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Improving Gonbes GBS-8200 Scart RGB-to-VGA output: Mod-Guide

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Improving Gonbes GBS-8200 Scart RGB-to-VGA output: Mod-Guide

Postby SmokeMonster » Wed Sep 10, 2014 5:39 am

2015 Major Update: A huge thank you to wheelman282 for the help!

Here is wheelman282's schematic that includes the entire circuit for building a GBS-8200 w/Scart RGB input and an LM1881N sync cleaner. This brings together all of the best methods and is the only guide with a correct sync circuit.
GBS-8200 mod schematic REV2.png
Mirrors: Postimg | Cloudstor | Mega | Imgur

My original post:
I've spent the last week working on improving the video output from my Gonbes GBS-8200. Even after wiring it for scart and adding an LM1881 sync stripper, I still ran into problems. Mainly, the video was very noisy and flickered out of sync, sometimes completely dropping the signal. This guide will explain how I overcame these problems:

1. Your power supply choice is vital to overcoming flickering. What seem like sync issues can be related to the voltage. I found that by using a 5v 2.5a power supply (2.5mm, center ground), the picture was much cleaner and stable than the 5v 2a that I first tried. Both were high-quality regulated-voltage/switching PSUs, so I don't get it. I suggest trying a few PSUs to find the best match.
Update: Ian Stedman suggests attaching a snap-on ferrite filter on the power supply's leads to reduce noise. Ian also found that the GBS uses a maximum of 0.4a, so a lower voltage power supply could actually be used.

2. Replace every cap on the GBS-8200 with high quality replacements. This discussion identifies that all GBS-8200s use low quality caps (mine were all Cheng garbage), which are a terrible choice given how sensitive the GBS is to bad voltage. I have a v4.0 GBS8200 (possibly a clone), which required six 100uf 16v, and two 470uf 16v Electrolytic caps. I replaced them with 105°rated 100uf 16v and 25v (what I had on hand) and 470uf 35v caps. Seriously, spare no expense and research to ensure that the caps you choose are top-quality. I went with Nippon and Illinois Capacitors. After recapping, the picture was finally clean and stable.
Update: Ian Stedman found that only one of the original caps was problematic. I'm happy with the extreme clarity that I'm getting though, so I don't regret replacing the caps. Take this as a warning though that it might be overkill to replace them all.

Genesis/Megadrive Users: I had problems with my Genesis Model 2 Boosted Sync Scart cable. I replaced it with a raw sync cable and that did the trick. Run the GBS's autoscan feature (hold its down button for 5 seconds) if you have problems with the Genesis's video.

Notes:
-A 680 ohm resistor is absolutely necessary on the LM1881's sync output. See Ian Stedman's research for more details.
- Scart Pin 16 should not be connected to +5v, no matter what you read elsewhere. Many tutorials suggest connecting +5v on the GBS-8200 to Scart Pin 16. Voltage on Pin 16 is what older EU TVs used to boot to the proper video mode. Connecting pin 16 to +5v on the GBS is redundant since this signal is read by neither the console nor the scaler. Thanks to wheelman282 for this correction.
-If you get any initial distortion or weirdness, hold down the GBS's down button for six seconds to run autoscan.
-Set "Clamp ST" to 3, and "Clamp SP" to 4 in the GBS's Geometry Menu
-Keep the GBS's sharpness setting between 3 to 5. Going higher results in artifacts.
-Use shielded wire for the Scart RGB video wires, and ground them on both ends
-The $20 T-SLG v1.4 Scanline Generator is worth its weight in gold, and is in production again after a long hiatus.
*This entire guide also applies to the Gonbes GBS-8220.

The final result is a beautiful picture with rock-solid stability and no sync issues or noise. I mean, no noise whatsoever, which seemed impossible given how bad it was to begin with. On 240p consoles, the 8200 performs best at 800x600. I'm now convinced that a recapped GBS-8200 is the best picture you can get without stepping up to an XRGB.

