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Jailbars fix found
Post Posted: Wed Aug 05, 2020 9:11 pm
Last edited by SLr on Tue Aug 18, 2020 7:55 am; edited 1 time in total
You read that title right !

That works, cost next to nothing and can be done in 5 minutes or less !

I've been experimenting with a theory (that isn't mine) and a complete guide is on its way, just waiting to get my hands on one last elusive PAL revision...

Here's the short, short version for people with enough know how and tools on hand who want to try it now :

Pre-requisite :
RGB Bypass (not just an RGB mod from the video encoder, a full-on RGB Bypass). An NTSC or PAL console that has been converted to NTSC WITH THE CORRECT CRYSTAL OSCILLATOR (if you have a PAL 50Hz or PAL "fake"60Hz console, please wait for the complete guide)

What you need :
-3.579545MHz crystal oscillator
-Something like 6cm of wire

What you need to do :
-Cut pin 33 of the VDP
-Cut pin 6 of the Z80
-Wire the new crystal to pin 6 of the Z80
And...
That's it.

The RGB bypass needs to be done correctly and these components (or their equivalent on other revisions) need to be removed :

SMS2 IC BD M4.Jr PAL - CXA1145P chip (IC 9), R12, R13, R14, R15, R16, R17, R18, C27, C28, C29, C31.
SMS2 IC BD M4.Jr PAL 2M - Fujitsu MB3514 chip (IC 9), R12, R13, R14, R15, R16, R17, R18, C27, C28, C29, C31.
SMS1 VA-3 - CXA1145P chip (IC 9), R51, R52, R53, R54, R55, R56, R57, R71, C18, C56, C62.

I'm new to this website but if I did it correctly, there should be a picture in attachment. This picture has been taken on a 50" (127cm) 4K LCD with a fully shielded RGBS cable, on an OSSC (5x mode, optimal timings, 9MHz LPF, 2.5MHz Sync LPF)...

For those waiting for the guide, it will be published on mmmonkey's website when it'll be ready. For updates, follow me on Twitter @SLR_Mod_Shop

I am NOT fishing for followers here, follow me, when the guide has been released, un-follow me straight away (shouldn't be too difficult, I'm a pretty boring lad ^-^). As for a release date for the guide... That will depend on when I get my hands on a PAL M4 SMS1... Hopefully soon... Know that I'm trying my hardest to get this done quickly. Basically, if I were to get one tomorrow, the guide would be available tomorrow...

If you try for yourself, please share the result here, that would be cool !

EDIT: I wasn't clear enough, by "theory that isn't mine" I meant "discovered by someone else". That someone else is Leah Rowe from RetroFreedom's website ! RetroLeah on this forum. (I asked the permission to share the name)
DSCF8548.JPG (3.61 MB)
DSCF8548.JPG

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Post Posted: Wed Aug 05, 2020 11:41 pm
That looks great, nice work on the mod :) Reminds me of the "remove the rf modulator" mod on C64 to get a better s-video/monitor picture.

best regards,
- dink
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Post Posted: Thu Aug 06, 2020 4:49 pm
NEW DISCOVERY MADE : Jailbar free Master system 2 without an RGB Bypass board !

Your console needs a Fujitsu MB3514 (Sonic built-in).

The gist of it : Bypass the clock signal from VDP to Z80 (as explained above) and lift subcarrier pin, do a simple RGB mod (NOT a bypass, there's no need)
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Post Posted: Thu Aug 06, 2020 10:23 pm
LOL looks amazing without jailbars. Thankyou!
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Post Posted: Fri Aug 07, 2020 5:08 am
Wouldnt disconnecting the clock pins and using shielded mini coax between the vdp and z80 instead of the pcb traces achieve the same thing? Why is a separate crystal really needed?
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Post Posted: Fri Aug 07, 2020 8:11 am
Hmmm, that's a good point, I would assume so.
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Post Posted: Fri Aug 07, 2020 9:33 am
Oh no, brain issue, that's a good idea but it wouldn't work. The noise comes from the signal within the VDP and not on it's way to the Z80 (although it plays a role obviously).

The 3.58Mhz signal (for 60Hz consoles) is generated in the VDP, so unless you can drill a hole in the chip and bypass it from there, it wouldn't make the cut.

Nice thinking though, I remember trying but my brain just didn't work properly earlier (it was like 9am and I had yet to drink my coffee).

Still working on a better solution though, that would be compatible with both NTSC and PAL and wouldn't require to source crystal oscillators.

