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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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Joined: 26 Jul 2016
Posts: 109
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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Joined: 21 Jul 2015
Posts: 159
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
Posts: 5
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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Joined: 10 Jan 2014
Posts: 431
Location: USA
Have you considered a bilateral switch or a FET to handle the job?
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For console modding, please send all inquiries to sales@voultar.com
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 Post subject: Re: A simple PCB for better RGB TV mods
PostPosted: Wed Aug 09, 2017 8:45 am 



Joined: 18 May 2017
Posts: 38
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 



Joined: 07 Aug 2006
Posts: 424
Location: Sydney, Australia
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: 8
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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Joined: 15 Apr 2016
Posts: 51
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
Posts: 4
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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