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-   -   Zx81 (https://www.vintage-radio.net/forum/showthread.php?t=164648)

John Earland 9th Mar 2020 7:41 pm

Zx81
 
Hi everyone, I wonder if anyone has any advice about a TV tuning issue with a ZX81? I have a CRT TV and I can tune it into the ZX81 no problem, but..the picture quality is such that the image wobbles slightly so that the words (I've written a short BASIC program to check to see if the machine works) look like they are intalics - if that makes sense. The TV has picked up the ZX81 signal but it's not 100% correct and I've tried fine tuning too. Could it be a problem with the signal output from the ZX81?

SiriusHardware 9th Mar 2020 8:27 pm

Re: Zx81
 
Hello again, John.

I imagine you have tried adjusting both the brightness and contrast as well as the tuning (if not, do so) but I suggest also turning the colour down to zero - yes, I know the ZX81 is not a colour computer, but presumably the TV is a colour TV? It might be producing colour fuzz / noise due to the absence of any colour signal to pay attention to.

There was an issue with early to mid period ZX81s outputting an imperfectly formed video signal - technically speaking the video output lacked the 'back porch' portion of the video signal.

The final version of the ZX81 ULA IC finally fixed that problem, but as time went by TVs became more fussy about the video input signal and the output from an early to mid period ZX81 with an earlier ULA can often look poor / dim on a later CRT or flatscreen because the TV is being fooled into setting the 'black level' to the wrong level.

If simple adjustments do not solve your problem, can you look (carefully, without stressing the keypad ribbon connector) to see which version of the ULA IC your ZX81 has?

fetteler 9th Mar 2020 9:32 pm

Re: Zx81
 
Sounds to me like hum on the video output of the ZX81. This is likely caused by hum on the power supply which in turn (given the age of these devices now) is likely to be down to the smoothing capacitors in the power supply unit.

Cheers,
Steve.

Paul Stenning 9th Mar 2020 9:54 pm

Re: Zx81
 
Fortunately the PSUs come apart easily, four screws under the self adhesive feet. Unregulated linear supply with, from memory, a 2200uF and a 1000uF capacitor in parallel. Regulation is by the 5V linear regulator inside the ZX81. You could perhaps try a different PSU as long as it's around the same voltage and high enough current.

John Earland 9th Mar 2020 10:12 pm

Re: Zx81
 
That’s brilliant guys-thank you. I haven’t tried messing with the TV so I’ll give it a go. It’s interesting you mention the background noise-that’s exactly what I’m getting. I tried saving a simple BASIC program and the noise on the cassette was terrible-a real buzzing sound with a faint trace of the program bleeps. The screen displayed the characteristic stripes as it was saving but the noise recorded was awful. Nothing wrong with the cassette player because I’ve recorded other things on it fine. But it does sound like interference from the power supply.

John Earland 9th Mar 2020 10:12 pm

Re: Zx81
 
Quote:

Originally Posted by SiriusHardware (Post 1223975)
Hello again, John.

I imagine you have tried adjusting both the brightness and contrast as well as the tuning (if not, do so) but I suggest also turning the colour down to zero - yes, I know the ZX81 is not a colour computer, but presumably the TV is a colour TV? It might be producing colour fuzz / noise due to the absence of any colour signal to pay attention to.

There was an issue with early to mid period ZX81s outputting an imperfectly formed video signal - technically speaking the video output lacked the 'back porch' portion of the video signal.

The final version of the ZX81 ULA IC finally fixed that problem, but as time went by TVs became more fussy about the video input signal and the output from an early to mid period ZX81 with an earlier ULA can often look poor / dim on a later CRT or flatscreen because the TV is being fooled into setting the 'black level' to the wrong level.

If simple adjustments do not solve your problem, can you look (carefully, without stressing the keypad ribbon connector) to see which version of the ULA IC your ZX81 has?

Hello again! Will have a go at this! Thank YOU again!

JohnBG8JMB 9th Mar 2020 10:34 pm

Re: Zx81
 
AFAIremember, and it is nearly 40 years ago, there was a simple mod - change of a resitor value in the outpt of the zx to increase the level - that fixed the record noise problem.

