Metal strap purpose and why don't they all have one?

[Dukes909]

So I am still stuck with my non working ZX81 (viewtopic.php?f=7&t=4043) until I get a second one from eBay to swap parts with. In the meantime I am diddling around with this one and wondering what the big metal strap across the bottom of the board is for. Why do some models have it and not others? I have watched a bunch of yt videos and I haven't noticed it on any others. The solder connection for it near the big heat sink is broken; should I solder it back? (Note I have removed the cardboard piece that was under it to show the solder disconnect).

Cheers
Dukester

[1024MAK]

Very simple answer: in the computers intended for sale in the U.S.A. the inside of the case is sprayed with a conducive coating. The purpose of the metal strap is to connect the computers 0V power rail to the conducive coating to try to reduce the unwanted radio frequency emissions.

Regulations were less strict elsewhere, hence European ZX81 computers don’t have a conducive coating on the inside of the case, hence no need for the metal strap.

Mark

[bola_dor]

Just Like Mark Said..
Beware yours mispositioned.. an lacking an insulator to avoid shorting with the solder points..

[bola_dor]

the insulation part is just a plastic strip

The metal strip is soldered to a point at the edge near the heat sink.

The other tip should be placed to the other side, like this, to reach the other part of the case.

Then both halves of the case are connected to ground.
Silver painting on the case has a rough part where the metal makes contact.