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Vintage Computers Any vintage computer systems, calculators, video games etc., but with an emphasis on 1980s and earlier equipment.

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Old 9th Mar 2020, 7:41 pm   #1
John Earland
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Default 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?
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Old 9th Mar 2020, 8:27 pm   #2
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Default 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?
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Old 9th Mar 2020, 9:32 pm   #3
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Default 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.

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Old 9th Mar 2020, 9:54 pm   #4
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Default 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.
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Old 9th Mar 2020, 10:12 pm   #5
John Earland
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Default 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.
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Old 9th Mar 2020, 10:12 pm   #6
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Default Re: Zx81

Quote:
Originally Posted by SiriusHardware View Post
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!
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Old 9th Mar 2020, 10:34 pm   #7
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Default 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.

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Old 10th Mar 2020, 4:06 pm   #8
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Default 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.
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Old 10th Mar 2020, 4:28 pm   #9
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Default Re: Zx81

Thank you everyone for your suggestions-I shall work my way through them and report back!
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Old 10th Mar 2020, 7:29 pm   #10
John Earland
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Default Re: Zx81

Quote:
Originally Posted by SiriusHardware View Post
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
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Old 10th Mar 2020, 7:46 pm   #11
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Default 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

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Old 10th Mar 2020, 7:52 pm   #12
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Default 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.
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Old 10th Mar 2020, 7:57 pm   #13
John Earland
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Default Re: Zx81

Image inside-looks very clean! ULA is 2C210E 8234
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Old 10th Mar 2020, 8:01 pm   #14
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Default 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?
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Old 10th Mar 2020, 8:07 pm   #15
John Earland
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Default Re: Zx81

Just tried the power supply, no load- it does read 14v. Yes it is the original Sinclair one.
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Old 10th Mar 2020, 8:10 pm   #16
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Default 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
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Old 10th Mar 2020, 8:17 pm   #17
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Default 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.
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Old 10th Mar 2020, 8:21 pm   #18
John Earland
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Default Re: Zx81

Yes-I have a friend who has some old TVs so will try him first-thanks for that.
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Old 10th Mar 2020, 8:32 pm   #19
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Default Re: Zx81

I'd love to see what the display on the screen looks like - a picture is worth a thousand words
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Old 10th Mar 2020, 9:56 pm   #20
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Here is a picture-not great!
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