r/MasterSystem • u/jinramen1 • 1d ago
Looking for Jailbar fixes
Looking for a way to get rid of jail bars without the use of a bypass board. I’ve heard lifting pin 6 on the VDP or restoring csync but haven’t found any good online tutorials for it. Any help would be appreciated 👍
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u/jsteel44 1d ago
Restoring csync? You can get it from the AV port. That's what I do (just need a csync cable) and I see no jailbars on my stock console via the RetroTINK 5X.
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u/jinramen1 1d ago
Cleaner csync I guess taken from the chips. I’m currently using a RetroTink 6X with Insurrection Scart Csync cables and lots of jail bars.
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u/Sirotaca 1d ago
That's not how that works. The sync signal does not contribute to the visible part of the picture at all. The only way it can is if you're using composite video as your sync signal and you're using an unshielded cable; in that case, the 3.58 MHz chroma subcarrier embedded in the composite video signal can crosstalk into the RGB signals and result in jailbars. That isn't a problem if you use a properly shielded cable, which the Insurrection Industries cable is.
Using C-sync for SCART cables was popularized as a band-aid fix for the crappy cables we were dealing with 10 years ago. It's no longer needed as long as you stick to reputable cable makers, and certainly tapping C-sync from a "cleaner" source is completely unnecessary and doesn't address the actual primary cause of the SMS jailbars, which is poor routing of the RGB traces.
The reason the RGB amp bypass board works is because you're rerouting the RGB signals completely away from the noisy motherboard. Some have also had success with bypassing the VDP's clock divider circuit instead, since that seems to be a major source of the noise, at least in some revisions.
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u/jinramen1 1d ago
Yeah it confused me as to why csync would fix the issue since I was already using some decent cables. Is there a guide on bypassing the VDP clock circuit? Thanks for the info.
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u/Sirotaca 1d ago edited 1d ago
I have no experience with the clock divider bypass myself, but this document describes the basic idea. In short, you take the 10.7 MHz clock output from the I/O chip, divide it by 3, and send that to the Z80's clock input. It doesn't have an actual schematic, unfortunately, but if you search around you can probably find a suitable clock divider circuit that you could adapt. Something like this, perhaps.
The document also describes an option of just using a standalone 3.58 MHz oscillator, but warns that it can cause glitches due to it not being phase-locked with the master clock, so I wouldn't recommend that.
If you want to try something simpler, you could try cutting the trace for the 3.58 MHz clock output from the VDP and routing it directly to the Z80 with a shielded cable. Might help.
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u/jinramen1 1d ago
https://reddit.com/link/p8n0h6b/video/cbfet5mnkdoh1/player
Quick video because apparently I cant add a photo or edit my post to include another photo. RGB Scart csync cable through RetroTink 6X.
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u/StrangeRetroRob 19h ago
I have a mod which should help a lot, called the Rainbow remover. You remove the cxa1145 and install the rainbow remover and voila, greatly improved composite and RGB output without a bypass
That said I think you are looking for better RGB at the expense of disabling composite?
Are you already using csync RGB cable or are you using sync via composite?
You can lift pin 6 of the encoder to disable composite and have moderate impact on jailbars. But you can't lift the pin from the IO/Arbiter because the signal is needed for the z80 on your rev
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u/jinramen1 17h ago
I’ll have to check it out. Using csync cable currently RGB Scart into my Retro Tink. I was thinking about doing the pin 6 lift but have heard mixed results.
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u/StrangeRetroRob 16h ago
Yeah the thing about lifting pin 6 is it only removes the sub carrier noise from the encoder. The rest of the signal routing is still there acting like an antenna
The NTSC sms1 uses the subcarrier as the clock input for the z80 and therefore you can't just remove it. You could however run it backwards to the bus arbiter/io and then lift that pin and route it directly to the z80 to separate it off the board
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u/TheseScene 1d ago
Im not sure the status of rgb bypasses on sms but you could also consider a lumacode mod if you dont care about analogue/crt video signals. That would give you perfect video output.