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 Post subject: Re: A simple PCB for better RGB TV mods
PostPosted: Tue Aug 08, 2017 8:19 pm 


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Seikenfreak wrote:
Edit: Oh yea, almost forgot. When I saw you post about the LT1675, I did a search for it and one of the first things Google had was a datasheet or something about a direct replacement. It does say on the datasheet bullet points "DROP-IN UPGRADE TO LT1675"

Texas Instruments OPA3875 - Triple 2:1 High-Speed Video Multiplexer

Which I was then able to find available at Mouser, where as the LT1675 was not.

I didn't read into the details because I'm sure it wouldn't make sense to me, but perhaps this is a suitable alternative if the LT1675 is harder to come by?


It's funny you mention that TI chip, I just found out about it myself a few days ago. If TI says it's a drop-in then it should indeed be fine to use instead of the LT1675.

That said, I'm coming to the unfortunate realization that despite my prototype being a success, the layout of the board as it is right now will most likely not do the job. I'm having issues getting a clean OSD overlay and I don't think it can be bodged into working and still meet the goal of being a cleaner, simpler, better alternative to the 4PDT switch. I guess when it comes to analogue signals, board layout is very important.

The external RGB looks very nice and crisp just as it should be, but the multiplexed OSD only looks ok when the external RGB blanking is on but the external console RGB off. If the external blanking is off, the OSD shows up as intensely white and very bloomy. As if the RGB levels are just just maxed out. When external blanking is on and the external RGB is active (that is, console RGB video is on the screen), the OSD is faded, washed out with colours shifted orange. If there's interest, I'll try to get some video demonstrating what's going on.

I tried hooking up my oscilloscope but frankly, I'm rather inexperienced with it and I wasn't really able to get any useful insight looking at the waveforms once I was able to get them to show up. Adding additional bypass caps to the LT1675's power inputs improved things somewhat, but I think the issue is just too much stray capacitance from the ground plane and the two other ICs being too close to the LT1675's I/O. In other words, I think I just packed everything too tightly and this batch of boards is most likely a write off. Live and learn I guess.

I'm going to do some tests to see if I can find what exactly is the culprit. I'll make a board with just the LT1675, another with just the negative voltage supply and another with just the OR gate. By isolating each I can see if the interference is eliminated. If none of that does it, it may mean the ground plane will also need to be moved away from the LT1675. This was mentioned in the TI OPA3875 datasheet's design recommendations but not the LT1675's. Thanks Linear.

I've sent a tech support request to Linear Technology and hopefully they'll get back to me and help me sort things out better. If they don't, I'll post on TI's forums haha. Once I have a better understanding of the problem, I'll redesign the layout, add some of the improvements that have been recommended by viletim and Voultar and get another batch made. I'll try to keep the size small and the components will likely remain unchanged, but a few more caps and resistors might be needed. I'll be out a hundred something dollars, but hey I want this to be a success and I want to share something with people who share my enjoyment in good old analogue RGB video. Hopefully those that have expressed interest in this project will still want them once I've got the layout worked out.
Sorry if I let people down; believe me, nobody's more disappointed than I am.


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 Post subject: Re: A simple PCB for better RGB TV mods
PostPosted: Tue Aug 08, 2017 9:10 pm 


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Thanks for the update. Sorry to hear about the issues, but it's always better to do it right in the long run.


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 Post subject: Re: A simple PCB for better RGB TV mods
PostPosted: Wed Aug 09, 2017 12:27 am 



Joined: 30 Jul 2017
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Well you already know waaaaay more about this than me so I trust your judgement. No rush. Maybe something magical will come to you while you're laying bed thinking lol

I'm still waiting on my 4PDT setup parts to arrive. Who knows when the SCART socket will show up.

And yea I'll still be interested. I get an email when I get a PM here so I'll know whenever you figure it out.

