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

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.


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