John G8JMB

Sideband 10th Mar 2020 4:06 pm

Re: Zx81
 
I seem to recall that the sync output of the ZX81 was less than perfect (as already suggested by SiriusHardware in post 2). Some TV's were sensitive to this and I had to try several before I found one that worked. Some early TV's had a 'VCR' position and this also helped as the sync time-constant is increased when the VCR position was selected.

John Earland 10th Mar 2020 4:28 pm

Re: Zx81
 
Thank you everyone for your suggestions-I shall work my way through them and report back!

John Earland 10th Mar 2020 7:29 pm

Re: Zx81
 
Quote:

Originally Posted by SiriusHardware (Post 1223975)
Hello again, John.

I imagine you have tried adjusting both the brightness and contrast as well as the tuning (if not, do so) but I suggest also turning the colour down to zero - yes, I know the ZX81 is not a colour computer, but presumably the TV is a colour TV? It might be producing colour fuzz / noise due to the absence of any colour signal to pay attention to.

There was an issue with early to mid period ZX81s outputting an imperfectly formed video signal - technically speaking the video output lacked the 'back porch' portion of the video signal.

The final version of the ZX81 ULA IC finally fixed that problem, but as time went by TVs became more fussy about the video input signal and the output from an early to mid period ZX81 with an earlier ULA can often look poor / dim on a later CRT or flatscreen because the TV is being fooled into setting the 'black level' to the wrong level.

If simple adjustments do not solve your problem, can you look (carefully, without stressing the keypad ribbon connector) to see which version of the ULA IC your ZX81 has?

Hi, I've tried tinkering with the TV - no luck I'm afraid. The problem is still there. Nothing is simple is it??

I shall take a look inside at the ULA

fetteler 10th Mar 2020 7:46 pm

Re: Zx81
 
Please check that the DC coming from the power supply is the correct voltage and is smooth?

Also a photo of the 'wobbly' text you describe would be helpful

Steve.

SiriusHardware 10th Mar 2020 7:52 pm

Re: Zx81
 
John doesn't have a scope as far as I know, just a meter.

Addendum for John: If you measure the output voltage from a power supply like this with no load, ie, not plugged into the ZX81, the output voltage will be much higher than the voltage (9V?) which is marked on it, possibly as high as 12 to 14 volts. That is normal for these old fashioned unregulated power supplies.

We're assuming the power supply is the original Sinclair supplied item with the 'Sinclair' logo embossed on it.

Agree, an image of the problem would maybe help us to narrow things down.

John Earland 10th Mar 2020 7:57 pm

Re: Zx81
 
1 Attachment(s)
Image inside-looks very clean! ULA is 2C210E 8234

SiriusHardware 10th Mar 2020 8:01 pm

Re: Zx81
 
Well, that's actually good news - the 2C210 is the 'Late' (desirable) version of the ULA, so whatever the cause of your video problem may be, that probably isn't it.... although the ULA is what generates the video display, so it could be faulty in a different way.

For the time being I would assume the ULA is OK.

Do you have any other TV you can try this with? A flatscreen TV which still has an analogue tuner perhaps?

John Earland 10th Mar 2020 8:07 pm

Re: Zx81
 
Just tried the power supply, no load- it does read 14v. Yes it is the original Sinclair one.

John Earland 10th Mar 2020 8:10 pm

Re: Zx81
 
I don’t buy I know someone who does. It could be the old TV of course-it’s a Daewoo so not that old, with a built in VCR. I modified my ZX Spectrum so it works on a flat screen composite signal. I guess I could try that? Although can’t seem to tune the TV into the Spectrum via the SCART

SiriusHardware 10th Mar 2020 8:17 pm

Re: Zx81
 
I would suggest starting another thread about the Spectrum SCART problem, if you want to pursue that.

The ideal TV for a ZX81 is an old 12" Ferguson, Ultra, Binatone or what have you black and white portable TV with an analogue tuning knob on the front but your Daewoo should be usable. You just need to try the '81 with one or two other TVs to see if the effect is the same on those as well. Only if it is can you say that there is definitely a problem with the '81.

John Earland 10th Mar 2020 8:21 pm

Re: Zx81
 
Yes-I have a friend who has some old TVs so will try him first-thanks for that.

fetteler 10th Mar 2020 8:32 pm

Re: Zx81
 
I'd love to see what the display on the screen looks like - a picture is worth a thousand words;)
Steve.