Basically, a clock divider (by 15) that you use as the bypass, no matter what the crystal oscillator value would be, 53.2 or 53.69MHz, it would give you the correct signal for the Z80 (3.58 or 3.54MHz).

So many things to check, it's a bit of a long process. I only have four 60Hz NTSC crystals at the moment to do my tests (I'm in PAL territory) and like 12 Master System, some have a switchless mods, other don't, some with a 60Hz Xtal others with a 50Hz, some with a bypass others with a RGB mod etc. And I still have other consoles to mod in the meantime...

The goal is to make a definitive guide... which I keep updating as I'm testing different revisions and combinations of mods. Believe it or not, that's already a 16 pages pdf... The goal is to find a clever solution (a bit like what you've proposed) that would make the fix cheap, easy to source and with a guide that would hold on less than 10 pages for all consoles no matter the region of said console. (the guide covers RGB mod and RGB bypass too, no point sharing that fix if people introduce noise while doing those other mods incorrectly, so I have to cover all the angles)
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Post Posted: Fri Aug 07, 2020 10:35 am
SLr wrote
The 3.58Mhz signal (for 60Hz consoles) is generated in the VDP, so unless you can drill a hole in the chip and bypass it from there, it wouldn't make the cut.


By disconnecting pin 33 on the vdp, isnt the clock divide still happening internally within the vdp anyway?
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Post Posted: Fri Aug 07, 2020 10:53 am
Yes but it goes nowhere as it has nowhere to go. So, it doesn't interfere with video signals. If you probe the pin, then the signal has somewhere to go and you see the jailbars come back. But as long as you don't give it anywhere to go, the signal won't run through its intended path.
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Post Posted: Sat Aug 08, 2020 2:08 pm
what if you make that signal go from the VDP to the CPU by a flying wire (shielded?) instead of using the PCB trace?
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Post Posted: Sat Aug 08, 2020 3:58 pm
See IC5 as a tiny pcb, even if you find a way to bypass the signal on the motherboard, you still have the "tiny pcb" that has that signal coexist with RGB lines.

I invite anyone to try the shielded wire method for himself (make sure to have the needed crystal or being able to put everything like it was before).

We're working on a "universal" solution that would involve a DFO. But this would be an alternative.

Trust me, I would love that fix to be : cut pin 33, cut pin 6 and run a shielded wire from pin 33 to pin 6... The core of the issue happens in the VDP itself.
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Post Posted: Sun Aug 09, 2020 9:04 pm
I see your point. It's not simply the SMS PCB.
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Post Posted: Mon Aug 17, 2020 8:45 pm
Last edited by retroleah on Tue Aug 18, 2020 7:56 am; edited 1 time in total
EDIT: SLR thank you for crediting me in your original post.
My original complaint was below:

So hey SLR please can you reference the fact that I discovered the oscillator bypass method for SMS2 jailbars ;) And mention my twitter, @n4of7 while you're at it ;)

I'm sure this fact in an innocent omission on your part :P

(having said that, my main priority is that people learn about this trick: and you've shared this knowledge, which is what I wanted, so thank you for sharing).

However, in your first post on this thread, you make it look like *you* came up with it xD

(proof that I came up with it first can be found on my YouTube channel. Search "Leah Rowe" and you'll find my channel, full of mods. The date of my first SMS-related jailbar research vid predates anything else you'll find on the internet, pertaining specifically to the oscillator bypass trick for Z80 CPU clock)

PROOF that I did it first:


video dated May 21, 2020. In the video, I show the oscillator bypass. Find me a publication on reputable site with earlier date than this :)

I'm not a petty person, but I DO expect to be credited for my work. If someone merely shares my work, that's fine, but if they make it looks like they did it, that really grinds my gears, you know?
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Post Posted: Tue Aug 18, 2020 7:29 am
new post rather than edit thread because I think it deserves new post:

SLR and I have written a fully comprehensive guide. see:
https://retrofreedom.com/jailbarsms/jailbar.pdf
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Post Posted: Fri Aug 21, 2020 8:46 pm
The PDF guide has been updated. Those who did those mod previously must be made aware:

Cutting CPU pin 6 cuts the 3.58mhz clock going to cartridge slot pin 47. Most carts don't use it, but you should go back and solder a wire jumper between pin 6 (connected to your oscillator aswell) to cartridge slot pin 47 on the solder side of the board.