Good luck! Also, I love that you're getting in touch with tech support and stuff haha


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 Post subject: Re: A simple PCB for better RGB TV mods
PostPosted: Wed Aug 09, 2017 4:38 am 


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Have you considered a bilateral switch or a FET to handle the job?
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 Post subject: Re: A simple PCB for better RGB TV mods
PostPosted: Wed Aug 09, 2017 8:45 am 



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Chin up, these things happen when you attempt to do something cool. I'm sure you'll work it out one way or another.


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 Post subject: Re: A simple PCB for better RGB TV mods
PostPosted: Thu Aug 10, 2017 2:31 pm 



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KnuckleheadFlow,

I think the problem is you are doing fast switching between two signals with a different DC level. The output will show the absolute difference each time. To make matters worse, the input signal is AC coupled, so the DC level depends on video content. The DC level of the OSD signal may vary depending on TV design also.

To make this work you will need to a video clamp circuit (aka DC restore) circuit for both the OSD and video input signals. Once the are referenced to the same point (same black level), you can switch between them at high speed without consequence.

A video clamp requires a clamp signal, some CMOS switches, coupling capacitors, and a reference voltage. The clamp signal may be floating around in the TV somewhere, but it's probably easier to make it yourself (LM1881, etc).

Your board layout looks fine to me. It's not all that critical for video of less than 10 MHz. Just look at the TV itself. Crappy single layer phenolic board most likely and it works OK.


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 Post subject: Re: A simple PCB for better RGB TV mods
PostPosted: Fri Aug 11, 2017 6:54 pm 



Joined: 11 Aug 2017
Posts: 43
This finally put me over the edge to registering - but he software won't allow me to PM KnuckleHeadFlow. I would love to buy this. I have a 36" Sony Wega sitting here waiting mod, plus 3 x 27" CRTs that I was going to mod and just give away to people (plus my costs - I got all of these free off of eBay).

KnuckleHeadFlow, if you can PM me, I would appreciate it!


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 Post subject: Re: A simple PCB for better RGB TV mods
PostPosted: Wed Aug 16, 2017 3:35 am 


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Hey man thanks for all your work. While it hasn't worked out yet the way you wanted, I'm sure you'll figure it out. And with me being on the injured reserve with a broken wrist, I'm in no hurry.

Thanks


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 Post subject: Re: A simple PCB for better RGB TV mods
PostPosted: Wed Aug 16, 2017 12:38 pm 



Joined: 15 Aug 2017
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Keep it up man!!!! This device could literally save thousands of old CRT's!!!! LMK when I can buy one :D

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 Post subject: Re: A simple PCB for better RGB TV mods
PostPosted: Sat Aug 19, 2017 5:15 pm 



Joined: 11 Aug 2017
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I am slowly putting together a tutorial on how to make this hack work, with pictures.

Holding back a bit to see the solution to the muxing issue to give a complete solution.

Image

As opposed to the 4PDT solution. I really need to find quick disconnects sized correctly so I don't have to keep soldering these. Would speed them up. Have 5 in the backlog right now.


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 Post subject: Re: A simple PCB for better RGB TV mods
PostPosted: Wed Aug 23, 2017 12:54 am 


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Sorry for keeping quiet, my real job's been keeping me busy, plus I've now got an OG white PC Engine CDROM^2 and IFU I need to get a THS7374 into. I'm a bit frustrated and my head's spinning from having been reading up AC coupling, DC bias, etc. the datasheets for the LT1675 and the TI equivalent OPA3875. I start reading things here and there and because I'm insufficiently familiar with the terms... AC coupling seems like it ought to be called DC coupling, DC offset, DC bias... Ugggh! This should be simple, but I'm seemingly missing something. I appreciate people's words of encouragement though.

Voultar wrote:
Have you considered a bilateral switch or a FET to handle the job?


For the blanking switching you mean? I'm not sure if that's the issue though. TI's page for the OPA3875 has a the 3 input version of this or gate listed as a "shit people who got the OPA3875 buy" so I think it's the right way to go.

viletim wrote:
I think the problem is you are doing fast switching between two signals with a different DC level. The output will show the absolute difference each time. To make matters worse, the input signal is AC coupled, so the DC level depends on video content. The DC level of the OSD signal may vary depending on TV design also.

