Hello,
New here, hi everyone!
The ZX81 was the first 'computer' I ever laid hands on many many years ago and may have just been the thing that got me where I am now...
Anyway, long story short, I now have a ZX81 'sold as seen' off eBay, which has now arrived, but has a black screen when powered on.
I am reasonably technical minded, I have tested the input into the RF modulator (composite video feed) and it also gives just a black screen.
With no experience with the ZX81, I'm wondering if this is a common issue and where to start looking at what the issue could be?
If anyone could help with where to start on troubleshooting this issue and whether it is a common fault that could be many different things or a potential single issue.
Thank you in advance and I look forward to posting more things as I progress with getting the ol' ZX81 into life.
ZX81 Black screen
Re: ZX81 Black screen
Try any tv you can find. The older the better. CRTs are preferred. The video signal from the ZX81 is not as precise as newer tvs want. That’s why you don’t get a picture. You have to try other TVs.
-
1024MAK
- Posts: 5118
- Joined: Mon Sep 26, 2011 10:56 am
- Location: Looking forward to summer in Somerset, UK...
Re: ZX81 Black screen
Hello and welcome 
We do have a welcome area if you want to introduce yourself
There are three different versions of the ULA chip used in ZX81 computers. The first two versions do not produce a video signal that is fully compatible with colour TVs. The part of the video signal that is not generated is called the “back porch” and this part enables a colour TV to determine the black level of the video signal, so that it can auto adjust to show the picture correctly.
The third and final ULA version does include circuitry to produce the “back porch”.
So the first thing to do, is to turn the contrast and brightness controls up to see if there is a inverse K on a dark plain screen. If this does not work on the TV you are using, try again on as many TVs as you can. Modern LCD TVs are less tolerant of the video signal from old computers than the old CRT TVs.
The video output from the ULA on pin 16 is composite video, but the ULA is not designed to drive a TV (or monitor) directly. You should use either a transistor buffer (like this one) or a clean up circuit that includes a “back porch” generator (e.g. a ZX8-CCB (seller currently on holiday) or a ZXVid or a DIY solution using a NE555 timer chip).
It is also possible that the ZX81 has a fault. For a black screen, the most common failure is the ULA. New spare ULA chips are not available
But there are modern replacement ULA modules
Mark
We do have a welcome area if you want to introduce yourself
There are three different versions of the ULA chip used in ZX81 computers. The first two versions do not produce a video signal that is fully compatible with colour TVs. The part of the video signal that is not generated is called the “back porch” and this part enables a colour TV to determine the black level of the video signal, so that it can auto adjust to show the picture correctly.
The third and final ULA version does include circuitry to produce the “back porch”.
So the first thing to do, is to turn the contrast and brightness controls up to see if there is a inverse K on a dark plain screen. If this does not work on the TV you are using, try again on as many TVs as you can. Modern LCD TVs are less tolerant of the video signal from old computers than the old CRT TVs.
The video output from the ULA on pin 16 is composite video, but the ULA is not designed to drive a TV (or monitor) directly. You should use either a transistor buffer (like this one) or a clean up circuit that includes a “back porch” generator (e.g. a ZX8-CCB (seller currently on holiday) or a ZXVid or a DIY solution using a NE555 timer chip).
It is also possible that the ZX81 has a fault. For a black screen, the most common failure is the ULA. New spare ULA chips are not available
But there are modern replacement ULA modules
Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp
Standby alert
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer later in the year.
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp
Standby alert There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer later in the year.
Re: ZX81 Black screen
Hi Both,
Thank you for your replies, In understand the suggestions. I thought that the composite video would be a better way of checking but I guess I did not fully understand.
I have an old CRT but I used it to build a cocktail table Pacman machine and it's had loads of bits taken off it so that won't work.
I'm not sure I have anything with an analogue RF inout in the house anymore!
I'll do my best to test on other devices and if not, will try the options suggested.
Thanks again.
Thank you for your replies, In understand the suggestions. I thought that the composite video would be a better way of checking but I guess I did not fully understand.
I have an old CRT but I used it to build a cocktail table Pacman machine and it's had loads of bits taken off it so that won't work.
I'm not sure I have anything with an analogue RF inout in the house anymore!
I'll do my best to test on other devices and if not, will try the options suggested.
Thanks again.