We have discovered that doing the oscillator bypass to Z80 pin 6 results in a slight timing discrepancy between CPU and VDP. This is because not all oscillators have the exact same frequency, even for a given specified rate. In practise, this just means that games will show random garbage sprites on random parts of the screen, but only for a split second or two. If you want fully accurate game execution, it is recommended instead to wire a PLL that takes the VDP's clock input 10.7mhz and divides by 3 to create the CPU clock; this will ensure perfect sync between VDP and Z80.

The PDF guide has been updated to reflect this.
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Post Posted: Mon Aug 24, 2020 12:40 am
are these jailbars only visible when run through scalers etc, to LCD?

I haven't had any issues with PAL Master System II jailbars using a simple RGB mod and 60hz switch (pal crystal)

https://imgur.com/gallery/TAcROnt
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Post Posted: Mon Aug 24, 2020 3:27 am
They should be visible on any LCD screen. Go to your TV settings, and disable the low pass filter (might be called "noise filter") and you should see them. They should be more visible on certain colors and almost invisible on others.

That's the kind of thing you don't want to investigate though. Jailbars are nothing but a video issue and not one that will damage your TV. So, if you can't see them, don't look after them and enjoy your games.

Getting the best video and audio quality out of your old game systems is a rabbit hole that should be avoided when possible. Before you know it, you'll have a £130 OSSC, £35 SCART cables for every console, all your consoles recapped and modded and a wife that hates you ^-^

If you can't see them, just enjoy your games.
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Post Posted: Mon Aug 24, 2020 4:19 am
I've already fallen into that rabbit hole because I enjoy fiddling with old tech, my missus already hates me because of it, and I've already spent stupid amounts of money achieving nothing all for something to do during another covid lockdown lol

The jailbars are visible using RGB on megadrives on my CRT, very visible until lifting the suggested pin on the VDP (well, cutting the trace which was easier). Other guides suggested lifting a pin on the sony encoder (other end of connection) which did nothing. But I haven't really had issues with the Master system.

I'm surprised no one has used the schematics available on the site for the barebones french model which has very few components, and unused footprints for some caps on some address/data lines which could be omitted.

You could squash this design down to a tiny PCB leaving a stack of room for your own video circuit and other modifications, like a switchable BIOS for example.

For me, I get a headache just looking at the discrepancies between the different revisions... for instance the BIOS IC is the same across the models, but some show A15 used as EXM2 (french model), and another uses OE pin as EXM2 and shows a VPP line to +5v as well as VCC. What's going on with all these differences.

Even the the BIOS guide says tie A15 to ground along with other unused address lines if you use a different ROM IC for the BIOS.

You *can* get your hands on the proprietary IC's from overseas and with the VRAM etc readily available online, you could build a new mainboard from scratch, routing lines wherever you see fit. I will get an OSSC soon, see how it goes. Not happy with the MEGA SG as it has graphical glitches on a lot of games and there is unbearable controller lag on pretty much most games and every master system game I've tried.
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Post Posted: Mon Aug 24, 2020 5:32 am
The jailbars are generated from within the VDP itself in the Master System, that's why the solution is to simply bypass the clock signal. You could use all the tricks in the book on the motherboard, nothing will be as effective as simply preventing the 3.58MHz (or 3.54MHz for PAL consoles) subcarrier signal from creating jailbars.

So, even if you re-create the entire Master System from scratch using the best practices in the world to make the motherboard, all that would be thrown out the window if you re-use the SEGA VDP.

On CRT, a simple RGB mod will do wonders (RGB straight out of the video encoder, CSync from pin 10).

On SMS2 PAL 2M (Sonic built-in), you don't even need an RGB bypass at all in the end when using an OSSC. A simple RGB mod and the jailbars fix will yield perfect results even in 5X mode using an OSSC.

Note that the best results are achieved using optimal timings, a 2.5MHz Analog Sync LPF and a 9MHz LPF.

You're probably aware already but French Master Systems have all the video circuitry embedded within the AV cable itself so, I'm not sure those are desirable in the end. All the discoveries we've made on PAL and NTSC Master Systems might not apply and all the work would probably need to be done all over again just because of that. So, in the end, if I were to buy a French Master System, I'd probably install an RGB bypass and turn it into a simple PAL SMS rather than retro-engineering the "RVB Adapteur" (the name of that cable, means "RGB adapter" as you might have guessed already ^-^).

