ZX81 Power supply specs

[Philsan]

Hi guys, this is my first!

I've just received a nice ZX81 without PSU and I found that my ZX Spectrum PSU doesn't work because it has a different plug.

Do you think the universal PSU I have is OK, in particular regarding the fact it has 1600 mA instead of 1400 mA (I attach a photo)?
If so, can you tell me which polarity do I have to set? The first of the second one of the attached scan?

Thank you very much.

[gozzo]

Set it for 9v with a positive 'pin' /'centre' polarity and it will probably be ok, - but - if using some 16k rampacks(cheetah, viscount, not sure about the sinclair ones, maybe some others), they may need a slightly higher voltage - about 10 to 11v - to work (the zx81 psu actually produces a bit more than 9v - well, mine certainly did!)You can use a spectrum psu if you like but you will have to make an adaptor lead up, and I think ,If I remember correctly ,the spectrum 'centre' polarity is negative(??) - be careful!

[Philsan]

Thank you.
So no problems with 1600 mA.

But to have a center positive which configuration of the attached scan do I have to make, the upper or the lower one?
I think the lower configuration.

[gozzo]

The lower one - which shows the positive (+) going to the 'dot' in the diagram. Although I said some add-ons need a bit more than 9v, do NOT be tempted to try the 12v setting as it is as this will probably cause the zx81 regulator to overheat, and maybe kill your add-ons, unless you wire in 1 or 2 3amp diodes , 1N5401 or similar, in series with the power supply!

[Philsan]

Thanks again!

No problems with 1600 mA?

[gozzo]

No, thats just the MAXIMUM amount of current you can draw from this PSU. The zx81 uses quite a bit less than this, just don't use a power supply with a current rating LESS than 1.2amp, especially if using the ZX printer or some other add-ons as it will overheat that PSU.

[gozzo]

It should be possible to pick up a genuine zx81 psu eventually as they do turn up on various 'auction' sites now and again - just look for the 1.2 amp version not the older 700mA (0.7amp) version! Unfortunately I cant supply you with one as I only have one 1.2amp type ,and I am keeping it, otherwise if I had a few spare ones I would willingly sell you one!

[gozzo]

Before trying your zx81 out for the first time I would advise carefully opening it up to see if the keyboard membrane 'tails' are ok, and then check if any of the chips are in sockets - if so, carefully remove each chip, squirt the chip pins and the sockets with a good contact cleaner which has a lubricant in it - do not use isopropyl alcohol or similar stuff, though - and replace the chips. - DO NOT get the contact cleaner on the membrane tails, as it may make them crack - unfortunately I ruined my QL membrane this way! Be careful when turning the board over and back or you may snap those membrane tails!

[Philsan]

It works!
Thank you very much!

In fact I have a genuine ZX81 PSU but it's part of a new ZX81 kit that I would like to keep unused.
Moreover it is a 110v version.

If you think my universal PSU is OK I can continue to use it instead of an original 1400 mA PSU.

[Philsan]

Thanks for your tips too.

This ZX81 seems new and I received it from a nice guy.
He tested it before sending.

He offered me the power supply but, having a spare Spectrum one, I tought it was unnecessary...