In my setup, the picture now pops like an arcade CRT and the scaling is light-years ahead of my old Scart RGB-to-HDMI converter. It's the best my old consoles have ever looked on a non-CRT, and the total cost is $55 for a GBS-8200, T-SLG, and parts.

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Re: Improving GBS-8200 Scart RGB-to-VGA output: Mod-Guide

Postby Shadow_StarLust » Wed Sep 10, 2014 7:11 am

Nice work! :D glad it works!
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Re: Improving GBS-8200 Scart RGB-to-VGA output: Mod-Guide

Postby thewheelman282 » Wed Sep 10, 2014 6:44 pm

i remember discussing with you about getting one of these for myslf a while ago and i did but i have yet to get it modded for scart. Its been sitting on the shelf tempting me to just get it finished already. Im glad you wrote this guide just in case i run into problems when i finally get around to this :) good job man! Bookmarked and thumbs up!
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Re: Improving GBS-8200 Scart RGB-to-VGA output: Mod-Guide

Postby SmokeMonster » Fri Sep 12, 2014 11:34 pm

@wheelman I had mine sitting around for like a year before I decided to give it a go too. I kept putting it off because of the unimpressive youtube videos of the gbs-8200 in action. In person, it looks damn good and has me excited to start playing some older games again.

I need to test it with an Xbox and DC to see how it handles 480p, which I'm guessing is a lot better than 240p. update: after adding the sync stripper, the only input that still works on the GBS is scart, this makes XBOX and DC component input impossible unfortunately.

I'm still an s-video evangelist, simply because of how easy it is to get a beautiful picture quality. RGB is more the domain of DIYers (GBS8200) and those with more disposable income (XRGB).

Comparing the GBS-8200 to my old favorite scaler, the Hall TVB-200PLUS Composite-to-VGA converter, after more 240p testing I still strongly believe that both have their merits:

The Hall TVB200PLUS is a simple pixel doubler, and was made for 240p-to-480p. Therefore, you get no filtering artifacts or weirdness from it, and the picture is stellar with/without a scanliner. The colors are not as vivid, and it is not as sharp as RGB, but the picture is steady, relatively sharp, and unadulterated.

The GBS-8200 has luscious colors and the perfect sharpness/clarity that you want out of 240p. The downside is that since it treats 240p as 480p, there are some rare artifacts (lines and sprites occasionally flicker). The filter is picky too, sometimes blurring the edges of sprites in movement (depending on the console/game). For example, Super Mario World is quite blurred, but Darius Gaiden on the Saturn looks nearly perfect--gorgeous in fact. Any filtering is cleaned up beautifully by a scanliner, which may be an absolute must for some titles and unnecessary on others.
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Re: Improving GBS-8200 Scart RGB-to-VGA output: Mod-Guide

Postby SmokeMonster » Sat Sep 13, 2014 1:53 am

Also, here is the exact 5v 2.5a PSU that I use. They're only $1.29, so you might want to grab one for future use. Very nice 1st-party quality, fits power strips, and it has the exact 2.5mm tip that the GBS uses. Good stuff.
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Re: Improving GBS-8200 Scart RGB-to-VGA output: Mod-Guide

Postby thewheelman282 » Sat Sep 13, 2014 2:40 pm

Thanks for that link smokemonster. I went ahead a bought one for when I get around to this :) where'd you get the capacitors? I looked around and found those brands but not those values
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>Helpful Tutorials<
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How to properly burn Dreamcast games

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Re: Improving GBS-8200 Scart RGB-to-VGA output: Mod-Guide

Postby SmokeMonster » Sat Sep 13, 2014 6:21 pm

Here are the great Nippon 470uf 35v caps (2 for $3 shipped), and here are some beautiful Illinois 100uf 25v (make an offer of $1 and the seller will accept).
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Re: Improving GBS-8200 Scart RGB-to-VGA output: Mod-Guide

