ZX81! Issue 1! 1980!

[VMSZealot]

Yes! 1980! I have a very, very, original ZX81. I bought it to break for spares - but this one is a keeper. It's one of the OGs, and it's entirely working, right down to its horrible, horrible, keyboard. So that's the good news. The bad news is that it's entirely working - I was hoping that it might still have its original ROM, warts and all, but (very sadly) PRINT SQR 0.25 gives 0.5 as the answer. And I was really hoping that it wouldn't. Still, it's nice to have such an original survivor in the collection, complete with manual and (working) PSU. Even nicer that it only cost me a tenner. I still need a donor for spare parts though!

 

[Snial]

Yes! 1980! I have a very, very, original ZX81. I bought it to break for spares - but this one is a keeper. It's one of the OGs, and it's entirely working, right down to its horrible, horrible, keyboard. So that's the good news. The bad news is that it's entirely working - I was hoping that it might still have its original ROM, warts and all, but (very sadly) PRINT SQR 0.25 gives 0.5 as the answer. And I was really hoping that it wouldn't. Still, it's nice to have such an original survivor in the collection, complete with manual and (working) PSU. Even nicer that it only cost me a tenner. I still need a donor for spare parts though!

Amongst my numerous ZX81s, I have a very early one, with the ROM chip fix. It's a very different one to the later ZX81s. It's worth checking it out. I thought I'd written a blog post about it, but I can't find it. The upshot is this though. As you know (as revealed by Dr Ian Logan), the original ZX81 ROM had a division bug which caused SQR 0.5 to be calculated incorrectly (I think it was the division bug that did that).

Sinclair updated the ROM, but also found a fix using a 74LS12N and 74LS27N pair of TTL chips. It turned out that the specific bit pattern (or enough of it) was only found in a few ROM locations and in only one of these (where the ROM error was) resetting a single bit would correct the ROM bug (in other cases, the bit was already reset or resetting the bit was benign).


From: https://www.binarydinosaurs.co.uk/Museum/Sinclair/oddzx81/index.php


This 68KMLA comment brought to you from a 2.4 GHz MacBook 2008 C2D running Mac OS 10.7.5 !

 

[VMSZealot]

After peering closely at that board, it appears to be identical to mine - albeit that mine doesn't have the bodge on it (it's been given an updated ROM). Note that there was a little exaggeration in my definition of "entirely working", although this might be down to the Sony Smart TV I had it plugged into (one of the last made with an analogue tuner). The bottom of the display is VERY unstable, to the point of being nearly illegible. I suppose I could pull out the old CRT to test with…

 

[Snial]

After peering closely at that board, it appears to be identical to mine - albeit that mine doesn't have the bodge on it (it's been given an updated ROM). Note that there was a little exaggeration in my definition of "entirely working", although this might be down to the Sony Smart TV I had it plugged into (one of the last made with an analogue tuner). The bottom of the display is VERY unstable, to the point of being nearly illegible. I suppose I could pull out the old CRT to test with…

Good point. The ZX81 also didn't generate correct HSyncs and so pretty much every TV in the past 30 years won't work properly. It ought to generate, I think, 0V for about 4.5µs then 0.3V (black) for 4.5µs; which the TV uses for calibration. The ZX81 goes straight from 0V to 1.0V for the white margin at the left. So most more modern TVs think a ZX81's black is white and the ZX81 gets run over at the next Zebra crossing.

Do you have a Ram Pack or just 1kB?

 

[VMSZealot]

The real ZX81 is just 1K. The one I built has 32k on board. No RAM pack wobble! Also no modulator, and it displays perfectly - so that's the one I use when I want my crap old computer fix!