With a direct current (DC) supply that is derived from the mains alternating current (AC) supply, the DC can be crudely processed or finely processed. What do I mean? Well alternating current flows from the power station to the powered device, then the direction of flow reverses and the current flows back the other way. Because current has to flow in a circle, this does not mean that energy is returned to the power station. Instead power is still delivered to the powered device. It is just the direction that the current is flowing is changing every 20mS, at a frequency of 50Hz (UK power network frequency. If you were able to stop time and trace the waveform of the mains, it would be a sine wave. This is because the power generators are normally rotating turbines.
Power supplies for electronic devices like computers have to convert this AC power to a nice steady direct current with no hint of the AC sine wave that it came from. But practical power supplies have limitations due to cost.
With the ZX81, Sinclair actually used a three stage power supply system. In the Sinclair "9V" power supply unit, there is a transformer to convert the mains voltage down to a lower voltage (and provide isolation from the mains voltage). This is the first stage.
Forming the second stage are some diodes and one or two capacitors. The diodes convert the AC output of the transformer to DC. BUT this DC changes voltage value in line with the changing AC voltage from the transformer
. The capacitor(s) act like a temporary storage tank, saving some of the electricity from the transformer and then when the voltage from the transformer falls, the capacitor(s) supply the DC output for a short time. This is better, but still far from perfect and the voltage will still rise and fall, but not as much. We can detect this voltage change, as it is at double the frequency of the mains AC voltage. We use the AC range on the meter. The meter should have been designed to ignore DC voltage when switched to measure AC voltage.
So to test and confirm that the power supply is working correctly, we need to measure any AC that is still mixed in with the DC power. This is referred to as a ripple voltage. There are two ways of doing this. The best, is to use an oscilloscope. But these are expensive and you need to learn how to use them, as they are complex items of test equipment. On an oscilloscope, you can see the waveform, and it looks a bit like ripples, hence the name.
The cheapest way to see what the approximate amount of ripple is, is to use a multimeter that has AC voltage ranges. The performance of different multimeters varies rather a lot. Generally the more expensive meters have more sensitive AC voltage ranges. However, when testing the output of the Sinclair "9V" power supply unit, most meters will give a useful indication.
Now you may be thinking, where is the third stage? Well it is inside the computer, and is known as a series voltage regulator chip. In the ZX81, it is the 7805 type voltage regulator. This converts the unstable, unregulated, nominal "9V" from the power supply unit, to a nice stable (regulated) +5V DC supply suitable for powering logic circuits including a CPU and memory
I hope that helps
Mark