I got a working ZX Spectrum 48k (issue 6a) with the original power supply.

Before hooking up the power supply I tested with my multimeter to see if it was good. The power supply specifies it outputs 9V but my multimeter says 15V. To be safe I didn't use it.

Is the power supply malfunctioning? Or is this normal?

3 Answers 3


This is relatively normal for the original PSU.

The supply unit is a "soft" one that will output a much lower voltage under load when connected to the ZX Spectrum. 12 or even more Volts are normal.

  • 5
    If it is an unregulated supply, it is normal, and will output rated voltage under rated load condition. If it is a regulated supply, then it's damaged.
    – Justme
    Nov 11, 2020 at 14:09
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    @Justme - or, differently put, if there is no load (and a multimeter is not a load) then there is no regulation. Pretty standard for most power supplies, particularly from that era.
    – Jon Custer
    Nov 11, 2020 at 15:40
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    And, more to the point, there are regulators inside the computer itself to provide the correct voltages to the logic.
    – Chromatix
    Nov 11, 2020 at 16:14
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    All the Sinclair 9V DC PSUs have a nominal voltage rating of 9V DC. But the output voltage will only be near this value at the units rated full load current. As they are the unregulated type. With no load (no computer connected), the output voltage can be between 12V and 16V. With a computer connected, the voltage should not be below 9V. The typical voltage is between 10.5V and 12V. Nov 12, 2020 at 9:46
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    @JVLobo: Many devices use a "linear regulator" circuit which is designed to take an input which stays between about 8 and 18 volts under whatever load conditions they will face, and will dissipate an amount of heat which is proportional to the amount of current drawn through them times the difference between the input voltage and five volts. At any particular level of current, feeding the regulator 18 volts would cause it to dissipate more than three times as much heat as as 9 volts, and feeding the regulator 18 volts at maximum current would likely cause it to overheat, but...
    – supercat
    Nov 12, 2020 at 16:01

A 48K ZX84 uses between 0.4 and 1Amp (https://www.bytedelight.com/?page_id=3610). A 22.5Ohm resistor will simulate a 0.4Amp load. The nearest standard values are 22Ohm or 24Ohm.

If you have (or can buy) a resistor of this value (and at least 7 Watt power rating) you can use the resistor as a dummy load to check the PSU.

To test on full load you'll need a 9.1Ohm, 16W resistor. NB, the resistance values are for 9V, but the power ratings are for 12V.

  • 1
    A 22 ohm 5% resistor may be anywhere between 20.9 and 23.1 ohms, so a standard 22 ohm resistor might happen to be precisely 22.5. For 1% resistors which are specified to three significant figures, the closest standard values to 22.0 ohms are 21.5, 22.1, and 22.6 ohms; 22.6 ohms is conveniently almost exactly what's needed here.
    – supercat
    Nov 12, 2020 at 16:17

Original PSU is "soft", which means the real voltage depends on current consumption. I.e. 9 volts (nominal) is under the full load. With no load, it could be much higher.

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