"Waving" RGB output - cause?

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troed
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"Waving" RGB output - cause?

Postby troed » Tue Feb 20, 2018 10:14 pm

I recently sold one of the best looking STEs in my collection. Very modern motherboard (square DMA), updated with new TOS and recapped PSU. Tested and working nicely with an UBE video adapter and the flatscreens I have.

But for the buyer it's not working as it should. When cold the image is fine, but after 10-15 minutes it starts to wave oh so slightly from side to side in an oscillating motion (see three videos linked further down).

The buyer has tested with two flatscreen TVs and one CRT monitor, all using an RGB SCART from Retro Computer Shack (https://www.ebay.co.uk/itm/250978089310). The monitors work fine with other retro computers, and the cable gives a rock solid picture with another STE the buyer already had. The issue looks exactly the same on all of them.

I'm at a loss really to explain what could cause this. I would tend to guess at some HSYNC instability, but don't really know how to pinpoint it better. I've offered to take the computer back but the buyer would like to fix it if possible (it is, really, in excellent condition otherwise ... )

Any suggestions?

videos: https://troed.ddns.net/d/13ca2308da33464fa445/

/Troed

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Re: "Waving" RGB output - cause?

Postby siriushardware » Wed Feb 21, 2018 6:56 pm

It does look like the Hsync is being slightly advanced and retarded at a slow sinusoidal rate. With everything being derived from crystal clocks it is hard to imagine how that could be. Sorry, I know that doesn't help much. Does this effect appear when an RF lead is used instead?

How switched on is the buyer, could he try the PSU from his other STE (would you want him to do that anyway, as it would involve taking the machine you sold him apart)?

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Re: "Waving" RGB output - cause?

Postby troed » Sun Feb 25, 2018 8:56 am

PSU has been replaced by another one, the issue stays just the same (and the original PSU works fine in another STE)

I'm still at a loss to explain how this could happen. There's only a resistor and inductor between the GST-MCU signal and the output and I can't see how a low oscillation signal can interfere there ... ?

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Re: "Waving" RGB output - cause?

Postby mrbombermillzy » Sun Feb 25, 2018 11:08 am

If you say the LCD is doing this as well as the CRT then maybe he is placing the monitors/cabling/STE next to something emitting RF interference (unshielded speakers)??

An LCD screen would not be affected like this unless the actual signal into it (cable or STE) was being modulated by external interference. Are there any internal addons/missing shielding?

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Re: "Waving" RGB output - cause?

Postby Andega » Mon Feb 26, 2018 5:43 pm

Cheers guys !
I am the proud buyer of this nice machine and we managed to sort the video issue out yesterday. After looking at the schematics i found in the PAL video cirquitry on the mother board a small variable capacitor named C117 (2,7-10pF) i gave that component some contact spray and twitched the adjustment scew very gently back and forward. That did the trick :-) , the picture immediately got rock solid. I have runned the machine in bench for some hours now and it works perfect.

I am reaching out to you to share this fix since i understood from Troed that this is a very unusual problem so if any of you STe owners get a horisontal disturbance in your RGB signal this might be a good place to start.

Thankś also to Troed for his great support.

Greets from Anders.

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Re: "Waving" RGB output - cause?

Postby troed » Tue Feb 27, 2018 9:03 pm

So, after Anders found the problem it becomes a bit easier to backtrack to how it could happen :)

I only use ubeswitch VGA-adapters, that is, HSYNC and VSYNC signals that come directly from GST-MCU through an inductor and resistor to the monitor port. http://info-coach.fr/atari/hardware/int ... olor_cable

RGB SCART, which Anders used, uses CSYNC (composite video / composite sync) instead for its sync signals. http://info-coach.fr/atari/hardware/int ... itel_cable

So, knowing that I didn't see the problem and Anders did, the logical conclusion would be to search for errors in the composite video part of the output signal :) I'm guessing CCLK, based on the schematics.

Screen Shot 2018-02-27 at 22.01.42.png


Screen Shot 2018-02-27 at 22.08.24.png


I've never seen an error like this before, but now we know ...
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Re: "Waving" RGB output - cause?

Postby czietz » Tue Feb 27, 2018 9:41 pm

The function of the complicated looking circuit in your second screenshot is actually quite simple: it's a PLL that syncs the 32 MHz clock to the 4.43 MHz (PAL color subcarrier) crystal. When this sync fails, I have however never seen wavy output before, either. Just weird color artifacts -- and on the composite video signal only, not on RGB. Also when the sync fails this means that the STe was probably running on a slightly off 32 MHz clock.

EDIT: The pulling range of the varicap in the 32 MHz oscillator won't be that big, maybe in the order or 0.1-0.2%. This means that a PAL STFm or STE where this sync circuit failed might for example run on 32.09 MHz instead of the nominal 32.085 MHz.

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Re: "Waving" RGB output - cause?

Postby FedePede04 » Sun Apr 29, 2018 8:58 am

thx for this post. Mine starts to dance left and right(a couple of pixel) after a round 1 hour use. Sound like the same problem
Last edited by FedePede04 on Mon Apr 30, 2018 6:55 pm, edited 1 time in total.
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Re: "Waving" RGB output - cause?

Postby telengard » Mon Apr 30, 2018 3:49 pm

I too am experiencing this issue w/ my 1040STF, however it seems more random while the video posted, the pixel offsets seems very uniform, or maybe periodic is a better word.

I looked at the schematic and it seems the trimmer cap, C117 is PAL only. Not sure where to look next.

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Re: "Waving" RGB output - cause?

Postby czietz » Sun May 13, 2018 9:32 am

Andega wrote:After looking at the schematics i found in the PAL video cirquitry on the mother board a small variable capacitor named C117 (2,7-10pF) i gave that component some contact spray and twitched the adjustment scew very gently back and forward. That did the trick :-)


As it turns out, the guide for the Atari workers on how to adjust C117 survived. Thanks to this find of mine one is now able to set the correct value of the C117 trimmer in a PAL STE, like fresh from the factory, using only an oscilloscope.

pal-pll-adjust.png


PS: In a PAL 1040STFm the component designators will be different, but since the circuit is similar, the procedure should be applicable as well.
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Re: "Waving" RGB output - cause?

Postby jamesfmackenzie » Wed Jun 06, 2018 6:35 pm

Very interesting! I use RGB SCART and have a very similar issue with my STE - video starts to wobble after being powered on for 10-15 minutes.

Can you confirm this issue disappears with the ST2VGA or ubeswitch adapters? If so I’ll get one - don’t have an osilloscope to tweak the capacitor unfortunately

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Re: "Waving" RGB output - cause?

Postby jamesfmackenzie » Sun Jun 17, 2018 7:23 am

jamesfmackenzie wrote:Very interesting! I use RGB SCART and have a very similar issue with my STE - video starts to wobble after being powered on for 10-15 minutes.

Can you confirm this issue disappears with the ST2VGA or ubeswitch adapters? If so I’ll get one - don’t have an osilloscope to tweak the capacitor unfortunately


Any thoughts on this?

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Re: "Waving" RGB output - cause?

Postby troed » Sun Jun 17, 2018 12:07 pm

Well, yes, if it's the same error. Those adapters use hsync, not csync.

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Re: "Waving" RGB output - cause?

Postby jamesfmackenzie » Fri Aug 10, 2018 10:30 pm

To close the loop on this - I purchased a VGA cable (to replace my RGB SCART cable) and the video instability is completely gone. So (at least for my STE), HSYNC works way better than CSYNC


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