John Earland 10th Mar 2020 9:56 pm

Re: Zx81
 
1 Attachment(s)
Here is a picture-not great!

fetteler 10th Mar 2020 10:19 pm

Re: Zx81
 
Thank you.
That looks very much like like hum on the video to me - i.e. there is a 50/100Hz waveform superimposed on the video output. Try powering the ZX81 from a battery...
Steve.

John Earland 10th Mar 2020 10:26 pm

Re: Zx81
 
Quote:

Originally Posted by fetteler (Post 1224223)
Please check that the DC coming from the power supply is the correct voltage and is smooth?

Also a photo of the 'wobbly' text you describe would be helpful

The voltage was 14v - seemed steady though I don’t know if it’s smooth because I don’t have an oscilloscope.

John Earland 10th Mar 2020 10:28 pm

Re: Zx81
 
Quote:

Originally Posted by fetteler (Post 1224290)
Thank you.
That looks very much like like hum on the video to me - i.e. there is a 50/100Hz waveform superimposed on the video output. Try powering the ZX81 from a battery...
Steve.

Thatís a great idea! Will do and Iíll let you know-thank you.

SiriusHardware 10th Mar 2020 11:09 pm

Re: Zx81
 
The heavy diagonal black bars in the picture are presumably just artefacts caused by the TV frame rate and the camera refresh rate interacting. I would imagine they are not present when viewed with the Mk1 eyeball, is that right?

Unfortunately those artefacts are so prominent they make it a little bit difficult to see the effect you are really trying to portray.

That buckled, washed out effect is what I would expect to see if I was tuned to the edge of the signal or the wrong side of the signal rather than the centre of it. You've definitely tuned manually, in very small steps, all the way across the signal from one side to the other?

John Earland 10th Mar 2020 11:41 pm

Re: Zx81
 
Hi-yes those artefacts can’t be seen by the naked eye. I did fine tune it the best I could and this was the result. It’s bang in the middle of the tuning area with the colour reduced to zero.

Andrew2 10th Mar 2020 11:50 pm

Re: Zx81
 
Another vote for a tuning error. It looks just like the TV needs a tweak.

nigelr2000 11th Mar 2020 12:02 am

Re: Zx81
 
Are you around channel 36 because I used to pick up a similar false signal further up the band on my old ZX81 and it looked just like that washed out and pretty unwatchable but tune back down the band and it was fine

SiriusHardware 11th Mar 2020 12:03 am

Re: Zx81
 
What sort of tuning does that Daewoo have, John, is it twiddly wheels or knobs or on-screen tuning via the remote?

I'm wondering if you are tuned to an 'image' of the signal rather than the actual signal. Try tuning completely off the signal you are currently tuned to and keep going for some way first higher than and then lower than the signal you know about.

According to my ZX81 manual which is conveniently to hand, the signal output from the ZX81 should be on UHF channel 36. However your TV is tuned, it should give you some indication of where in the channel range you are currently tuned to.

Edit: Cross posted with nigelr2000, who is thinking along the same lines.

fetteler 11th Mar 2020 12:45 am

Re: Zx81
 
I wasn't refering to the big diagonal lines, just the wobbly image. Having seen it though I do think SiriusHardware's suggestion of being tuned to an image is a good one and as he suggests you should have a tune through the entire band to see what you find.

I wish you well with this, there's a lot of fun to be had with a ZX81:)

Steve.

John Earland 11th Mar 2020 7:58 am

Re: Zx81
 
Quote:

Originally Posted by nigelr2000 (Post 1224333)
Are you around channel 36 because I used to pick up a similar false signal further up the band on my old ZX81 and it looked just like that washed out and pretty unwatchable but tune back down the band and it was fine

Yes, channel 36 is where it is sitting. Iíve been up and down the band and this is the best slot.

John Earland 11th Mar 2020 8:01 am

Re: Zx81
 
Quote:

Originally Posted by SiriusHardware (Post 1224335)
What sort of tuning does that Daewoo have, John, is it twiddly wheels or knobs or on-screen tuning via the remote?

I'm wondering if you are tuned to an 'image' of the signal rather than the actual signal. Try tuning completely off the signal you are currently tuned to and keep going for some way first higher than and then lower than the signal you know about.

