Voltages from Original ZX Power supplies

lesoleil70

Hi. I've recently picked up a few power supplies in boot sales, one for a ZX81 and another for the ZX Spectrum.

For the first time ever I decided to test the voltage on the jacks since I've just bought a multimeter.

The ZX81 is reading 16vDC and the Spectrum one 12.3vDC.

The face plates say both should be 9v. Has something gone bad over time or is there a tolerance?

Should I change the capacitors inside the power supply?

Thanks for reading.

1024MAK

The Sinclair Power Supply Units (PSU) are unregulated. The output voltage varies as the mains input varies AND also varies depending on how much current is drawn from them.
With no current flowing from the output, the capacitor in the PSU will charge up to a higher voltage than if the power supply is at full load.

The 16V on the ZX81 is about right for an off load voltage.
The 12.3V for the Spectrum one is actually a little lower than I would have expected (assuming it is not connected to a load).

In order to work out if the main smoothing capacitor is faulty, or if one of the rectifier diodes is faulty, you need a test load. I suggest a light bulb.

More details in this thread... http://www.worldofspectrum.org/forums/discussion/51116/testing-zx81-power-supply

Mark

1024MAK

Also see...
http://www.worldofspectrum.org/forums/discussion/48536/spectrum-zx81-psu-strange-voltages
http://www.worldofspectrum.org/forums/discussion/49412/silly-question-i-know
http://www.worldofspectrum.org/forums/discussion/49514/zx81-power-supply
http://www.worldofspectrum.org/forums/discussion/comment/818872/#Comment_818872

Mark

lesoleil70

Thank you very much Mark!