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I have a ZX-81 in my collection, but do not know what revision it is. The board text just says "Sinclair ZX81".

As far as I know, there were 3 revisions of the ZX-81. What are the landmarks I can use to determine which of the revisions I have?

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The easiest way to tell is by looking at the traces on the board.

If they look curvy, as though hand-drawn, it's an issue 1.

If they're in more or less straight lines, it's a 2 or 3.

Seeing as issue 2 boards are exceedingly rare in the wild, it's almost certainly the latter. You can confirm this by looking for the issue number on the board, which should be between the voltage regulator and a bank of resistors.

Edit: Just to confuse matters there are also boards for the US market labeled as "Issue One" with straight traces. These are clearly marked as "Sinclair ZX81" and "USA" by the edge connector though, and should be relatively easy to distinguish from either of the UK boards. Later US models were sold as Timex Sinclair 1000 or 1500 machines.

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