Data Cassette Mod for ZX81

To use the tape recorder for storing data from a ZX81, the audio signal has to be extremely clean. There should not be any kind of noise in the audio or else the show is off. Usually the ZX81 will reject any program that has corruption through noise.

After connecting an oscilloscope to the audio output, I noticed a small amount noise from the motor. Hence, this was something that required filtering through a capacitor.

In addition, the DC input to the circuit had a small 25 µF electrolytic capacitor, which was clearly not enough; hence, I decided to solder an additional capacitor in parallel. Since I was going to use this tape recorder to store digital data from the Sinclair ZX81, I decided to sort this out, because it is vital that the audio signal is pure and free of any noise.

Recycled Capacitors

A bag of recycled capacitors is handy for trying out different values. I tried out many capacitors whilst the oscilloscope was hooked to the output, and found some, which filtered out the noise completely.

Decoupling Capacitor

I connected a 1000 µF electrolytic capacitor across the DC power planes, which performs a nice job of smoothing the power.

Suppression Capacitor

The motor noise in the audio signal was clearly due to there not being sufficient suppression of the back EMF noise, or it could be due to a worn out motor. After trying a few different capacitors across the junction of the motor terminals, I found a 0.22 µF capacitor that made a huge difference in suppressing the noise.

Conclusion

With these very simple and small modifications, I was able to make a huge improvement to the data recording reliability of this cassette recorder. I was able to "LOAD" data into the ZX81 without any problems and it always worked the first time round.

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