To make this work you will need to a video clamp circuit (aka DC restore) circuit for both the OSD and video input signals. Once the are referenced to the same point (same black level), you can switch between them at high speed without consequence.

A video clamp requires a clamp signal, some CMOS switches, coupling capacitors, and a reference voltage. The clamp signal may be floating around in the TV somewhere, but it's probably easier to make it yourself (LM1881, etc).


Would the clamp you're proposing get the sych of the jungle IC? I had a look at the lm1881's datasheet and see the application circuit this could be based off.
However I'm worried about making it more complex. It's not that I doubt you (naturally, given the circuits you've designed), but why can't moving the decoupling caps from before the jungle IC to before the mux board do the trick? The external video should have caps at the output or in the cable so doesn't that mean it's without DC bias going into the TV?

I feel rather dumb that I just had AC coupled video from the micro going into the LT1675. With the negative voltage requirement and instructions not to add capacitance to the output, it looks to meant for use with DC coupling.

Adding 0.1uF caps on the OSD inputs gave a more "stable" but still very white and "bloomy" OSD. Analog Devices tech support engineer seemed to think I'm on the right track but he doesn't seem too familiar with the subject matter.
Figuring those inputs are supposed to be terminated, I added 75 ohms to ground and got proper colour OSD but too dim. Of course I still have the damn caps on the TV before the jungle IC. I can't pull the board from the set tonight but, could that do it?

Then there's the matter of the RGB channel select. I've added a pull down resistor to the RGB on DC blanking input and that helped. However, having gone over the TV's schematic again, I noticed that the pull down resistor on the YS is after this board and I've probably got that input floating as well. I added a 1k resistor but that just turned off the switching entirely. Welp. I then saw in the LT1675's development board documentation that 1k to ground on the select line will do exactly that; keep the input set to RGB channel 2. Ok then. The OPA3875 dev board's doc shows that has a 50 ohm resistor to ground to set the impedance. Now, I'm thinking of trying that but I'm somewhat concerned if the micro YS source can provide the current needed.


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 Post subject: Re: A simple PCB for better RGB TV mods
PostPosted: Mon Sep 25, 2017 6:32 pm 



Joined: 15 Aug 2017
Posts: 12
I bought a bunch of these recently, they showed up (to US) in about a week and a half.

http://www.ebay.com/itm/4-PIN-Female-Ma ... 1589589571?

the connectors have very little purchase when connected, which I actually like because the slightest yank will disconnect instead of mess with the 4p6t switch I have them hooked up to. But that also means they don't stay connected very well in an environment where they are being jostled.


I might be completely off the mark with what you mean disconnects :D


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 Post subject: Re: A simple PCB for better RGB TV mods
PostPosted: Mon Sep 25, 2017 9:49 pm 


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There shouldn't be a problem having the Blanking lines linked, I do that for most of my sets I mod.

The problem is feeding 2 inputs at once to the Jungle when both those inputs are not 100% equal.

Another thing is the 10n OSD cap will backfeed the 100n console cap and cause extreamly bad console ghosting.
Your better off just using the 100n for both OSD and Console. If you can the signals at the same level..
The OSD can require up 430ohm terminations or it will be ghosty white.

The only successful mix ive seen is when OSD and Teletext/CCD are on the same line.
They both share the same v-pp, terminations and caps. BUT because of the way teletext works only one can ever be on at a time I'm pretty sure.

If you get your OSD and Console RGB lines and tie the Jungle side of their caps together you get a perfect picture with a strong OSD and Console as the caps isolate the different terminations.
But you get that ghosting I spoke of earlier.
If you could figure out a way to stop that ghosting...(caps talking to each other or something)


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 Post subject: Re: A simple PCB for better RGB TV mods
PostPosted: Tue Sep 26, 2017 4:02 am 



Joined: 18 May 2017
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Syntax wrote:
The only successful mix ive seen is when OSD and Teletext/CCD are on the same line.
They both share the same v-pp, terminations and caps. BUT because of the way teletext works only one can ever be on at a time I'm pretty sure.


