacitor (47 pF)
6. Resistor (22 kΩ)
7. To pin 15 (LM1889)
8. To the negative terminal of capacitor C65 (22 mF)
9. To pin 16 (LM1889)

On the computer’s circuit board, pins 14, 15, and 16 of IC 1889 are connected to 12 V. Disconnect pin 15 (pins 14 and 16 must continue to receive 12 V). Add the components shown in the schematic. Tune the oscillator circuit to 5.5 MHz. The built-in speaker can be disconnected, but you MUST put a 1 kΩ / 0.25 W resistor in its place. When everything is connected correctly, switch on the computer and tune the oscillator circuit once more, using its core, for the best sound from the speaker.

The capacitor and resistor values are not critical.

The 100 pF capacitor should simply be of the highest possible quality, preferably polystyrene.

Wind the coil on a former 5–8 mm in diameter, with a ferrite core. The exact number of turns depends on the diameter of the former and the core, so it is not specified—and it does not matter.

The only important thing is that the coil and the 100 pF capacitor resonate precisely at 5.5 MHz.