Another problem - image tuning via pot

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Mulbin
Vic 20 Newbie
Posts: 4
Joined: Mon Jun 12, 2017 2:14 pm
Website: http://www.goforlaunch.co.uk
Location: Plymouth

Another problem - image tuning via pot

Post by Mulbin »

Hi all, so I've established the other issue (disappearing characters) needs a replacement Character Rom but can anyone help with this issue?

When I got the Vic, the image looked like this...

Image

So I have adjusted the pot to improve image sharpness (only one pot as this is a PAL machine). The trouble is, I have turned it as far as it will go and it still doesn't quite get the image as sharp as it should be. Se below...

Image
Image

So my question is - is this pot trying to compensate for another chip that's just too far gone? If this Vic actually needs 2 replacement roms I'll probably just sell it on for spares.
norm8332
Vic 20 Nerd
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Location: USA

Re: Another problem - image tuning via pot

Post by norm8332 »

That's "Dot Crawl" and "ringing", very common issue with the video signal, not a chip issue. Normal on some machines. There are various fixes by modifying the video output circuitry. It looks much worse on a LCD monitor. Turning up the video singal just masks it, and when you get it working stuff will look better with it turned down a bit.
“In religion and politics people’s beliefs and convictions are in almost every case gotten at second-hand, and without examination... whose opinions about them were not worth a brass farthing.”

-Autobiography of Mark Twain
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mrr19121970
Vic 20 Nerd
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Occupation: IT service manager

Re: Another problem - image tuning via pot

Post by mrr19121970 »

S-Video mod is needed. Disconnect 6561 PIN2 from the mainboard, add a 100uF ceramic capacitor + 360 ohm resistor in series and bring this to the chroma line in a modified S-Video cable.
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MCes
Vic 20 Afficionado
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Re: Another problem - image tuning via pot

Post by MCes »

if your monitor has the tuning "sharp" try to reduce its value
"Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe." (Albert Einstein)
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