I'm French (I live in the UK though) and I always assumed that French Master Systems were common because they were to me when I was a kid. But turns out, those are hard to come by and pretty expensive.
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Post Posted: Mon Aug 24, 2020 2:06 pm
I heard to use Pin 10 (csync in), its written in capital letters somewhere on the internet, with exclamation marks... but I use pin 11. Pin 10 doesn't work.. not on my consoles. That was kind of the point with the French Master system containing external video circuitry... it also only has the 1 crystal on the board. Theres heaps of room to implement your own video circuit, add the pause switch mod... switch out the BIOS to the 32pin Flash ROM with a socket.. etc etc. I'd like one to just hardwire an everdrive into without messing around removing the cart slot etc
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Post Posted: Mon Aug 24, 2020 2:20 pm
Csync at pin 10 is TTL Csync, I think that was your device (TV, scaler etc) that wasn't happy with it. I take it straight from pin 10, there's a 470 Ohm resistor and a 220µF in my SCART cable and that runs fine on everything (random LCD, OSSC, PVM).

YOU CAN TAKE CSYNC FROM PIN 10 !!!! I don't know why it didn't work on you before but it should do the trick this time ;)

If you want to do that (hardwiring your Everdrive), think about adding a microUSB connector to power the console in 5V from USB. Make a little box that you can hide behind your TV and use a controller cable extension that you run somewhere in the front of your TV or TV stand. That way, it's like you have a "secret-built-in" Master System in your TV.

About USB powered Master system, I made myself a 3D printed part to get a no cut microUSB connector but I haven't had the time to properly test that. It might introduce audio and video noise depending on the USB source... So, it might be a bad idea actually.
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Post Posted: Mon Aug 24, 2020 2:23 pm
I’m not sure, I have used TTL on megadrive/ SNES and Famicom and they were ok. I didn’t really look into it as such, just tried the other pin.

This guy has some full on Frankensteined consoles that are really good looking on LCD.



https://www.smspower.org/forums/17750-ModdedMasterSystem2sAndGenesisMegadrive12s...
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Post Posted: Mon Aug 24, 2020 2:24 pm
As for USB maybe USB3 would be ok... I think USB2 is 300ma and the console wants 500ma??
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Post Posted: Mon Aug 24, 2020 2:32 pm
Ah ! I know Leah, we've been working on the SMS for a month now. I was the guinea pig for the jailbars fix ^-^ I tested all possible combinations of mods. When Leah had an idea, an hour later I came up with the answer. I absolutely love spending time with this console and lately, I wasn't happy with jailbars so I held on to all my SMS (instead of selling them) and it paid off big time because I discovered Leah's video about the fix that went under the radar of everyone else If you're using Twitter, here's my profile : @SLR_Mod_Shop I've been posting some of my SMS too. Also testing some new mods that should be available soon on Consoles Unleashed. (yep, I've been pretty lucky lately ^-^)

Yes, USB3.0 but depending on what's behind that USB3.0, it might cause some issues. I wouldn't recommend to use a cheap Chinese USB HUB to power that console. Although I'd like to see the result of that. Maybe it'd be fine... Who knows...
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Post Posted: Tue Aug 25, 2020 12:25 am
Cool as you’ve really done your research!

I’ve thrown together a schematic of the French RGB mainboard. Ive the BIOS IC to the 32pin dip for flashable BIOS, The SMS 2 in France retains the smaller SDIP28 I/O controller off the SMS 1 I believe ? I sourced a new VDP... it doesn’t look like a pull so NOS sega ICs are still out there.

I managed to find the SDIP28 I/O IC too.. it wasn’t cheap.

essentially I’m building a master system from scratch to fit in a model 2 shell.

I got the RAM traces all interfacing with the VDP. Everything has the correct footprint, labels etc including the 64QIP and basically ready to throw together and test as a prototype. It will take some time though as I work 5/6 days a week.

Any suggestions for the board, throw them at me. I will be using a cleaner linear 5v regulator that matches pins with the 7805 and I’d like to throw the RGB bypass straight on the board but they’re attenuated for the megadrive not the master system... Maybe you could help with resistor values.

I’d be happy to send you a board for cost of postage when it’s done as they come in MOQ of 5

I have 18 master system 2s (from trying to find the elusive sonic model) and 4 model ones (I wanted one of each bios, I have one double up) but I don’t have the heart to use them as organ donors lol especially the model ones which have the correct IO IC

*i meant 42 pin SDIP
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Post Posted: Tue Aug 25, 2020 7:47 am
SLr wrote
They should be visible on any LCD screen. Go to your TV settings, and disable the low pass filter (might be called "noise filter") and you should see them. They should be more visible on certain colors and almost invisible on others.