According to my ZX81 manual which is conveniently to hand, the signal output from the ZX81 should be on UHF channel 36. However your TV is tuned, it should give you some indication of where in the channel range you are currently tuned to.

Edit: Cross posted with nigelr2000, who is thinking along the same lines.

Unfortunately there are no knobs to twiddle and itís all through the remote. I use the manual tuning to find the signal on ch 36 then fine tune it and this is the best. I donít know what you mean re: tune to an image?

John Earland 11th Mar 2020 8:45 am

Re: Zx81
 
This is the tv I have :
https://rover.ebay.com/rover/0/0/0?m...2F202881667062

SiriusHardware 11th Mar 2020 9:54 am

Re: Zx81
 
An 'Image' is a false or secondary weaker signal which a receiver can sometimes 'see' on a frequency some distance away from the frequency which the signal is really on. We were thinking you might be tuned to an image signal rather than the real signal.

Can you check the 'aerial' lead for continuity?

Check that there is zero ohms between the centre pin of the TV end of the lead and the centre pin of the ZX81 end of the lead, and also check that there is zero ohms between the outer part of the TV end of the lead and the outer part of the ZX81 end of the lead.

Finally, check that there is NO connection between the inner and outer parts of the TV end plug (All measurements to be made with the lead just by itself, not plugged into anything).

John Earland 11th Mar 2020 10:19 am

Re: Zx81
 
1 Attachment(s)
Will do-thank you. I took this this morning

John Earland 11th Mar 2020 10:33 am

Re: Zx81
 
1 Attachment(s)
LOL! This is what I get if I apply slight pressure on the ZX81 RF input connector! Almost perfect!

John Earland 11th Mar 2020 10:49 am

Re: Zx81
 
Interesting. Now when I go to type something everything disappears and all I have is a blank white screen. Unplugging it and plugging it back in and the prompt is back. When I go to type though-same thing happens. Overheating causing this? Doesn’t feel very hot.

John Earland 11th Mar 2020 10:53 am

Re: Zx81
 
Okay, now it’s switching itself off and resetting to prompt all the time. I’m going to switch it off and let it cool down and try in an hour.

Dave Moll 11th Mar 2020 11:00 am

Re: Zx81
 
Quote:

Originally Posted by John Earland (Post 1224427)
LOL! This is what I get if I apply slight pressure on the ZX81 RF input connector! Almost perfect!

Sounds like a loose contact to me.

SiriusHardware 11th Mar 2020 12:23 pm

Re: Zx81
 
Possibly the connecting lead has an intermittent break in it, if so, this would usually be a break in the inner conductor of the lead just at the point of entry/exit to one of the plugs as that's where they tend to get bent around the most - also people have a bad habit of pulling them out by gripping the lead close to the plug and pulling, rather than gripping the body of the plug and pulling that.

Do you have an alternative aerial / RF lead you can try?

John Earland 11th Mar 2020 12:38 pm

Re: Zx81
 
Hi, yes - same result. I have tested the ohms and all seems well with the inner and outer connections and there is no short between them.

I haven't switched it back on yet because it was almost like it was short circuiting causing it to reboot. Picture went really off signal and the prompt kept disappearing. Strange.

John Earland 11th Mar 2020 1:43 pm

Re: Zx81
 
3 Attachment(s)
Iíve just switched it back on and placed the ref cable on half way-first two images-fairly decent. However after 10 mins on-3rd picture! 😱

SiriusHardware 11th Mar 2020 2:22 pm

Re: Zx81
 
The thing to realise about the mini transmitters (modulators) used in tune-in tech from this era (like the ZX81) is that the little signal generator inside is quite crude and simple. The transmitter / channel frequency is determined by the adjustment of a coil, nothing more, and as they warm up the frequency tends to drift a little.

TV tuners of the time were also quite prone to drift so they incorporated a feature called AFC (Automatic Frequency Control) which would automatically keep the TV's tuner centred on any signal it was tuned to, so, as the frequency of the incoming signal slowly drifted, the TV would self-retune to keep itself centred on the signal.

Modern TVs have incredibly stable tuners and they expect any incoming signal to which they are tuned to be the same, so they don't track drifting signals the way older TVs do.
They stay precisely tuned to whatever frequency you tune them to.

