ZX81 review and repair on Adrian's digital basement

[David G]

V I D E O : Is the ZX81 / Timex Sinclair 1000 actually bad? Let's fix these to find out.

4 minutes in and I've already learned two new things:

• ZX81 cases differ. USA model has the video jack farther back than UK model

• Timex/Sinclair 1000 models were made in various countries. He has two 1000s made in Portugal and made in France

ZX81 cases, USA has video cutout in a different place

TS/1000 vs ZX81 USA models

[David G]

I looked up photos on here for difference in the video output location. Since I never had a UK model, this was news to me

created this comparison image

ZX81 USA vs ZX81 UK

RF modulators:

• ZX81 USA model: ASTEC UM 1082 LA2/3. Also used by the Timex/Sinclair 1000 machines

• ZX81 UK model: ASTEC UM 1233

USA modulator uses VHF channel 2 or channel 3. What channel does the UK modulator use?

[David G]

As usual, Adrian is fairly thorough. He compares the boards to each other

Two of the boards generate video signals, but the third does not. He checks for 5V, then check the clock signal, which is 0V. Is it a bad crystal? He swaps the ULAs and it starts generating video. He does all this without a TV, just using an oscilloscope

One machine had a cracked keyboard cable, so he cut it shorter, but it broke again

The other two machines work fine. He added a composite mod to one

I noted from the video:

• ZX81 USA has an ISSUE ONE board, nearly identical to other ISSUE ONE boards but has the word USA silkscreened on it -- and straightened traces

• TIMEX 1000s have ISS 3 boards, same as ZX81 ISS 3 boards. France-made machine has green board, Portugal has red board with yellow silkscreen markings

[stroebeljc]

Thanks for the link, David. Even though he is unfamiliar with these machines, it was a good video.

[rmbrowngr]

It was interesting, at least to me, that the RAM packs do generate noise on the video signal. Never actually having owned a RAM pack back in the day, it came as a surprise.

Also, taking the video directly from the ULA, and then sending it directly to the PiRgbToHDMI, produced such a good picture, maybe this is better than converting the video to composite before connecting it to the RGBToHDMI. Of course this would only work if your ULA produces the back porch, which of course mine doesn’t.

[swensont]

I've been looking into the USA ZX81's and the T/S 1000. I found ebay to be a good source of pictures of the T/S 1000. Besides those built in Portugal and France, I found one built in the UK (I'm assuming the Dundee plant). I've queried the US T/S group for details on their hardware to gather more info in what ULA and ROM versions the US ZX81 and T/S 1000 had. I will probably have a write up in the next ZXzine.

As for Adrian, at the start of the video he was holding a US ZX81 and said that it was introduced in 1982 and cost $100. He was really referring to the T/S 1000. That is inaccurate as the ZX81 was introduced in late 1981 (and I got mine Mid-Nov 1981). To me that was not a small mistake.

Tim