dbGrafx Booster – Not Every SCART Cable is Created Equal

Now that many dbGrafx Boosters have found new homes with happy customers, I received several complaints that the dbGrafx Booster did not cooperate well with the XRGB-Mini Framemeister and several models of PVM. This was very surprising to me since I test and verify each dbGrafx Booster in my workshop with a Sony PVM and an HDMI upscaler before shipping – could the Boosters be deffective on arrival?

Dark Images

bonk-scart-no-caps

The main complaint was that the image was very dark on RGB outputs. Here’s an example to the right: screenshot of a dbGrafx Booster connected to a Framemeister exhibiting the dark image problem. This could maybe point to a deffective CXA1645 (video encoder on the booster) but all of these customers also verified that the S-Video output worked correctly which also comes out of the CXA1645. Dark images could also be caused by improper resistance on the RGB lines, but this is not the case since the Booster’s RGB outputs are directly DC-Coupled (just like the Genesis 2, hence use the Genesis 2 SCART cables) which essentially means the chips RGB pins are connected directly to the output connector. Maybe the problem lies in the SCART cable then.

SCART Cable Differences

I had run into an issue before where Genesis SCART cables I had purchased from several Chinese sellers on eBay did not include any 75ohm resistors on the RGB lines. This makes for a very bright image and is totally incompatible with the Genesis’s RGB outputs – stay away! After this experience I set out to find a reputable manufacturer of retro SCART cables, stumbling upon www.retrogamingcables.co.uk – looks good right! I bough a few cables, tested on my PVM, perfect!

Retrogamingcables.co.uk

SCART-with-no-caps

The common thread emerging from everyone who was experiencing issues with their dbGrafx Booster was the Genesis 2 SCART cable from retrogamingcables.co.uk. – ok so what’s up with this cable?

To the right is a picture of the insides of a Genesis 2 SCART cable from retrogamingcables.co.uk. As you can see, it includes 75 ohm resistors on the RGB lines (hidden underneath heat-shrink). But wait! There are no AC-coupling capacitors on the RGB lines! This cable directly DC-couples the Genesis’s RGB outputs to whatever your connect it to (upscaler, PVM, etc…). Ok now, most equipment “should” be able to accept a DC-Coupled signal but this is not guaranteed nor is it particularly good design practice to rely on this fact. All in all, this cable should have 220uF capacitors on the RGB lines and without them does not respect the original specifications of the Sega Genesis 2 RGB output. It may “work” with most equipment on a Genesis 2, but evidently, on a new design with a different video encoder this cables shows its weakness.

Other Genesis 2 SCART Cables

I ordered a bunch of other Genesis 2 SCART cables for comparison. As I expected, all other cables (not from China) included both 75 ohm resistors and 220uF AC-coupling capacitors on the RGB lines. This particular cable below was purchased from the eBay store retro_console_accessories (which was highly recommended by reputable people). This particular cable meets the output specifications for Genesis 2.

SCART-with-caps

AC-Coupling vs. DC-Coupling

What is this sorcery? Can’t we include both like the rock band and all be happy? Well no. I don’t really want to bore anyone with mountains of circuit theory (nor am I a video expert by any means). So I figured it’s probably best to link a professional article for this. All in all, the outputs on the CXA1645 (and CXA1145 for that matter) are not designed to drive DC bias current into a 150 ohm load; this happens when there is no capacitor in the cable to prevent this.

Using a Proper SCART Cable

bonk-scart-with-caps

With a proper SCART cable including AC-Coupling capacitors the dark image issue is no longer. The screenshot is a dbGrafx Booster through a Framemeister using a proper SCART cable. A big thank you to RetroRGB for testing with a Framemeister because I don’t own one. Many other users have also confirmed to me that switching away from retrogamingcables.co.uk cables has solved their problems with dark images output from the dbGrafx Booster. Awesome!

 

Conclusion

This problem has left several obvious questions:

Why Make a Cable Without Capacitors?

The most obvious reason is cost and labour. Even though 3 capacitors like the ones required are dirt cheap, they cost time in labour to mount and install inside the cable. I figure not installing the capacitors probably saves at least 2-3 minutes per cable assembly. Also, retrogamingcables.co.uk is only concerned with Genesis compatibility with his cables. I 100% disagree with “oh it works without capacitors therefore it’s ok to remove them” – this is not good design practice in my opinion.

Why Didn’t Sega Include the Capacitors in the Console?

Other consoles, like the Neo Geo for instance, include the capacitors and 75 resistors required on RGB lines directly in the console. So if they’re required, why not just put them there and thus only require a cheap SCART with no built-in components? After all, electrically speaking, it is best practice to place these components as close as possible to the video encoder – not 6 feet away in some cable. Again, the answer is probably cost. Let’s say there are 20 million Genesis consoles in the US – almost all of which never saw the use of their RGB outputs. By not including those capacitors and resistors Sega saved on purchasing and installing 60 million capacitors and 60 million resistors.

René

Electronics engineer and retrogaming fanatic!

25 thoughts on “dbGrafx Booster – Not Every SCART Cable is Created Equal

  • July 1, 2016 at 05:13
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    Thankyou for this informative guide – much appreciated.

    Am I correct in my understanding that if 220uF capacitors are added to the RGB lines of a retrogamingcables.co.uk cable (which I own), then it will resolve the dark screen issue?

    Reply
    • July 1, 2016 at 05:16
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      …. I meant 220 uF AC coupling capacitors

      Reply
    • July 1, 2016 at 09:06
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      It should yes, if you’re up to it you can try.