That fits exactly with my experience, the designers of the chassis expect teletext or OSD, not both.

I'd be happy if you can just get a "useable" OSD to display when in RGB mode using your board. People really don't care about having a perfect OSD. The only care about having a perfect RGB and being able to adjust the TV. That's how I focus my efforts when modding.


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 Post subject: Re: A simple PCB for better RGB TV mods
PostPosted: Mon Oct 02, 2017 10:56 am 



Joined: 18 May 2017
Posts: 349
MarkOZLAD wrote:
Syntax wrote:
The only successful mix ive seen is when OSD and Teletext/CCD are on the same line.
They both share the same v-pp, terminations and caps. BUT because of the way teletext works only one can ever be on at a time I'm pretty sure.


That fits exactly with my experience, the designers of the chassis expect teletext or OSD, not both.


...and then the very next set I mod defies this assumption.

I modded a TV with a chinese generic chassis and an la76810a Jungle this week.

I am yet to find a TV that requires me to snip the OSD lines to inject RGB, This set was no different. I was able to find an unused port for external RGB input (in fact I used some holes for un-populated jumpers leading to this port) and injected the RGB there. The chip only has a single set of RGB inputs designed for OSD input. Therefore my RGB lines are sharing the same lines as the OSD, in fact with the board layout I am really just pumping the RGB directly into the caps that are before the Jungle's OSD RGB inputs with the OSD stiill connected to the same caps.

At first I had used the standard 75ohm termination to ground and the picture was terrible, dark and impossible to colour correct with the service menu...AND the OSD was washed out and barely useable. As I mentioned on the main RGB Mod thread I found this link. It said to use a 330Ohm resistor in line on the RGB lines and then have 580 ohm termination to ground. I was prettty desperate for any lead at this point. Long story short, this approach worked and I got the picture bright and the colours corrected.

The thing that makes this applicable to this thread is that after I added the 330 Ohm resistors to the RGB lines, the OSD was no longer washed out by the RGB input. It was very clear, had colour, and most importantly usable no matter what picture colours were showing.

Image
Image

I wonder whether the approach of adding resistance in line and then experimenting with the termination resistance is a valid option for more sets? I know the next set I mod I will be trying a similar approach. If I can't find unused ports to inject my RGB I will solder directly to the underside of the caps that lead to the Jungle RGB lines, thereby leaving the OSD inputs intact.

Note: I never found a service manual for this set, I had to rely on schematics for similar sets. I grew in confidence when I stumbled onto the schematics of another set with the same jungle and RGB inputs. It too had 330Ohm resistors on the RGB lines from the scart.

Also...I have previously done sets that share the OSD with my RGB lines and just "put up with" a washed out OSD. I possibly should've used this technique.

Hope this info helps someone else.


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 Post subject: Re: A simple PCB for better RGB TV mods
PostPosted: Mon Oct 02, 2017 12:08 pm 


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Did you remove the original OSD termination resistors?
Did you wire the RGB line in this order
RGB 330ohm inline 550ohm termination original osd cap jungle?

If the original OSD resistors were left it should of stuffed with their termination which could be why you had to tune it to 550. With OSD resistors removed you may be able to find a lower value.

Keen to see results on another set that seems to require a 4pdt.

Nice work dude :)


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 Post subject: Re: A simple PCB for better RGB TV mods
PostPosted: Mon Oct 02, 2017 11:06 pm 



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Syntax wrote:
Did you remove the original OSD termination resistors?


No, but I don't think this set has termination to ground via a resistor, instead it has inline resistor and termination to ground via a cap. I don't have a schematic for this set but I did make a diagram after inspecting the chassis.

Image

Image

Hope they make sense to you

Syntax wrote:
Did you wire the RGB line in this order
RGB 330ohm inline 550ohm termination original osd cap jungle?