The upshot of all this is that if you tune a modern TV to the ZX81's output while it is cold you may have to retune the TV in little steps from time to time as the ZX81 warms up.

That's why an older TV with a simple analogue tuning knob on the front is the best type to use for the ZX81. Up to a point the AFC will try to maintain a lock on the signal, and then if it drifts out too far it's just a simple matter to reach out and retweak the tuning for best picture.

You could bypass all of this by converting the output to composite video , as you did on your Spectrum. Only you will know whether you feel confident enough to do that.

John Earland 11th Mar 2020 8:25 pm

Re: Zx81
 
Hi, yes I could give that a go. It looks trickier when I've looked on YouTube that it was to convert the Spectrum - any tips? It may be the best solution!

SiriusHardware 11th Mar 2020 9:29 pm

Re: Zx81
 
In theory you could do it in the same way as on the Spectrum, by disconnecting the input and output wires from the modulator inside the housing and wiring the video-in signal straight through to the centre pin of the UHF-out terminal.

The only problem with that is that it directly exposes the output of the ULA (an exotic, expensive IC) to the outside world. It might be better to build a simple buffer circuit to go between the ULA and the outside world into the modulator housing, as in this article:-

https://www.classic-computers.org.nz...o-for-zx81.htm

That shows the basic circuit and general principle of a buffered composite video modification, although I would hope you would try to make yours look neater than the end product in that article.

Dave Moll 11th Mar 2020 11:04 pm

Re: Zx81
 
As an aside, it also passes on a useful tip about that notoriously fragile ribbon cable between the keyboard and the motherboard.

SiriusHardware 11th Mar 2020 11:35 pm

Re: Zx81
 
Let's take one more punt at trying to get it to work as built.

John, can you flip the lid off the modulator and check that the wire going to the lug on the centre terminal of the UHF output socket has not come loose? When it works, it appears to work quite well.

John Earland 12th Mar 2020 8:54 pm

Re: Zx81
 
Yes - that's a great idea - will do and I shall report back

John Earland 13th Mar 2020 10:57 pm

Re: Zx81
 
Quote:

Originally Posted by SiriusHardware (Post 1224641)
In theory you could do it in the same way as on the Spectrum, by disconnecting the input and output wires from the modulator inside the housing and wiring the video-in signal straight through to the centre pin of the UHF-out terminal.

The only problem with that is that it directly exposes the output of the ULA (an exotic, expensive IC) to the outside world. It might be better to build a simple buffer circuit to go between the ULA and the outside world into the modulator housing, as in this article:-

https://www.classic-computers.org.nz...o-for-zx81.htm

That shows the basic circuit and general principle of a buffered composite video modification, although I would hope you would try to make yours look neater than the end product in that article.

Iíve just noticed in the first picture in this article, showing the set up, the cassette recorder is the one I have, a Sony!

Iíve read this article and it involves using a resistor and a transistor. I have a 100R resistor and Iíve ordered a transistor so will attempt the mod. However, I did this on the Spectrum without using either of these. Would this approach work here do you think?

SiriusHardware 13th Mar 2020 11:56 pm

Re: Zx81
 
I think the best way to answer your question is to suggest you Google around and see how many different ways people have done the composite mod on the ZX81. If you see enough articles independently suggesting that it be done the same way as you did it on the Spectrum then it probably works OK.

If the general consensus is that you need to use a buffer transistor, then I would go with that option. As I suggested before, using a buffer provides a degree of protection to the exotic and difficult to obtain ULA IC by sitting between the ULA and the outside world - if something untoward happens, it hopefully takes out a twelve pence transistor instead of the ULA IC.

I would favour the buffer method, and in particular this version of the buffer circuit which comes already made on a little PCB which is small enough to fit inside the modulator without removing any of the original modulator parts. All you have to do is add four wires, Power, 0V, Video in, Video out.

https://www.thefuturewas8bit.com/sho...video-mod.html

Although the headline describes as being for Atari it also works with the ZX81.

However, I would not discourage you from trying either the 'simple' video pass through mod like on your Spectrum or the homebrew transistor buffer version - that's how you learn.

John Earland 14th Mar 2020 12:11 am

Re: Zx81
 
Thank you for this. Food for thought.


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