Zx81 - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

fetteler · 10th Mar 2020, 10:19 pm

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

Quote:

Originally Posted by fetteler

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

Quote:

Originally Posted by fetteler

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.

That’s a great idea! Will do and I’ll let you know-thank you.

SiriusHardware · 10th Mar 2020, 11:09 pm

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

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

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

nigelr2000 · 11th Mar 2020, 12:02 am

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

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

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

Quote:

Originally Posted by nigelr2000

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. I’ve been up and down the band and this is the best slot.

John Earland · 11th Mar 2020, 8:01 am

Quote:

Originally Posted by SiriusHardware

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 it’s 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 don’t know what you mean re: tune to an image?

John Earland · 11th Mar 2020, 8:45 am

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

SiriusHardware · 11th Mar 2020, 9:54 am

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

Will do-thank you. I took this this morning

John Earland · 11th Mar 2020, 10:33 am

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

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

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

Quote:

Originally Posted by John Earland

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

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

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.

10th Mar 2020, 10:19 pm	#21
fetteler Octode Join Date: Jul 2011 Location: Staffordshire Moorlands, UK. Posts: 1,407	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. __________________ Those who lack imagination cannot imagine what is lacking...

10th Mar 2020, 11:09 pm	#24
SiriusHardware Dekatron Join Date: Aug 2011 Location: Newcastle, Tyne and Wear, UK. Posts: 11,082	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?

10th Mar 2020, 11:41 pm	#25
John Earland Hexode Join Date: May 2013 Location: Middlesbrough, North Yorkshire, UK. Posts: 442	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.

10th Mar 2020, 11:50 pm	#26
Andrew2 Nonode Join Date: Mar 2010 Location: Dukinfield, Cheshire, UK. Posts: 2,019	Re: Zx81 Another vote for a tuning error. It looks just like the TV needs a tweak. __________________ Andy G1HBE.

11th Mar 2020, 12:02 am	#27
nigelr2000 Heptode Join Date: Nov 2008 Location: North Walsham, Norfolk, UK. Posts: 876	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 __________________ https://www.facebook.com/gntrading

11th Mar 2020, 12:03 am	#28
SiriusHardware Dekatron Join Date: Aug 2011 Location: Newcastle, Tyne and Wear, UK. Posts: 11,082	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.

11th Mar 2020, 12:45 am	#29
fetteler Octode Join Date: Jul 2011 Location: Staffordshire Moorlands, UK. Posts: 1,407	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. __________________ Those who lack imagination cannot imagine what is lacking...

11th Mar 2020, 8:45 am	#32
John Earland Hexode Join Date: May 2013 Location: Middlesbrough, North Yorkshire, UK. Posts: 442	Re: Zx81 This is the tv I have : https://rover.ebay.com/rover/0/0/0?m...2F202881667062

11th Mar 2020, 9:54 am	#33
SiriusHardware Dekatron Join Date: Aug 2011 Location: Newcastle, Tyne and Wear, UK. Posts: 11,082	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).

11th Mar 2020, 10:19 am	#34
John Earland Hexode Join Date: May 2013 Location: Middlesbrough, North Yorkshire, UK. Posts: 442	Re: Zx81 Will do-thank you. I took this this morning Attached Thumbnails

11th Mar 2020, 10:33 am	#35
John Earland Hexode Join Date: May 2013 Location: Middlesbrough, North Yorkshire, UK. Posts: 442	Re: Zx81 LOL! This is what I get if I apply slight pressure on the ZX81 RF input connector! Almost perfect! Attached Thumbnails

11th Mar 2020, 10:49 am	#36
John Earland Hexode Join Date: May 2013 Location: Middlesbrough, North Yorkshire, UK. Posts: 442	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.

11th Mar 2020, 10:53 am	#37
John Earland Hexode Join Date: May 2013 Location: Middlesbrough, North Yorkshire, UK. Posts: 442	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.

11th Mar 2020, 12:23 pm	#39
SiriusHardware Dekatron Join Date: Aug 2011 Location: Newcastle, Tyne and Wear, UK. Posts: 11,082	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?

11th Mar 2020, 12:38 pm	#40
John Earland Hexode Join Date: May 2013 Location: Middlesbrough, North Yorkshire, UK. Posts: 442	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.