So this relates more to lcd monitors and or scalers rather than crt monitors? Composite video on a crt is terrible for it on certain consoles but rgb is normally free from it.
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Post Posted: Tue Aug 25, 2020 8:16 am
Exactly wasup, if you're using a CRT (even if it's a PVM), a simple RGB mod (without a bypass board) will give you perfect results. On a Master System 1, you don't have anything to do at all. French Master System 2 either.

I've seen Jailbars free RGB bypassed Master System on my PVM, I cannot see the difference with a SMS that has simple RGB.
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Post Posted: Tue Aug 25, 2020 1:58 pm
Come on sLR , at least show us a photo of your voultar board so I can see the resistors ! I know it only passes through one and then has another pulled down to ground. I wana throw a THS7316 on this proto
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Post Posted: Tue Aug 25, 2020 2:29 pm
I'm using a RetroGamingCable UK Megadrive 1 cable which has a 470Ohm in-line resistor which gives me 450mV when using the SMS. On the SMS side, (still only talking about the resistors), you have R18 that pulls the signal up. There is indeed an in-line resistor on the Voultar bypass board but it's meant for Megadrive, I kept some components on there but also changed others (if you're using the components for Megadrive, I think that the picture will be too dark, you might not get sync on some TV either). On other bypass boards (like the one 8bitmod is selling), the only thing you have on the Csync line is a pull up.
DSCF8627.JPG (3.38 MB)
DSCF8627.JPG

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Post Posted: Wed Aug 26, 2020 2:08 am
I guess if I ever get around to it I will just leave a solder pad for RGBs with some optional components you can bypass depending on whether the component required is in the cable , if not you can put it on the board. I have paid for a decent oscilloscope locally but the lady won’t let me get it until lockdown is over. Once I’ve got that I can work things out properly myself.

Not going to lie either. I got stuck straight away interfacing the 2 VRAM ic’s with the VDP. The schematic is very straight forward

D0 to D7 on IC6 as AD0 to AD7
D0 to D7 on IC7 as AD8 to AD15
A0 to A12 tied together on both IC6/IC7 As AD0 to AD12

Then A0 to A15 as AD0 to AD15 on the VDP

All “Bus’d” together

But when I correctly label and number each pin then transpose the schematic in Kicad into the PCB, match placement and orientation the same way as on the segas original mainboard my traces aren’t able to be routed out as neatly and perfectly as on the bottom side of the photos of the French mainboard posted on this site.

I mean some have to criss-cross and needed a couple vias so I feel I’ve made a mistake

Maybe it’s way too over my head for a beginner, but I shall persist when possible :)
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Post Posted: Fri Aug 28, 2020 8:15 am
what is it about everyone having a "missus" (presumably a housewive) that hates them for doing all this modding ;) If you hate your wife that much, divorce her :P

I'm a trans person myself and single so I don't have the burden of a "missus" telling me not to do modding. I do research on hardware every day in my lab!

I'm so glad to see people interested in the jailbar fix I've discovered!

In answer to someone's question: is the jailbar issue only a problem on scalers on LCDs: pretty much yes. On a CRT you won't notice the jailbars as much (though in my opinion, you should still do the jailbar fix)

I have a video here for Alex Kidd SMS2 showing jailbar fix with THS7374 RGB bypass... WARNING: I work slow. I take lots of breaks and goof off watching anime or taking endless coffee breaks while modding, so while this video is about 10 hours long, it's only like, not that much modding



Here's one for VA3 SMS1 (same chips as SMS2) but NOTE: I replaced boot ROM with power strike ROM, not realizing that built-in ROM on master system does some magic to make cartridges work... so after I shot this video, I restored the built in ROM to the default boot ROM that also has hang-on in it:



For shits and giggles here is an SMSFM install video :)

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Post Posted: Fri Aug 28, 2020 8:41 am
Regarding sync:

On PAL/NTSC consoles, pin 10 to video encoder is TTL sync (it's like 3.5v p-p on my scope).

Between VDP 315-5246 pin 30 and CXA/MB chip pin 10 is the TTL sync; on that line is a 2.2k pull-up resistors. You can grab sync there for RGB mod; use 470ohm series resistor and 220uF series capacitor in SCART cable. NOTE: There's a separate csync and composite video pin on your DIN, per megadrive pinout. For composite video one, split it off from the sync pin, with series 470ohm+220uF. Then normal SCART cables that get sync on composite video will work, but specialized cables that get TTL sync (with 470ohm+220uF in SCART cable) will also work.