No I didn't and I am interested to know if it makes any difference. I wired them to the terminated to ground before adding the inline resistor then to cap. I know this is different to the link but it fitted with the wiring I had prepared and it worked so I haven't revisited yet.

Syntax wrote:
If the original OSD resistors were left it should of stuffed with their termination which could be why you had to tune it to 550. With OSD resistors removed you may be able to find a lower value.


Even more interesting is that if you read the link he actually DOES snip the OSD lines and sends to a switch so there shouldn't be any interference on his setup.

I'm an electronics novice so I won't pretend to understand that. My skills are in research, troubleshooting and perseverance!

Syntax wrote:
Keen to see results on another set that seems to require a 4pdt.


Yes, me too. Don't know if I've just got lucky here or what. I'm a pretty lucky guy in general :-)

Syntax wrote:
Nice work dude :)


Thanks. I nearly spewed when I got this set and realised I didn't have a schematic (I had the schematic for a similar set from NEC but it turns out the chassis was totally different) or way to get into the service menu (see my post about hacking the remote). To reverse engineer this set and then get a result after things looked hopeless feels pretty darn good.

Don't mean to hijack this thread, I'm hoping the learnings here can help with the design of the OSD muxing board. Maybe there isn't a need for an amplifier/combiner in the design.


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 Post subject: Re: A simple PCB for better RGB TV mods
PostPosted: Tue Oct 03, 2017 12:07 am 


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There's a good chance that inline resistor is keeping the console termination isolated from the OSD lines..


I have not tried doing this for isolation, only my failed ghosted attempt with 100n caps.

The inline resistors would of pulled the volts on the RGB lines down so your jungle must need a low vpp.

On any other set that would take about half your color out...


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 Post subject: Re: A simple PCB for better RGB TV mods
PostPosted: Tue Oct 03, 2017 12:49 am 



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Syntax wrote:
There's a good chance that inline resistor is keeping the console termination isolated from the OSD lines..


That thought ran through my head too. Might be lucky I wired it up the way I did with 330ohm after the termination.

Syntax wrote:
I have not tried doing this for isolation, only my failed ghosted attempt with 100n caps.


Syntax wrote:
The inline resistors would of pulled the volts on the RGB lines down so your jungle must need a low vpp.

On any other set that would take about half your color out...


I was thinking that too but the datasheet for the LA76810A shows 0.7vpp so I don't know what the go is.

Initially when I set up my test I only put the 330Ohm resistors in line and left the termination at 75Ohm. This resulted in a very dark image. (I expected this). The change in termination resistance then corrected it. (I won't pretend to understand why).


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 Post subject: Re: A simple PCB for better RGB TV mods
PostPosted: Tue Oct 03, 2017 1:13 am 


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MarkOZLAD wrote:
Syntax wrote:
There's a good chance that inline resistor is keeping the console termination isolated from the OSD lines..


That thought ran through my head too. Might be lucky I wired it up the way I did with 330ohm after the termination.

Syntax wrote:
I have not tried doing this for isolation, only my failed ghosted attempt with 100n caps.


Syntax wrote:
The inline resistors would of pulled the volts on the RGB lines down so your jungle must need a low vpp.

On any other set that would take about half your color out...


I was thinking that too but the datasheet for the LA76810A shows 0.7vpp so I don't know what the go is.

Initially when I set up my test I only put the 330Ohm resistors in line and left the termination at 75Ohm. This resulted in a very dark image. (I expected this). The change in termination resistance then corrected it. (I won't pretend to understand why).


Should of checked OSD lines to ground before starting the mod. Multimeter doesn't lie and will tell you if there are any there.

Beats playing "Where's Wally?" With smd components.


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 Post subject: Re: A simple PCB for better RGB TV mods
PostPosted: Tue Oct 03, 2017 2:07 am 



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Syntax wrote:
Should of checked OSD lines to ground before starting the mod. Multimeter doesn't lie and will tell you if there are any there.

Beats playing "Where's Wally?" With smd components.


Everything is set up on headers with jumpers so easy to dismantle and check.


