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 Post subject: PC Engine jailbar research
PostPosted: Sun Jul 08, 2018 2:59 pm 


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Joined: 02 Oct 2010
Posts: 2962
Location: UK
For some time now behind the scenes I've been researching the jailbar issue that seems to affect Core Grafx and Core Grafx 2 consoles when RGB modded. I don't like to offer mods in my store unless I'm really happy with them and while the RGB mod is fantastic on the Supergrafx and (I'm assuming) other PC Engines, we can't seem to shake these jailbars entirely on the CoreGrafx machines.

So here's exhibit one, PC engine with 4 caps replaced - C121,C131,C132,C135

Imagebas-pc-engine by videogameperfection, on Flickr

Honestly it's not bad.. but you can definitely still see the jailbar effect. Next up we tried completely re-capping a machine

Imagematt-pc-engine by videogameperfection, on Flickr

Recapping did seem to have an effect but again can't seem to get rid of the jailbars entirely.

We also tried electrolytic rather than ceramic capacitors for the jailbar fix, theoretically electrolytics do a better job at AC decoupling (I'm just repeating what I've been told here) but in practise it didn't seem to make much difference. We're also always testing using a linear power supply.

Anyway, since it's always worth throwing this out to the community, that's where we're at. At the moment we can't get rid of the jailbars on these Core Grafx machines entirely and we've tried multiple consoles.
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 Post subject: Re: PC Engine jailbar research
PostPosted: Sun Jul 08, 2018 8:28 pm 


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I assume this is with proper 9MHz video LPF like on the OSSC, right?

I'm not familiar with the layout of the Core Grafx consoles, but one thing I'd check for is to see how the subcarrier for the composite video is routed. Sometimes removing that source does the trick. Other times is it's because the RGB lines run too close to a crystal clock or some other coupling issue, and lastly, it can be encoder itself (though I assume you are bypassing the encoder in an RGB mod).

Edit: Also I assumed you tried upgrading C121 and C135 with 4.7uf?
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 Post subject: Re: PC Engine jailbar research
PostPosted: Mon Jul 09, 2018 1:24 pm 


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Joined: 09 Aug 2017
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Location: Australia
The bypass board needs its own power supply to be rid of jailbars it seems.
Depending on where you grab 5v from the jailbars can get really bad.


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 Post subject: Re: PC Engine jailbar research
PostPosted: Tue Jul 10, 2018 10:56 pm 


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Joined: 02 Oct 2010
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Quote:
I assume this is with proper 9MHz video LPF like on the OSSC, right?


Yep that's through my OSSC.

Quote:
I'm not familiar with the layout of the Core Grafx consoles, but one thing I'd check for is to see how the subcarrier for the composite video is routed. Sometimes removing that source does the trick. Other times is it's because the RGB lines run too close to a crystal clock or some other coupling issue, and lastly, it can be encoder itself (though I assume you are bypassing the encoder in an RGB mod).


Yeah he says he's checked for most of that. I'm actually tempted to cut composite video altogether see if that makes a difference.

Quote:
Edit: Also I assumed you tried upgrading C121 and C135 with 4.7uf?


Yes that's been tried.

Quote:
The bypass board needs its own power supply to be rid of jailbars it seems.


Interesting, I'll pass that along.
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 Post subject: Re: PC Engine jailbar research
PostPosted: Wed Jul 11, 2018 2:10 am 


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Location: Australia
Changing the input output caps from 47uf to 220uf seemed to make a difference too. "Seemed..."

Still chasing my tail.


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 Post subject: Re: PC Engine jailbar research
PostPosted: Wed Jul 11, 2018 5:22 pm 


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Joined: 10 Jan 2014
Posts: 527
Location: USA
I spoke about this a long time ago on TheRetroRoundtable.

CVBS on the PC-E is generated kinda outside of the silicon, but it looks really nice.

The color-carrier and chrominance data are interleaved between pins 26 and 35.


Image

I don't have a CoreGrafx, so I can't do a proper analysis. But get out your scope and look for AC noise and capacitance coupling between these signal outputs, and composite video on the routing. By nature of the expansion connector, composite video is routed adjacent to the analog RGB output.

Otherwise, just lift both of those pins that I marked on the HU6260 and monitor. Is there a change? If so, there you go.
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 Post subject: Re: PC Engine jailbar research
PostPosted: Thu Jul 12, 2018 6:12 am 



Joined: 26 Feb 2010
Posts: 383
Hello,

Tim Worthington suggested that changing caps C133 and C135 should fix the issue in Core II systems. I haven’t had a chance to do this yet, but I thought I’d share. A markup supplied by Tim is included below.

DImage

Curious to hear if this fixes the issue.


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 Post subject: Re: PC Engine jailbar research
PostPosted: Fri Jul 13, 2018 6:33 am 



Joined: 26 Feb 2010
Posts: 383
I decided to install Voultar’s RGB into my Core Grafx 2. The board is great, but there are definitely issues with jailbars. I tried the following:

1. Replacing C131 and C135 per Tim’s direction

2. Replacing C121 and C135 per the info from Console5’s Wiki

3. Lifting the pins Voultar specified in his post

Unfortunately none of the above significantly reduced the jailbars.

If anyone has any additional idea, please let us know.

Thanks


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