Vertical white stripes when using a ST to VGA cable

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marcello
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Vertical white stripes when using a ST to VGA cable

Postby marcello » Sat Jul 28, 2018 3:38 pm

I bought a ST to VGA adapter so I could play low-res games on a flat display, but no matter the resolution, low, mid or high, I have annoying vertical white stripes which kind of denature the gaming experience.


This is the picture I get when using Xenon II in low res, and Uniterm im High Res.
https://imgur.com/a/rrtdlew
https://imgur.com/a/pA8iwzB

What could be the cause of this ?

I was wondering if electronic interferences could cause this because the cable is quite long.
This is the cable I am using: http://www.best-electronics-ca.com/monadapt.htm

Some notes:
* switching monitor did not help
* I have also a Video Scaler Box from Ambery that I tried to put in the chain, output is similar
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Re: Vertical white stripes when using a ST to VGA cable

Postby ctirad » Mon Jul 30, 2018 12:18 pm

This kind of noise is usualy caused by a signal disturbance from the mains caused by either a ground loop, or weak capacitors in ST PSU, or both. I'd also check the cable for a proper impedance on RGB lines and termination.

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Re: Vertical white stripes when using a ST to VGA cable

Postby marcello » Wed Aug 01, 2018 6:53 am

Thanks, I read a bit about what are ground loop, will see if I can avoid that, then look into a replacement PSU.
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Re: Vertical white stripes when using a ST to VGA cable

Postby Smonson » Wed Aug 01, 2018 9:34 am

Are those lines spaced exactly every 16 pixels?

To me that doesn't look like mains interference - the signal is approx 500KHz, and is exactly synchronised to the video. To me it looks like the interference is digital in origin. If you have an STFM it may be worthwhile to check that all the ICs are properly plugged into their sockets. I'd definitely look into replacing the capacitors though, it's a common problem for video.

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Re: Vertical white stripes when using a ST to VGA cable

Postby ctirad » Wed Aug 01, 2018 11:35 am

marcello wrote:Thanks, I read a bit about what are ground loop, will see if I can avoid that, then look into a replacement PSU.


You need only to replace the electrolytic capcaitors, not the whole PSU.

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marcello
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Re: Vertical white stripes when using a ST to VGA cable

Postby marcello » Wed Aug 01, 2018 8:18 pm

@Smonsom: I counted 20 of those line in middle and low res, which would then bring to a perfect 16 pixels distance (320 / 20 )

Which calculation did you use to come to the 16 pixels ? Curious here, but I am bit of a noob in electronics.
The machine in question is a STE. I changed the TOS roms one year ago, but I was only using High Res before that and did not notice these stripes in High Res.
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Re: Vertical white stripes when using a ST to VGA cable

Postby Smonson » Sun Aug 05, 2018 10:22 am

In low res mode, the shifter outputs pixels at 8MHz, same as the CPU clock speed, so 500KHz is every 16 pixels. At 50Hz, the length of each scanline is an exact multiple of 16 pixels (512 = 320 for the normal video plus 192 for borders). It's not exact in 60Hz though - I think you'd get diagonal lines if you were to try that mode. It is exact in 71Hz: 896 pixels = 224 clock cycles (it produces pixels at 32MHz) = 14 x 16 clock cycles.

The ST has a 500KHz clock that is needed by the keyboard and MIDI serial interfaces, as well as other clocks that are all multiples of 500KHz (2MHz, 4MHz, 8MHz, 16MHz and 32MHz). Cyclic current draws aligned with these clocks could generate noise, and if capacitors that are there to keep voltages steady during cyclic current draws have lost their abilities over the years, you can get these types of signals in the video... Or at least that's my only theory.

Electrolytic capacitors have a limited life since contain a liquid, which dries out. But they are cheap and easy to replace, having only two legs.

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Re: Vertical white stripes when using a ST to VGA cable

Postby marcello » Mon Aug 06, 2018 12:07 pm

Very clear, thanks for the detailed explication, made my day !
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Re: Vertical white stripes when using a ST to VGA cable

Postby Smonson » Tue Aug 07, 2018 5:21 am

It's always a pleasure if I can assist in fixing something!

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Re: Vertical white stripes when using a ST to VGA cable

Postby marcello » Fri Sep 07, 2018 10:10 am

So I replugged ROMs and keyboard IC in their socket.
I had a look at the PSU at the capacitors look quite new, isn't that old capacitors have a curved top when they get old ?

Since this STE is second hand, I wonder if the previous owner changed the capacitors already

Picture of the PSU
https://imgur.com/a/etldwpC
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Re: Vertical white stripes when using a ST to VGA cable

Postby mpattonm » Fri Sep 07, 2018 3:32 pm

no, they are not changed.

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Re: Vertical white stripes when using a ST to VGA cable

Postby dhedberg » Fri Sep 07, 2018 8:00 pm

marcello wrote:I had a look at the PSU at the capacitors look quite new, isn't that old capacitors have a curved top when they get old ?

An electrolytic capacitor can be bad without a bulging vent.
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Re: Vertical white stripes when using a ST to VGA cable

Postby marcello » Mon Sep 10, 2018 6:16 pm

mpattonm wrote:no, they are not changed.


I know there would be people on this forum able to spot this :wink:
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