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 Post subject: Re: A simple PCB for better RGB TV mods
PostPosted: Tue Oct 10, 2017 3:23 am 


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So, turns out you can mix console and OSD rgb directly together for the majority of Sony sets. Probably other brands too but yet to test.

Just remove the osd inline resistors and change the stock 470ohm terminations to 75ohm ones. Then insert your console on that line

Far easier than 24 solder points across 12 wires, a clunky switch or a chip that still has a switch lol.


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 Post subject: Re: A simple PCB for better RGB TV mods
PostPosted: Mon Nov 27, 2017 7:08 pm 


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@OP: Any progress? Would love to hear an update.


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 Post subject: Re: A simple PCB for better RGB TV mods
PostPosted: Mon Nov 27, 2017 7:45 pm 



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Syntax wrote:
So, turns out you can mix console and OSD rgb directly together for the majority of Sony sets. Probably other brands too but yet to test.

Just remove the osd inline resistors and change the stock 470ohm terminations to 75ohm ones. Then insert your console on that line

Far easier than 24 solder points across 12 wires, a clunky switch or a chip that still has a switch lol.


You would still need a switch for blanking...right?


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 Post subject: Re: A simple PCB for better RGB TV mods
PostPosted: Mon Nov 27, 2017 8:10 pm 


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numbski wrote:
Syntax wrote:
So, turns out you can mix console and OSD rgb directly together for the majority of Sony sets. Probably other brands too but yet to test.

Just remove the osd inline resistors and change the stock 470ohm terminations to 75ohm ones. Then insert your console on that line

Far easier than 24 solder points across 12 wires, a clunky switch or a chip that still has a switch lol.


You would still need a switch for blanking...right?


No you'd wire blanking to scart console 5v feed like the scart standard..


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 Post subject: Re: A simple PCB for better RGB TV mods
PostPosted: Tue Nov 28, 2017 7:11 pm 



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Syntax wrote:
numbski wrote:
Syntax wrote:
So, turns out you can mix console and OSD rgb directly together for the majority of Sony sets. Probably other brands too but yet to test.

Just remove the osd inline resistors and change the stock 470ohm terminations to 75ohm ones. Then insert your console on that line

Far easier than 24 solder points across 12 wires, a clunky switch or a chip that still has a switch lol.


You would still need a switch for blanking...right?


No you'd wire blanking to scart console 5v feed like the scart standard..


OH! Gotcha, so you are trusting that the console cable has a 5v supply line. If it is omitted you won't get a picture.

Need to double-check to see if any of mine are missing it. I am wanting to say that the NESRGB does't provide it...


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 Post subject: Re: A simple PCB for better RGB TV mods
PostPosted: Tue Nov 28, 2017 9:28 pm 


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It is the standard so voltage should be present on scart pins 8 -16? Its early for me I thinks that's right :)

NESRGB if an AV model has it. Any other model doesn't even have a plug till the modder installs one.


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 Post subject: Re: A simple PCB for better RGB TV mods
PostPosted: Wed Nov 29, 2017 1:17 am 



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A standard NESRGB install uses an 8 pin mini-din connector. The signals it carries are the same ones that the XRGB-Mini has on it's input, which is red, green, blue, sync, ground, left audio, and right audio. There's an unused pin, but it would be unwise to commandeer that for 5v I suspect.

You're right - if you mod it to use the multiport, you would have 5v available, but for the moment, the vast majority of installs won't have that. That said, SCART is the standard, not NESRGB - but that would be a blocker for this to work for many.


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 Post subject: Re: A simple PCB for better RGB TV mods
PostPosted: Wed Nov 29, 2017 2:50 am 


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For what reason would it be unwise? Do you know what that pin is used for on the mini?

To veer from a standard is not something Tim does often.


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 Post subject: Re: A simple PCB for better RGB TV mods
PostPosted: Wed Nov 29, 2017 1:28 pm 



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I don't, which is why I would't do it. Blindly sending voltages seems like a bad idea - maybe it goes nowhere. Maybe not.


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