View Full Version : ZX81 video issues

October 25th, 2008, 10:35 PM
I inherited this computer from my uncle who died a few years back and it came pretty much boxed with the manuals and cables. I later found a bunch of program tapes for it (well for the Timex 1000 but they are pretty much the same) and all I need now is a matching Sinclair 16K ram pack.
Anyways, I plug the thing in to both a composite video jack and (obviously) get no picture. I also plug it into an RF adapter and try tuning it in but still get nothing. I can't refer to the person who built the computer either because he is dead so I don't know what to do.
Is this common or has something in it failed?

October 26th, 2008, 12:37 AM
The ZX-81's RF modulator is built into the machine. It's video output was designed to plug straight into a TV's antenna input. Try that and see if it shows up on a channel.


October 26th, 2008, 05:50 AM
And if you would prefer composite, you can check out my Timex thread.


linkage (http://www.vintage-computer.com/vcforum/showthread.php?t=12674&page=1)

October 26th, 2008, 10:14 AM
And if you would prefer composite, you can check out my Timex thread.


linkage (http://www.vintage-computer.com/vcforum/showthread.php?t=12674&page=1)

That is an interesting read and I guess it is worth a try but for now I would just like to get it to work. I have several B/W TVs as well as color TVs. Before I really start to hack away at how video is outputted, I first need it to work.

October 26th, 2008, 10:38 AM
Just connect the rf out to 300ohm antenna port. If all you have is 70 ohm plug, then you can buy a cheap adapter, here (http://cgi.ebay.com/UHF-VHF-FM-splitter-75-ohm-in-to-75-300-300-out-adapter_W0QQitemZ180255114957QQcmdZViewItem?_trksi d=p3286.m20.l1116) if you can't find one at a part store.

October 26th, 2008, 11:55 AM
Well I did that, set the tv to channel 34 (according to the manual) and plugged in. I got a little less static bust still no picture. I also checked the other channels and didn't find it.

October 26th, 2008, 11:56 AM
No, no, 3 or 4. That's your problem.

October 26th, 2008, 12:28 PM
Tried those ones as well and just got static.

October 26th, 2008, 12:30 PM
Are you sure that it's working?

maybe you could type in a couple commands then hit the save button, listen on the tape port for any noise. Can you test the PS?

October 26th, 2008, 12:46 PM
It s doing something. if I set it on channel 34 and plug it in, if I jiggle the power connector I can go from a fuzzy screen, to something a little less fuzzy to a solid shade of gray on the screen. the little heatsink in the ZX81 is warming up when plugged in as well and I am using the power supply that was found in the box with the computer which is rated for 9.75VDC, 650ma.

October 26th, 2008, 12:57 PM
The ZX-81 has no bootup message. All you get is a grey screen with an single black blinking cursor on the top left.


October 26th, 2008, 01:23 PM
Well then I am getting the gray screen but no blinking character.

October 26th, 2008, 04:58 PM
You can try hooking up jumper wires to the contacts in the link below to try a composite monitor.

October 26th, 2008, 05:21 PM
You can try hooking up jumper wires to the contacts in the link below to try a composite monitor.

Pretty much every image on that site is broken on me.

October 26th, 2008, 06:32 PM
Just to clarify, the "boot screen is a


at the bottom left.

Try this:

1. Set up a system (like an Atari 2600) that you know works with an RF switch and TV, adjust the picture, etc and get it nice and sharp.

2. Replace the Atari with the ZX-81, everything else left the same.

3. If you cannot see a K in the bottom left corner of the screen it *could* be that the Sinclair has a bad video output but, it's more likely that the TV is not equipped to handle the SInclair signal. There is something about how the synch/black signals work. Try another TV, an older one if possible. Only one or two of the TV's I own works with my Sinclair.

4. If you have another TV, start from step #1 again, with a different TV.

Make sure you have the correct channel, 3 or 4.

October 26th, 2008, 07:17 PM
I did this and still didn't get it.
Also, like I said before, I am getting absolutely nothing on channel 2, 3 and 4 (the manual does not even say to tune to any of these channels, ANYWHERE!) and I AM getting a visual response (but still no [K] in the corner] on Channel 34 (which the manual even says you have to tune to).
I have also tried with another B/w tv that requires manual fine tuning (and a screen that makes the one on the Osborne 1 look spacious) as well as two different color tvs that have automatic tuning and a pocket Casio color tv.

EDIT: aaargh!

October 26th, 2008, 08:03 PM
It IS a NTSC model, right?

October 26th, 2008, 08:07 PM
Just to clarify, the "boot screen is a


at the bottom left.

Opps, you are right Bill. Sorry. Too many different computers to keep track of them all.


October 26th, 2008, 08:20 PM
It IS a NTSC model, right?

I had a feeling something was different.
It could possibly be.
Did the North American manual talk about settings for the UK and europe as well as have excerpts on how The UK uses a UHF system using 635 lines at 50 FPS while the USA uses VHF using 525 lines at 60 FPS?
I have no idea where my uncle bought this thing.

EDIT: Google proves it. There is a PAL version. Is there any definitive way to see what video standard it is currently set for?

October 26th, 2008, 08:23 PM
Probably. I doubt that Sinclair would have printed two separate manuals for such an inexpensive unit.

However, since I've never owned one (and the manual wasn't necessary to repair them), I can't say for SURE

October 26th, 2008, 08:56 PM
I think you can see on the PCB which country it's for.
The 3 wires going into the modulator, you can trace them to pads labelled UK, USA, and France. You can probably trace back to see which it's soldered to put out for.

