Why was Sinclair selling UK model ZX81s in the USA?

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VWestlife
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Why was Sinclair selling UK model ZX81s in the USA?

Post by VWestlife » Thu May 17, 2018 10:46 pm

Hello all. I've been active on other vintage computer forums but this is my first post here because I have a very specific question and this looks like the best place to ask it.

When you look on U.S. eBay there's usually about a 50/50 mix of U.S. and UK Sinclair ZX81s for sale (from North American sellers). Maybe some of the UK models were personally brought over by British expats, but that can't possibly be the case for all of them, especially since many of the UK models are bundled together with a U.S. power supply, U.S. RAM expansion module ("ZX81 USA" printed on the back), and U.S. literature. That was the case with my ZX81 that I got on eBay a few years ago -- the computer itself is a UK model but everything else is from the USA, as shown in this video.

Just a little clarification: in addition to being rebranded and sold by Timex in the USA as the Timex-Sinclair 1000, the ZX81 was also sold via mail order from Sinclair Research in Nashua, NH. The U.S. model is distinguished from the UK model by having a channel 2/3 switch underneath for its RF modulator, whose output jack is towards the rear of the case, while the UK model's RF output is closer to the other jacks. The U.S. model also has the New Line button marked as Enter and the Rubout button marked as Delete. And on the bottom it has "ZX81 USA" printed on it, with an FCC ID.

Now here's the smoking gun: a UK model ZX81, in the original box, with the original shipping label, showing it being shipped from Sinclair Research in New Hampshire to a U.S. customer:
https://www.ebay.com/itm/112994034031

So the question is, does anybody know why Sinclair was selling UK ZX81s in the USA? The closest thing I've come up with is a letter I saw of Sinclair explaining to a U.S. customer who had bought a ZX81 kit that they had run out of kits and had instead shipped them a fully assembled machine at no extra charge. So is it also possible that during this time of peak demand for ZX81s, Sinclair was also selling UK models to U.S. customers as a substitution?

Back in the day of manually tuned TVs, it was no big deal to use a UK ZX81 with a U.S. TV set, because the UK channel 36 (which virtually all British computers use) matches up with U.S. UHF channel 34, and with an adjustment of the V-Hold knob, you can get an old TV to sync up to the 50 Hz refresh rate instead of the 60 Hz used by U.S. TV signals. (In this case the difference in color subcarrier frequency between PAL and NTSC is irrelevant because the ZX81 is of course black & white.)

So that's my theory of what was going on -- but I'd love to hear from anyone who either has proof of it, or an alternate explanation. Did anyone here in the USA or Canada order a ZX81 from Sinclair Research back in the day and get a UK model? Or know anyone else who did?
Last edited by VWestlife on Sat May 19, 2018 12:48 am, edited 1 time in total.

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1024MAK
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Re: Why was Sinclair selling UK model ZX81s in the USA?

Post by 1024MAK » Thu May 17, 2018 11:22 pm

I can't really answer your question. But I can add another possible theory!
Is it possible that Sinclair was selling UK spec ZX81 machines before the USA spec machines were in production? It is certainly true that Sinclair was selling ZX81 machines in the USA before Timex did a deal with Sinclair.

It is also true that the Sinclair issue 3 PCB was used in the UK, parts of Europe and some USA machines.

It may be possible to tell if someone who has one can take some photos of the PCB showing the text on the chips and on the modulator.

Mark

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Re: Why was Sinclair selling UK model ZX81s in the USA?

Post by spodula » Fri May 18, 2018 11:49 am

The ULA was designed to output both 50 and 60hz signals. Swapping between 50 and 60hz was done tying pin 22 low. (Floating for 50, Low for 60)

From what i gather from the schematics, USA models also required a different modulator, two resistors and a diode.
Edit: It may be that is is only if you needed VHF signals rather than UHF which the UK uses.

As the machine is black and white, the complicated video signal differences, such as Chroma et al, can be ignored. (PAL and NTSC are really different ways of how Colour is represented and dont really apply to B&W signals)

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VWestlife
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Re: Why was Sinclair selling UK model ZX81s in the USA?

Post by VWestlife » Fri May 18, 2018 2:47 pm

1024MAK wrote:
Thu May 17, 2018 11:22 pm
Is it possible that Sinclair was selling UK spec ZX81 machines before the USA spec machines were in production?
I don't think so, because Sinclair had already been selling NTSC ZX80s in the USA (mostly in kit form) before the ZX81 arrived.

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