Atari 520 ST video issues

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Oldskool
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Atari 520 ST video issues

Post by Oldskool » Sat Nov 21, 2020 10:28 pm

Dear Forum,

After installing a ST Blitterboard (Anders) in my 520 ST I was playing around with some things but found some strange video related issues.
My computer is an old short 520 ST (without a Modulator and internal floppy).
It has a 4 mb expansion and a Stblitter board with blitter and CF card.
The computer works OK with most software/games and for instance passes all memory test programs I tried.
The video related issues were already there before installing the exxos 4 mb board and or stblitter board (so are not related to these upgrades).
I also use a new power supply now but this does not change the video behaviour (old vs new PSU does not matter).

Video Issues

1 (see picture desktop and giana sister background)
Via scart on my LCD the video quality is OK but I see vertical static lines in the picture. They can be annoying. In some colours they are more obvious than in others. But it's not really nice. They are there in all resolutions from low,mid (scart) - high (via vga)

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2 (see demo pictures and Pang game)
Games and Demos which use software tricks to increase screen size are not displayed correctly.
For instance Pang has some additional stuff at the bottom of the screen.
Demos like Darwin Dilemma are also not displayed correctly at the bottom of the screen there should be something (but nothing is displayed).

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My question = Is this normal behaviour for such an old ST or is maybe the shifter or some other IC partially broken?
And if this is an hardware issue could this be fixed.
Maybe these issues have been there always but in the past I mostly used my ST with small b/w monitor which I don't own any more.

Thanks,

Chris

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Re: Atari 520 ST video issues

Post by exxos » Sat Nov 21, 2020 11:20 pm

You need to check the video shifter part number.. Some early shifters (I think marked -20 or -28) do not work with demos.. There is a thread on this *somewhere* but I would check you have a "normal" shifter C025914-38A.

As for those interference lines, that comes up every few weeks on various machines. Its down to PCB issues. Not really solvable easily, if you want a good picture you will have to get a H4/H5 board where those issues are fixed.
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Oldskool
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Re: Atari 520 ST video issues

Post by Oldskool » Sun Nov 22, 2020 12:00 am

Thanks for the feedback.

So the lines are normal. Probably not noticable on CRT’s we used to have.

The demo issue is shifter related.
My shifter has the following code: C025914-38A PH23-024 5L1 17 but that should be the OK one. Maybe it is broken.

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Re: Atari 520 ST video issues

Post by exxos » Sun Nov 22, 2020 12:16 am

The other chips may need to be -38 types as well.. That's all I can really suggest other than maybe running RAM tests.
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Re: Atari 520 ST video issues

Post by Cosmic Puppet » Sun Nov 22, 2020 1:55 am

I have those lines too @Oldskool

Despite my machine being new old stock and having all mandatory fixes done they still remain.

Like @exxos says it's a quirk of some ST machines. My last Atari which was of a later revision didn't suffer from this.

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Re: Atari 520 ST video issues

Post by tin » Sun Nov 22, 2020 10:15 am

exxos wrote:
Sat Nov 21, 2020 11:20 pm
You need to check the video shifter part number.. Some early shifters (I think marked -20 or -28) do not work with demos.. There is a thread on this *somewhere* but I would check you have a "normal" shifter C025914-38A.

As for those interference lines, that comes up every few weeks on various machines. Its down to PCB issues. Not really solvable easily, if you want a good picture you will have to get a H4/H5 board where those issues are fixed.
This looks alot like the bottom border does not open, in which case it might hint to your machine having an old Glue with short top border. Many modern demos don‘t check for old Glue and thus „miss“ the correct time to open the bottom border (i.e. it opens border in line 239 new glue style instead of line 213 for old glues). Check with older demos, I guess the L16 fullscreen in the union demo might support old Glues. (see e.g. here for details https://temlib.org/AtariForumWiki/index ... e_Atari_ST)

So my guess is: it’s not very likely that the Shifter is defective or even involved in that particular problem.

Regarding the stripes: sadly that is a well known issue still not solved. AFAIK many old machines suffer from this - IF connected to modern TFT or capture devices. Old SC1224 don‘t show these stripes indeed. I have a machine that always shows stripes pretty bad if connected to TFT/OSSC or a PEXHDCAP. No stripes via CRTs. I‘m beginning to guess that modern devices don‘t meet the 75 ohms impendance the ST requires from devices connected to the RGB port; but that‘s just a wild guess, did not have time to check that thoroughly.
Others report not seeing stripes any more on TFTs if the modulator on the STs motherboard is populated.

Oldskool
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Re: Atari 520 ST video issues

Post by Oldskool » Sun Nov 22, 2020 11:12 am

Thanks for all the hints.

Lines:
They are normal.

Memory test:
I let the YAART mem tester run for a while and it can’t find any errors (11 passes and running)

Chip set:
The shifter is version 38A all other costum chips are version 38 without the A.
The glue chip is C025915-38 JH23-38 5J2 77

So can I assume that the demo issues are glue chip related? Should i get a 38A glue?

The demo have indeed the border issue. Pictures which use tricks to get more colours are often not displayed correctly. Some demos bomb out (4 bombs)

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Re: Atari 520 ST video issues

Post by stephen_usher » Sun Nov 22, 2020 11:29 am

Most of my STs are fine, but one shows regular vertical bars, especially in green and blue. Again, this is merely "one of those things" and not a fault as such, it's a feature of the video circuitry, e.g.:
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Re: Atari 520 ST video issues

Post by tin » Sun Nov 22, 2020 12:02 pm

Oldskool wrote:
Sun Nov 22, 2020 11:12 am
The shifter is version 38A all other costum chips are version 38 without the A.
The glue chip is C025915-38 JH23-38 5J2 77

So can I assume that the demo issues are glue chip related? Should i get a 38A glue?
That‘s indeed odd because a -38 glue isn‘t an old glue (-20 would be an old one). Can you check the Sowatt Mainmenu (http://pacidemo.planet-d.net/archives/SOWATT.ZIP) and the dotball, voxel and end/text part on this one: http://absencehq.de/twatnfinal/twatnfinal.zip. Both work (aka open bottom border) on an old Glue, just re-checked.

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Re: Atari 520 ST video issues

Post by tin » Sun Nov 22, 2020 12:20 pm

stephen_usher wrote:
Sun Nov 22, 2020 11:29 am
Most of my STs are fine, but one shows regular vertical bars, especially in green and blue.
Is that image taken using a TFT? If not, do those bars show on a CRT? Esp. the SC1224?
I‘m asking because the lines I see on several machines are mostly grey (a change in overall brightness).

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