Video issue over rf. Changes with reset

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daverobzippy
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Video issue over rf. Changes with reset

Postby daverobzippy » Thu Jul 26, 2018 6:12 pm

So the QL is up and running.
I have the picture over RF, however sometimes the picture will be nice and clear and other times it wont be.
I know that the next step is to order an RGB cable and try that but has anyone ever come across anything similiar.
It has been re-capped and was doing this before being re-capped.
Image
Image
I hope these images attach.
They didn't so I will link to them
https://www.dropbox.com/s/uyhyuq7027c47l9/2018-07-25%2018.52.22.jpg?dl=0
https://www.dropbox.com/s/1autd4zvstl8n9v/2018-07-25%2018.52.41.jpg?dl=0

It isn't a massive difference, the colours look sort of washed out, with a touch of 'dot crawl' if you like
See on the second image the green is a lovely green sort of green. It isn't a tuning issue.


Martin_Head
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Re: Video issue over rf. Changes with reset

Postby Martin_Head » Fri Jul 27, 2018 10:45 am

Probably the RF modulator getting old. They were never that great even when new.

Once you go RGB, you wont want to go back to RF.


daverobzippy
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Re: Video issue over rf. Changes with reset

Postby daverobzippy » Sat Jul 28, 2018 9:54 am

No it is the same if I intercept the composite signal before it reaches the modulator.
It is always one way or the other, no inbetween, either a really great picture or a slightly not so great picture.
Of course using an RGB cable will hopefully help pinpoint where the issue is.


Martin_Head
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Re: Video issue over rf. Changes with reset

Postby Martin_Head » Sat Jul 28, 2018 10:09 am

It may be a dry joint. Use the handle of a small screwdriver, or a plastic pen. To tap gently around the the area of the MC1377 IC to see if you can make the fault come and go.


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1024MAK
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Re: Video issue over rf. Changes with reset

Postby 1024MAK » Sun Jul 29, 2018 4:17 pm

The RF output comes from the modulator. All the modulator does is to put the composite video signal onto a high frequency carrier. The main problem with the modulators and hence the RF output, is that these are analogue devices which use a capacitor-inductor to form the high frequency oscillator, so are not at the exact TV channel frequencies (so not all TVs can correctly tune in and lock on to the signal). Plus they drift as the inside of the computer warms up (the resulting symptom tuning drift).

However, the main problem is the composite video signal. Various compromises were made when coming up with a system of adding colour to the existing monochrome TV standards. So the colours on a TV picture from a composite video signal will always be nowhere near as good as RGB. This is the reason that the 'brown goods' (radio/TV/VCR etc) industry came up with S-Video, as it separated the colour signal from the rest of the video signal. As at the time there was no world wide standard for a RGB connector outside Europe.

One of the problems with composite video, is that parts of the monochrome video signal and parts of the colour signal may overlap. So the circuitry inside the TV has to try to distinguish between them. But it will not always be successful. The result is incorrect colour patterns or loss of detail (sharp edges become blurry).

In terms of quality, the various analogue video systems rank in this order (poor to excellent):
  • RF
  • Composite video
  • S-video
  • RGB

A RGB lead is the best solution.

Mark


QL, Falcon, Atari 520STFM, Atari 1040STE, more PC's than I care to count and an assortment of 8 bit micros (Sinclair and Acorn)(nearly forgot the Psion's)
daverobzippy
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Re: Video issue over rf. Changes with reset

Postby daverobzippy » Wed Aug 01, 2018 8:45 am

Indeed. It is the fact that the picture can be excellent and then not so.
This isn't down to the television it is down to the machine.
Hopefully it is the 1377 that is at fault.
I'm assuming that by using an RGB cable the 1377 will not come into play so a stable picture constantly at that point would point to the 1377 or at least in that area and a continuation of issues would rule that out and point elsewhere.
I'm hoping so anyway.
I have had the 1377 out and reseated it which didn't help.


Martin_Head
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Re: Video issue over rf. Changes with reset

Postby Martin_Head » Wed Aug 01, 2018 9:53 am

daverobzippy wrote:I have had the 1377 out and reseated it which didn't help.

I thought these were soldered in. If it's in a socket, it looks like someone's had problems around there in the past (The same fault?).


daverobzippy
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Re: Video issue over rf. Changes with reset

Postby daverobzippy » Wed Aug 01, 2018 6:16 pm

The ones I have been dealt with have been socketed. That doesn't mean they were from the factory, hopefully the issue is this IC and the RGB output will be fine.



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