Postby thewheelman282 » Thu Sep 18, 2014 1:46 am

i actually forgot to even look at those caps until now. and dang 20 caps for $2.10 is a great deal! i bought the 470uf and offered $1 on the 100uf. thanks for your help man :D if i had gotten around to getting my GBS-8200 up and running sooner and seen that the picture quality was crap i would have been pulling my hair out!
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>Helpful Tutorials<
How to properly burn PS1 games
How to use XboxHDM
How to install FreeMCBoot on your PS2
How to properly burn Dreamcast games

Image My YouTube Channel >> thewheelman282
Watch tutorials on console modding, repair, and more

The Golden Rule of Xbox Modding:
Backup eeprom.bin to your PC
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Re: Improving GBS-8200 Scart RGB-to-VGA output: Mod-Guide

Postby SmokeMonster » Thu Sep 18, 2014 4:33 am

Glad to be of assistance :) Keep in touch with your progress.

Those are quality caps too, which seems to be important for the GBS. I am eventually going to rewire my entire scart cable to get every single wire shielded/grounded. I also want to shorten the sync lines as much as possible.

Every system I have looks excellent, except the Genesis, which has sync issues. It may be its boosted-sync scart cable, but I want to make sure that it's not something on my end before ordering a replacement.
Update: Yep, it was the Genesis's boosted sync cable. I got a raw sync scart RGB cable and it looks and works perfectly 8-)
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Re: Improving GBS-8200 Scart RGB-to-VGA output: Mod-Guide

Postby Gelip » Fri Dec 05, 2014 5:12 pm

Hello all ! This is my first post on this forum. Has just bought a GBS-8200 to my ZX Spectrum clone "Byte". I test it and also noticed some white dots on black screen. I use 5V 2A PSU. Can I connect the 5V or 12V power supply AT or ATX?
P.S. To: SmokeMonster - please make photo and/or wideo how work your modded GBS :)
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Re: Improving GBS-8200 Scart RGB-to-VGA output: Mod-Guide

Postby SmokeMonster » Tue Jan 20, 2015 12:01 am

Wheelman282 contacted me with some improvements and corrections to the tutorial, so I've updated it to reflect them.

Gelip wrote:Hello all ! This is my first post on this forum. Has just bought a GBS-8200 to my ZX Spectrum clone "Byte". I test it and also noticed some white dots on black screen. I use 5V 2A PSU. Can I connect the 5V or 12V power supply AT or ATX?
P.S. To: SmokeMonster - please make photo and/or wideo how work your modded GBS :)

Wecome to TIZ Gelip! :D Have you replaced the capacitors? I know it's a major surgery, but I would bet that this is your problem. You could also test out a different PSU first to make sure of it. I had some issues with the first 5v 2.0a PSU that I tried.

As for a video..a little birdie told me that a user may already be working on one 8-)
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Re: Improving GBS-8200 Scart RGB-to-VGA output: Mod-Guide

Postby Gelip » Tue Jan 20, 2015 5:27 am

I wrote in this thread about "White dots". User "Tronix" made to modify the PCB and confirms that this solves the problem :-)
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Re: Improving GBS-8200 Scart RGB-to-VGA output: Mod-Guide

Postby SmokeMonster » Tue Jan 27, 2015 7:00 pm

I've updated the guide with some great findings by wheelman282 and Ian Stedman's thorough testing.
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Re: Improving GBS-8200 Scart RGB-to-VGA output: Mod-Guide

Postby IchiSumeragi » Sat Jan 31, 2015 8:18 pm

Hey,

First post here aswell.

Thanks for this guide. Provides really helpful informations,and I will probably replace the capacitors of this board in the future.