Between VDP pin 27,28,29(red,green,blue) on VDP 315-5246 and encoder CXA/MB pin 2,3,4 RGB: from VDP is 820ohm resistor on each line, and if memory serves, after that an 8200ohm pull-up on each line... then through 0.1uF ceramic cap to VDP. The resistors are to shape RGB lines to 1v p-p for the CXA/MB chip.

On French RGB master systems, I think the VDP RGB and Sync pins jsut hook directly to DIN? I do not know if the Sync line in French Master System has the 2.2k pull-up on Sync line, but this could be added inside the SCART cable if necessary!

For French RGB master systems I recommend buying the shielded RGB SCART cable for SMS1, from retrogamingcables.co.uk. BUT!!!!!! BUTz: Remove 75ohm resistors for RGB in SCART cable. on side to console, connect to an RGB bypass board! for voultar RGB board for megadrive, change RGB inline resistors to 82ohm on bypass board.... pull-downs to 2k. This slightly divides RGB voltages going into bypass board, giving ideal output. Then via 75ohm resistor from outputs on RGB bypass board... wire to anodes of 220uF caps in SCART cable, for RGB. In other words, your SCART cable itself will have an THS7374 RGB bypass built in, and this will be suitable for French RGB SMS

I wish to try the above, in a future youtube video.


IF SOMEONE HAS A FRENCH SMS2, I WANT IT! SELL IT TO ME! Message me @n4of7 on twitter. I wish to subject your French SMS2 to extreme amounts of experimentation!

EDIT:
to clarify, the THS7374 is a more modern IC for driving analog video. It can be used for all kind of vdeo signal. Generally, it simply takes an input and doubles it on the output. The french RVB cable does this aswell, with that little circuit board in the middle, but it does it using raw components like transistors, capacitors, resistors and whatnot.... the THS7374 is a much more modern thing to use for the same purpose. If you have adapteur RVB you could put a THS7374 in there instead. and it has LPF feature that you can use on the THS7374
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Post Posted: Fri Aug 28, 2020 9:08 am
retroleah wrote
For shits and giggles here is an SMSFM install video


You planning on doing many fm installs on sms2s? I made an adapter board exactly for this... takes 2 mins to solder in and your away laughing as it becomes as easy as an sms1 from there.
3_103.jpg (158.18 KB)
3_103.jpg
4_208.jpg (168.65 KB)
4_208.jpg

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Post Posted: Fri Aug 28, 2020 9:16 am
wasup wrote
retroleah wrote
For shits and giggles here is an SMSFM install video


You planning on doing many fm installs on sms2s? I made an adapter board exactly for this... takes 2 mins to solder in and your away laughing as it becomes as easy as an sms1 from there.


AHA!

I've seen that before. Where do you sell it? Yes, I'd love to buy some from you. I do SMSFM installs semi-regularly; it takes ages to arrive from Tim Worthington, and I usually only buy like 2-3 of them at a time.

With one of those little QSBs I can just drag solder it using a knife tip and some flux.... and off I go!
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Post Posted: Fri Aug 28, 2020 9:58 am
Last edited by L10N37 on Fri Aug 28, 2020 9:59 am; edited 1 time in total
Cheers for the info, I’ve only so far played around with megadrive and made a bypass board for it but I’m waiting for PCBs to perfect the attenuation. I also think I have a faulty amp as I get 2vpp R and B channel but green is at half I think (it’s been over a month since I was mucking around with that) the board allows you to trim these values with resistor adjustment which is part of the board

I just reversE engineered the pre amp signals on the underside of the megadrive one and tinkered with resistor values . Originally I had it working externally on both Pal snes / ntsc famicom/ Megadrive one externally via DB9 port and custom cable. The picture on both consoles was just a little dark and from there I changed the circuit completely ...like A LOT to get it right on the sega (now it won’t work at all with the Ni****do’s) It’s called the MDGFX haha. Can’t wait to finish it actually.

The megadrive Pre amp RGB signal passes by a 5k6 pull up , then through i believe a filter resistor which is 4.7k and then pulls down again on a 1.2k

From here it passes through a cap to block the DC component and into the CXA encoder.

