ZX81 power supply

[dougle25]

Hi I have a ZX 81 power supply UK700 .

Can any body tell me what the output should be but NOT plugged in to a ZX81 .

The one i have is given out 16 V DC .

I know that a ZX81 needs 9v

Dave

[TomD]

I believe the voltage drops under load, what load did you test this with?

Also as much as it is nice to keep these original I would seriously replace the PSU with a modern one. Not the easiest to find so I got a 9V one cut the cable and put a mono jack on it. Works great just be careful with polarity.

[1024MAK]

Hi I have a ZX 81 power supply UK700 .

Can any body tell me what the output should be but NOT plugged in to a ZX81 .

The one i have is given out 16 V DC .

I know that a ZX81 needs 9v

Hi Dave,

That’s normal. That type of power supply is unregulated. It only produces about 9V when the load draws 700mA (it’s rated maximum current) from it.

When connected to a ZX81, the 9V nominal voltage will be between about 10.5V to 12V.

The 7805 voltage regulator in a ZX81 can manage these voltages okay.

Mark

[dougle25]

Thanks mark

I should have asked it does not have a 9v plug on the power supply can you recommend one.

Dave

[1024MAK]

For use with a ZX81, it needs to be a 3.5mm mono jack plug. Like the one second from left (above the “2” on the ruler) in this picture:


The positive needs to be wired to the tip (signal) contact and the negative to the GND/screen/shield connection.

Mark

[dougle25]

Thanks Mark

You have been a great help

Dave