But I'm running across a problem with this board : I want to use it in a 31KHz arcade cabinet to play with 15KHz Jamma PCBs. Basically,I hooked up a VGA connector to the Jamma harness (with the Jamma sync hooked up to both the H and Vsyncs of the VGA connector) ; I plug in this VGA into the VGA input,and I plug the VGA cable of my arcade cab monitor to the VGA output. But I am having sync troubles with that wiring : on some of my PCBs,the picture is doing this :

https://www.youtube.com/watch?v=J0QGp6LU7ag

I really have NO idea why it behaves like that. I'm seriously starting to think that my board is just faulty,yet it is brand new... It does the same thing on an LCD screen.

If you guys have any idea why,any suggestion is appreciated :)

Thanks !
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Re: Improving GBS-8200 Scart RGB-to-VGA output: Mod-Guide

Postby SmokeMonster » Sat Jan 31, 2015 8:57 pm

Welcome to TIZ, Ichi! :D

Great idea to shoot a video of your problem. This makes it easy to diagnose. What you're experiencing is a sync issue. I've had almost that exact same occurrence with SNES and Genesis depending on the cables that I used. Your board isn't faulty, so don't go replacing it :)

To cure this problem, you'll need to add an LM1881 as a sync cleaner between your source and the GBS's sync input. When you do this, make sure to add a 680 ohm resistor to the LM1881's sync output to bring its voltage down to the correct levels for the GBS.
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Re: Improving GBS-8200 Scart RGB-to-VGA output: Mod-Guide

Postby IchiSumeragi » Sat Jan 31, 2015 9:00 pm

Hmm,what's strange is that I have one or two boards plus an MVS slot that work just fine without the sync cleaner. They sometimes loose the sync,but they are indeed working. Should I still make a sync cleaner?
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Re: Improving GBS-8200 Scart RGB-to-VGA output: Mod-Guide

Postby SmokeMonster » Sat Jan 31, 2015 9:12 pm

Yes, I would make it regardless as a first step. Each board probably has a slightly different sync output, which is why the LM1881 is a great addition. It will correct the sync for everything you plug in to it, and will possibly improve their video as well.

The issue of dropping the sync could be because the MVS boards are outputting too high of a voltage load for the GBS to handle (it accepts a max of 3.6v sync), and as a result the ESD protection diodes are being activated and the device will be damaged in the long term. Ian Stedman has documented this on his blog if you would like more details. The LM1881 even outputs 4v natively, which is why a 680 ohm resistor is necessary to bring the sync's voltage down to 3v.
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Re: Improving GBS-8200 Scart RGB-to-VGA output: Mod-Guide

Postby IchiSumeragi » Sat Jan 31, 2015 9:19 pm

Well,no,my MVS board works perfectly. This video was taken from an arcade PCB,not an MVS.
Simply put,only a couple of my jamma boards and my MVS slot are working ; I have 6 or 7 others arcade PCB that have this sync problem. I have a V4 board,by the way,if this can help.

I will try the LM1881 circuit anyway,hoping that it works. But it's really strange that only a couple of them work. I really have no idea why,all are proven to work fine on a 15KHz cabinet.
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Re: Improving GBS-8200 Scart RGB-to-VGA output: Mod-Guide

Postby SmokeMonster » Sat Jan 31, 2015 9:25 pm

I think the LM1881 will clear everything up. It's just an issue of different sync signals for different boards.

It's the same for old consoles. Before upgrading my GBS, I could plug in a Saturn and have a relatively stable image, but the SNES would flicker out of sync every 20-30 seconds, and the Genesis looked like the video you posted. To make things stranger, an SNES mini would work perfectly. After adding the sync cleaner, they're all perfect though.
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Re: Improving GBS-8200 Scart RGB-to-VGA output: Mod-Guide

Postby IchiSumeragi » Sat Jan 31, 2015 9:29 pm

Okay,so that seems to be the solution. Either way,it's not going to make things worse,so I might aswell give it a try.

Thanks a lot ! I'll check tomorrow if I have an LM1881 laying around (tho I really don't think so) or I'll buy one and post the result here. ;)

By the way,just to make sure,this sync cleaner works with a VGA input,right?
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