I think I did 5k6 pull up/ 2k filter as anything higher value would cause distortion and jail bars, then 1k3 pull down with 470uf caps (they didn’t start this high of a value but helped immensely with a trinitron CRT RGB I hacked) The caps as mentioned on the input were only as it helped on a fussy Sony telly...

I have a Loewe CRT that isn’t fussy with RGB or sync and I could make a million changes and always be able to get it almost perfect with OSD adjustment . As you mentioned yeah I pass the c sync through the recommended 470ohms and 220uf cap.

When I get a scope and my PCBs it won’t take long to nail it right into proper video standards.

I have a video on YouTube , VajSkiDs consoles
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Post Posted: Fri Aug 28, 2020 9:59 am
wasup wrote
retroleah wrote
For shits and giggles here is an SMSFM install video


You planning on doing many fm installs on sms2s? I made an adapter board exactly for this... takes 2 mins to solder in and your away laughing as it becomes as easy as an sms1 from there.


Awesome board ! I’d totally get one but I can’t justify it as I have one in one of master system version 1’s and they’re not that cheap
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Post Posted: Fri Aug 28, 2020 10:14 am
I subscribed to your channel... boomer ^-^ You're supposed to record in landscape not portrait mode =)
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Post Posted: Fri Aug 28, 2020 3:03 pm
If tuning THS7374 for megadrive, check on scope the RGB levels coming off of the DIN. On an SMS1 I see about 1.4 Vpp on all-white screen, for each line on RGB.... this works for for cables that have 75ohm series resistor. Voltages are measured on an oscilloscope.

Getting all-white screen can be done on megadrive using 240p test suite on a ROM cart. On SMS I boot Psychic World cart (or via ROM cart) and I ground the CPU clock when it hits a white screen during the intro to the game; VDP keeps running but at that point the game has crashed.

RGB voltages going into your ths chip should be about 710mV p-p on a scope, on all-white screen, per each line. measure voltage coming off of VDP (isolate the RGB pins on VDP so that they are out of circuit). This is becaus ethe THS chip doubles the input. Anything going into it gets doubled. So you input 710 into it if your SCART cable is using 75ohm resistors on the lines. I guess if you wanted straight-through SCART cable with no resistors (just capacitors), you'd input... about 360mV into the THS for each RGB line (in that case, having fully straight-through SCART cable is easier because then you just add the capacitors inside the console).

In general, any 220/470 uF capacitor rated at least 10V should work just fine in your scart cable. Rubycon make really nice quality capacitors; get one with low ESR and high ripple, don't use cheap chinese crapacitors

voltage division is like that:

r1 = series resistor
r2 is after r1, going to ground

between r1 and r2 is your output

before r1 is input

this is a voltage divider. formula is:

output voltage = input voltage * (r2 / (r1+r2))

75ohm RGB is (if I'm not mistaken) about 710mV (640-780 is tolerated range). That's why SMS1 is outputting about 1.4Vpp, because in TV you have 75ohm termination to ground for each input line.... so in SCART cable you have 75ohm resistor. 75/(75+75) = 0.5. So it halves the voltage.

In my SCART cables I like to have the 220uF cap *first*, and then between output of cap to TV I have my 75ohm resistor.

Most people have 75ohm resistor first, then 220uF cap, in the line.

Actually, you can go higher than 220uF... some people use 470uF. anything 220-1000 uF is fine.

The capacitor is just for AC coupling. 200uF is the minimum on video lines



sync, don't bother with sync on the THS chip. megadrive and SMS both have TTL sync coming off of their VDPs (with 2.2k pull-up already on the board). when I do RGB bypass I don't bypass the sync line, there's no need. for 75ohm sync you can just use a simple voltage divider. For TTL sync anything like around 3.5v is fine... for instance on a PS1 the GPU will output roughly 3Vpp sync from the GPU. I measured it on an SMS2 coming off of VDP, with 2.2k pull-up present on the board, and it was about 3.5Vpp.

This is why for TTL sync (like when connecting to pin 10 of CXA1145) you use the 470ohm resistor. because, remember the 75ohm termination resistor in TV? well, 75/(75+470) multipled by like 3.5, you'll get a voltage within tolerance (for sync line the voltage going into TV should be between 200-900mV p-p).... is this example you'd get about 480mV going into TV
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Post Posted: Fri Sep 04, 2020 1:47 pm
they only use 100uf on composite out on the early SMS1

Im confused as to how this mods any different to the mod that’s been getting around for 10 years or so.