      Reply
  • July 1, 2016 at 09:16
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    I’m fairly sure that it does, but just wanted to check this is the same for the UK version, Megadrive 2?

    I also have the retrogamingcables cable, and will fit the caps if it is.

    Reply
    • July 1, 2016 at 09:26
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      You could always open your SCART connector and share a few detailed pictures (here, Twitter, Facebook) – that way I could tell you if your UK cable has the same issue.

      Reply
  • July 1, 2016 at 09:35
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    Sorry, I should have been more clear!

    I have the exact cable as shown in your first picture from retrogamingcables.
    I’m using it with the dbgrafxbooster, and suffering the same fading issue. im sure it only needs the caps, but wanted to check there was no difference between a PAL(Megadrive 2) and NTSC (Genisis 2) cable pinout/voltages before I add the caps?

    Reply
  • July 1, 2016 at 18:28
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    Why don’t you ask RGC to fit capacitors into their future Megadrive 2 cables? I’m sure they will listen to you.

    On a side not my official French Megadrive SCART does not have capacitors on the rgb lines, so I don’t blame RGC if they have copied their design from an official Sega cable.

    Reply
    • July 1, 2016 at 18:43
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      I have spoken to them before. They have removed the caps because it “works” without them. What they don’t realize is the larger DC bias current which results from a cap-less cable is actually bad for the video driver whether it’s the CXA1145 in the Genesis or the CXA1645 in the booster.

      Reply
      • July 1, 2016 at 19:31
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        I have ordered some caps now, 220uf 6.3v radial electrolytic capacitors are required from what i’ve read.
        I’ll get them fitted and post back 🙂

        Reply
  • July 1, 2016 at 18:48
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    Maybe you should fit caps to your board, because if somebody uses an official Sega cable they will have the same problem found with RGC cables, or are you trying to save money just like sega did? 😉

    Reply
    • July 1, 2016 at 19:37
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      Well no, you don’t understand. If I fit caps onto my board I break compatibility with every properly designed SCART cable out there.

      Reply
      • July 2, 2016 at 02:13
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        I understand that the cause of the issue is the retrogamingcables and not your own pcb. If we were to attach the 220uf AC Coupling capacitors to the DB electronics pc engine rgb board, whereaboouts would we solder them too, to resolve retrogamingcables issue? I know that this would remove the compatibility with other genesis cables, but just looking at all approaches to resolve the issue without buying additional cables as I’m happy to get the soldering iron out

        Reply
        • July 2, 2016 at 09:02
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          There are three zero ohm resistors (R103, R104, R105) connecting the RGB outputs of the CXA1645 to the Genesis 2 AV connector. You could remove these three resistors and solder the 220uF coupling caps to those pads. Not an ideal solution but it should work.

          Reply
    • July 8, 2016 at 08:57
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      I did the exact same, added the caps to the cable I had already from rgc and it went dark almost immediately on the booster, but strangely it works perfectly on the megadrive 2?

      Reply
      • July 8, 2016 at 09:21
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        The fact that it works on Megadrive 2 is actually only a testament that the MD2’s video encoder is harder to drive into failure with excess bias current. At this point I would simply recommend to get a proper cable from retro_console_accessories. The retrogamingcables.co.uk simply don’t live up to the quality and design of the retro_console_accessories – not only for RGB caps.

        Reply
    • July 8, 2016 at 09:19
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      At this point I would simply recommend to get a proper cable from retro_console_accessories. The retrogamingcables.co.uk simply don’t live up to the quality and design of the retro_console_accessories – not only for RGB caps.

      Reply
    • August 12, 2016 at 10:25
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      Your guess is as good as mine. It “sorta works” with the Genesis without cables but it causing a large amount of DC current to flow on the RGB output signals – something the Genesis’ video encoder is absolutely NOT spec’d to handle but still does by shear luck!

      Reply
  • August 12, 2016 at 10:35
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    I have the cable from retro_console_accessories and it works perfectly.

    its strange that the cable i had before (that i added the caps in series with the resistors) doesn’t work?

    I can only surmise it is wired differently, I will pull them both apart and see whats going on, hopefully I can make a good schematic of the working cable for everyone.

    Reply
  • September 1, 2016 at 21:54
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    Well, unlucky me. About 8 days ago I received the DB Grafx Booster in the mail. I did not have any RGB cable options yet but I did have an S-video cable lying around. I hooked up my TG16 to my PVM via the S-video and it looks great, really sharp. A few days later I set about ordering the Genesis 2 SCART cable from retrogamingcables.co.uk. (I bought Sega Saturn cables from them and they work perfectly). Today I happened to be perusing db-electronics.ca when I came across this thread and find out the cable I ordered and which is on its way is likely going to not work properly. I’ll try it out anyway when it arrives. Is there any chance the retrogamingcables could damage the DB Grafx Booster?

    Reply
    • September 2, 2016 at 13:35
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      Yes, I would definitely not recommend using those cables, even in your Genesis.

      Reply
  • September 9, 2016 at 14:49
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    This is a follow up to my previous post. The Genesis 2 SCART cable from retrogamingcables.co.uk arrived yesterday. I connected everything and turned on the TG16. The image went almost completely black in about a second. The brief moment the image was visible I saw some nice scan lines, heh. Needless to say I’ll be getting the Genesis 2 SCART cable from retro_console_accessories.

    Reply
  • October 12, 2016 at 02:33
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    Oh my goodness! Impressive article dude! Thank you so much, However I am having issues with your RSS.
    I don�t understand why I am unable to subscribe to it. Is there
    anybody having identical RSS issues? Anyone who knows the answer will you kindly respond?

    Thanx!!

    Reply

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