exxos's Atari, Amiga & retro forum

A friend just donated this to me from their loft clear out. A boxed ZX81, with 16k ram pack, manuals, an early 80s set of computer course magazines and some tape software!

No idea if it works or what repairs it might need, but I'll give it a go :)

I'm looking forward to the amazing games - they'll be just like the pictures on the front of the cassettes right?!

I had one of those back in 1981 and have another now which I have to get round to testing. You'll need an old crt tv, preferably with manual tuning to get that to work , or do the composite mod. Apparently the video signal is rather basic and won't sync to a modern tv.

I was going to try this mod for getting a picture on a modernish tv:



I'll test the power supply and then try plugging it in and see what happens. Not for a couple of weeks though - one project at a time :)

Good luck, let me know if the mod works as I'll need to do something similar :D

That's a very late ZX81, so it should have the ULA which produces a reasonable display. (The early one forgot to add a bit at the beginning of the scan line which is used to gauge the black level. This meant that all colour TVs (and some B&W ones) showed a very dark display.)

I notice that the manual isn't ring-bound either. Probably to make it cheaper.

Looks like it's from late 1982 - the programming guide is a 1982 print and the software catalogue is the December 1982 edition. Is that a late model? I'll be honest I know little to nothing about the zx81 at this time!

stephen_usher wrote: 06 Aug 2018 20:58 That's a very late ZX81, so it should have the ULA which produces a reasonable display. (The early one forgot to add a bit at the beginning of the scan line which is used to gauge the black level. This meant that all colour TVs (and some B&W ones) showed a very dark display.)

I notice that the manual isn't ring-bound either. Probably to make it cheaper.

I have a very poorly ZX81 that suffers from this very problem, the display is just about unreadable, even on a period TV.

Something else I need to fix, but I have too much Atari stuff going on :shock:

While you're at it, check the 5V Regulator, these things were not very reliable. In my Speccy 128k I have replaced the 7805 with a L78S05CV, which allows more current and is cooler at the same time. My "toastrack" is now only a handwarmer ;)

And if you want to replace the power supply, check the polarity of the plug, these are reverse to what you might be used to. Afterwards check the polarity. And while you're at it, recheck the polarity ;)

If you're looking for enhancements, you can find some cool ones (like flash storage i.e.) here: https://www.bytedelight.com

PeteW wrote: 06 Aug 2018 21:05 Looks like it's from late 1982 - the programming guide is a 1982 print and the software catalogue is the December 1982 edition. Is that a late model? I'll be honest I know little to nothing about the zx81 at this time!

Yes, that's just about the time Sinclair stopped selling them. The price drop shown on the box was probably to shift the last of the stock.