As in, removing the clock signal from the video encoder altogether. So you only have RGB and composite won’t work ? On my megadrive I had to disconnect it from the VDP side , as it didn’t work cutting the pin on the encoder.


As for master systems I am waiting on OSSC to confirm as I’ve had no issues with my model 2’s with the CXA encoder




https://imgur.com/gallery/YeqhzmD
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Post Posted: Fri Sep 04, 2020 1:55 pm
When you cut the subcarrier signal from IC9, you don't have that signal going from IC5 to IC9 but you still have it going from IC5 to IC1. The "old fix" removes maybe 0.5% of the issue and mostly on NTSC consoles anyway.
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Post Posted: Fri Sep 04, 2020 2:01 pm
Ah cool. You should make a PCB with a new clock system altogether to feed each IC that needs it. It’s much neater to have a “product” and installation guide to install it. You can stick It on OSH park and people can get the board and populate it them self.
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Post Posted: Fri Sep 04, 2020 2:03 pm
I know the main fix for model ones is just switching to C sync , which mainly removes the bars but blue still has weird artefacts
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Post Posted: Mon Sep 07, 2020 1:25 pm
yeah with my fixes the composite signal will be black and white

I have been doing some new research into jailbars, but on early SMS1 using older VDP

see:

part1:

part2:

I also fixed brightness. On this older SMS1, VDP RGB lines were roughly 1.37Vpp. Going into video encoder, it should be 1Vpp. However, sega used a resistor divider circuit with series resistor of 3300ohm and after that, 3300ohm to ground. This halved the voltage to about 685mV.

Changing the series resistors to 1200ohm fixed the brightness issue.

I've yet to experiment with RGB bypass on the sms1
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Post Posted: Mon Sep 07, 2020 1:29 pm
Here are some before/after results on SMS1 jailbar research:

https://imgur.com/a/cwP4MBa

Before (jailbars. dark picture):


After (jailbars gone. brightness corrected):

jailbarsms1fixed.jpg (192.5 KB)
jailbarsms1fixed.jpg

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Post Posted: Mon Sep 07, 2020 1:35 pm
Here is a YouTube video demonstrating what the SMS1 or 2's quality will be like once you do the jailbar fixes:



Captured using this setup:
* OSSC line doubler. converts analog RGB to HDMI. OSSC tweaked for optimized pixel-perfect output

* Mirabox HDMI capture card

* OBS Studio, GNU+Linux version (used to correct colour/gamma issues. My HDMI capture card is a bit janky and screws up the colours)
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Post Posted: Wed Sep 09, 2020 5:32 am
I did some further experimentation.

NOTE: If you have CXA1145, please replace it with THS7314/7374 bypass with 710mVpp RGB lines going into the THS chip.

Anyway, I did some further experimentation. This came when I was re-capping a VA3 SMS1. I decided to replace all the 10uF electrolytic capacitors with 10uF ceramics; other electrolytics replaced accordingly, with corresponding capacitances.

Anyway, I'll get right to it:

Add 100nF, 470nF and 4.7uF DC coupling capacitors on all 5v pins on the VDP, and on the video encoder if using sonic1 SMS2 which has MB3514 encoder (on all other models, just do an RGB bypass, and have 100nF DC coupling on the THS chip... maybe also 470nF and 10uF)

For RGB bypass I prefer to use THS7314 on SOP to DIP adapter. If CXA1145, the RGB input/output pins are actually opposite each other on the chip, so remove CXA chip and solder the SOP to DIP adapter so that it aligns... poke component leads (thick ones) or use machine header pins, can do it nice and cleanly (I will have photos of this later, when I actually do this. Until now I was using a pre-made RGB bypass board but THS7314 on SOP to DIP adapter will actually be much cleaner if bypassing on console with CXA1145 - then just use TTL sync for sync, when wiring up the DIN)

Also make sure to re-cap your PSU. Official sega PSU uses a 3300uF 16v cap (I recommend using a 25v one. Rubycon/Nichicon/Panasonic brand with low ESR, 105C 10000hr spec, high ripple, etc)

It's the capacitors that make all the difference, in addition to the trick of using a separate 3.58mhz oscillator for the CPU clock.

The PDF guide has been updated. I also still need to add info about jailbar fix on pre-VA3 SMS1 models, though the fix is largely identical (with a few other considerations in mind, such as moving the default location of the 10.73mhz crystal wired into the VDP, so that it's further away from RGB pins/traces)
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