1581 repair
13 January 2023
ZX Spectrum +2 audio-video mod
30 January 2023
Adding the audio input
The Spectrum +2A, made by Amstrad in 1988, has a built-in tape recorder for reading and writing tapes.
If its tape deck is faulty, or we simply want to load our beloved programs from SD or elsewhere, we can add an external audio input and load games from a TZXDuino or similar.
At the bottom right we can see the 3.5mm jack used for audio output when using a composite or RGB video cable instead of the RF signal. This connector is stereo, but the left and right channels are connected together, since the computer outputs a mono signal.
The most practical solution is to use one of these channels as an audio input. To do this you need to desolder the jack connector, cut the trace underneath that joins the two channels, and use the one that is no longer connected to anything as the input.
Alternatively, if we don’t want to damage our Spectrum’s PCB, we can simply bend the jack’s pin (the one on the left seen from above) and insulate it both on the PCB (with insulating tape) and on the jack with heat-shrink, so that it no longer touches the pad underneath, and with a wire we can connect it to the point where the audio goes in.
Now all that’s left is to run our input channel to the + pin of capacitor C200 on the board.
Job done! Or rather, loaded!
Now close everything back up and enjoy being able to connect an external audio device to load programs.
The stereo jack will now have one channel for audio output and the other as input. To use them properly you need a splitter, but you can also build one yourself. First, take a 3.5mm male stereo jack and plug it into the Spectrum’s audio connector. With the multimeter in continuity mode, check which of the pins is connected to the audio output (the other will be the input; the GND is the long one). Now connect a female stereo jack with both channels on the pin checked earlier (the Spectrum’s audio output); GND always goes to GND, and the other free pin of the male jack should be connected to another female mono jack connector (stereo is fine too), with the signal pin on the free one of the male jack and, again, GND to GND (if you use a stereo one, always join the two L+R pins together).
Here’s an example of the splitter: (to find out which wire is the audio output, use the multimeter as described above)
Low or inverted video signal?
Here’s another thing that can happen on some motherboards: the video signal over an RGB SCART cable may be noisy or even appear in negative, with a black background and white characters.
Don’t worry, it’s not a fault; luckily it’s just down to the blanking signal being low, so the TV/monitor may not recognise it to switch into RGB mode.
To fix it, look for resistor R44, which is 1k ohm, and replace it with a 470 ohm one, or put another 1k resistor in parallel with it as in the photo.
Done, the video is now perfect. Watch the video repair of this computer and also of the Spectrum +2 (grey)