EDIT: As discussed any times, if you do this, be EXTREMELY careful with the keyboard ribbon, as most OEM one's are brittle, and VERY easily broken/cracked, and unless
you have a spare, don't even THINK of removing it! most likely, it's WAY too brittle to get back in.

And yes, I AM speaking from experience!



October 26th, 2008, 09:07 PM
Three wires? I only see two.
One goes to Fr/UK1 and the other to UK2.
Well that pretty much explains it. It's set to PAL.

now, how do I wire it back to NTSC? I see spots for USA 1, 2 and 3 but since I only have two leads I assume that my uncle trimmed one off.
It appears that it used to pass through the hole in the plastic grommet that is closest to the side of the board (nearest the jacks). Looking inside I an also see what appears to be a set of clipped leads. I don't know if that is normal either. I'll post a pic.

EDIT: Here you go.

October 27th, 2008, 01:58 PM
If you get some visual response around UHF channel 34-36, that's probably where it is.

As a matter of fact, recently I was given a broken Timex/Sinclair 1000 from a friend. I got as far as getting a ghost image around channel 2 and by pressing some key saw it kind of works, but it appears to keep resetting itself all the time. Also my TV is nowhere near displaying a sharp picture. Perhaps I'll be able to do something about it, or it remains a "display piece".

October 29th, 2008, 09:14 PM
Still waiting on an answer on converting this to NTSC. :/

November 2nd, 2008, 03:27 PM
From the other Marketplace (!) thread about Timex/Sinclair 1000:

Is the membrane keyboard ASCII or multiplexed? We should be able to design an adapter to use a standard keyboard. Does anyone have the pin-out of the keyboard connector?
I have finally scanned some pages. Those are rather low resolution and in Swedish, as promised.

http://www.cbm.sfks.se/tidning/byggutdinzx81.pdf (4.7 MB)

Since the resolution is so bad, I rescanned the vital tables and schematics on pages 2-3:

The keyboard logic consists of five outputs (0-4) and eight inputs (8-15):

Char | Input | Output
A | 9 | 0'
B | 15 | 4'
and so on.

This build project actually takes care of three issues about the ZX-81: a real keyboard, a better power supply and built in 16K expansion memory. The first three pages describe the keyboard, the last four are about PSU and memory.

Terry Yager
November 2nd, 2008, 03:53 PM
What, no translation? The only foreign languages I know are Englandish, Canadian, and a smattering of Australian (although after many years of deciphering instruction manuals, I'm starting to pick up a little Engrish too).


November 3rd, 2008, 08:46 AM
Given the Sinclair ZX81 is a British computer, I would suspect if you dig into early issues of Your Computer, possibly Sinclair User whatever they were called, you would find build projects in English too. Just that I don't know which issues you should look at. World of Spectrum has a rich set of scanned magazines, but perhaps most date from the ZX Spectrum era and forwards.

Anyway, here is a very quick and rough translation of the most important sections:

How it works
The address bus is driven by IC 7 (SN74LS244 octal buffers) and assigned to the keys through diodes. We are only interested in the low pulses. The function of the diodes is a bit hard to understand, but since Sinclair put them there, we'll keep them.

When the CPU wants to know the keyboard status, it holds low lines A8 to A9 and so on until A15 also has been held low. The keyboard sees this as a series of low pulses. If either of the "normal" key switches is closed, it results in a pulse on either of the lines 0' to 4'. The pulse goes to inputs KBD0 - KDB4 in the controller circuit on the ZX board.

If you press a key in the "shifted" area, we first need to simulate the SHIFT key. IC 1 finds a key is pressed down in this area. A positive flank triggers the monostabile multi vibrator IC 2a (SN74LS123 dual monostable). Its outgoing pulse goes through IC 4 (SN74LS02 quad NOR gate) and IC 5 (SN74LS05 hex open collector inverter) onto KBD0 where the pulse from the shift key anyway would have ended up, but only if A8 is low. The other input on IC 4 makes sure this happens. It is only during the A8 interval that a pulse on KDB0 results in the SHIFT function. Immediately thereafter the character information needs to be sent.

Q on IC 2a together with A8 gives a burst with trigger pulses to IC 2b, which trigs and opens IC 3 and IC 4 so the pulses on the lines 0' to 4' gets through and out to KBD0 - KBD4. Note that the shift pulse on KBD0 arrived first and indicates that SHIFT was pressed down.

The decoding logic needs to be connected to the ZX board in some way, preferrably connectors so it can be removed again. The connectors on the ZX board accommodates regular wire pins, e.g. a Minimodul contact list cutted in suitable bits. Thereafter you solder the lines from the decoding logic onto the list, push down the contacts and you're done. Mark all lines well, so they are not connected backways. Where the threads should go can be determined by looking into the schematic and the circuit board. 5V to the decoding logic needs to be pulled from a separate pair on the power supply.

nige the hippy
November 3rd, 2008, 11:08 AM
There isn't a wire missing from the modulator, the different models of the astec modulators had a separate input for audio which went in there. and the output is designed to go into 75ohm coax.

the schematic for the zx81 is here....


...But I suspect that there's more of a change between the US & UK models than just a diode/resistor or two. The timing of NTSC & PAL is completely different and done in software on the '80 & '81

If your ZX81 is UK & you want to run it, check out PC based TV tuners, mine does PAL, ntsc, secam & half a dozen other standards too, and it was pretty cheap, and you can screen capture too!

November 4th, 2008, 08:37 AM
Talk about luck.
I accidentally bought PAL WinTV tuner a few years back and never had a use for it. We will see how it likes the Sinclair once I get it up and running.