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 Post subject: A simple PCB for better RGB TV mods
PostPosted: Thu Jul 13, 2017 8:59 pm 


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As some of you may have seen in the TV RGB mod thread, I put together a little circuit that makes the mod easier, cleaner, maybe cheaper and definitely cooler. The RGB mux board (love the word mux, glad it's in my vocabulary again years after college) takes RGB video signals, the OSD RGB signals from a TV that's moddable (see mikejmoffitt's great summary post at the beginning of the mod thread) and "muxes" (mixes) the two so that:
1) you don't have to use a clunky, bigass 4PDT switch
2) you get to keep your OSD so you can see the volume indicator or adjust menu settings, etc

Did I mention it's really small?
Image

Anyway, to install, once you figure out what's what, you connect the board as if it were the 4PDT switch that is typically used with this mod. External RGB video signals from the input jacks of your choice go to the marked pins. I included pads for 30 Ω resistors so it'll attenuate the external 0.7 Vp-p video for TVs that have 0.5 Vp-p OSD signals. These pads are jumpered (0 Ω resistors or wire) on 0.7 V TVs. 75 Ω resistors properly terminate the external video.

The TV's OSD signals should be taken after any resistors on the TV's circuit, but before any decoupling caps on the jungle IC input. Perhaps I should have included pads for these caps on the board, but being that I've so far just used the pads on the TV's board for the mods that I've done, I did not. I'm open to feedback about this for any future revisions, of course.
You connect a +5V line from a voltage regulator on the TV's board and ground (take your pick), then a simple on/off switch and you're good to go! Once I get one of these I'll do a step by step with photos post.

I just submitted an order with a fab shop yesterday. It being my first circuit, I decided for a few fancy options so it could have been cheaper but it's still pretty good at $2 US each. I got some extra, so if people are interested, I can sell off a few of these. I don't care about making a profit with these, so cost +shipping will be what I'll ask for with the first batch. If there's enough interest, I could go for a second batch and depending on how many, we could get the price down to $1.10 or less. But lets not put the cart before the horse. I doubt the RGB TV modding industry is very big lmao.

I'm making it open source, with an ok to use it for commercial purposes so dudes can install these for people. It being my first circuit and all, I wanted to get the boards first before releasing the files. I'll post links to the files in this thread.



For the caps, what I meant by "voltage values used based on availablity" is that these were the voltage values of the caps I had in my inventory. So long as they're at least 10V and the right size, you're good.

The LT1675 is most expensive on Digikey (at $6.66 currently :twisted:), I only got it from them once when it was expedient. Mouser doesn't have it and seemingly doesn't stock Linear Technologies at all. The cheapest in the US I've seen is from Arrow at $3.95 currently, but as I found out, their shipping to Canada makes them significantly more expensive than that for me. I've also bought 10 from Ali Express for $23.60. They had free shipping but I paid and extra $5 to get it faster with an ePacket. I haven't tested these but they look absolutely the exact same as the real-deal from Digikey and Arrow. I've got them separated so one of the first boards I test out will be with one of these to make sure they're ok.

There's also a single channel LT1675 with 8 pins. You want to make sure you get the 3 channel, 16 pin SSOP package one.
Digikey
Arrow
Ali Ex (I am that one order they have listed)

Please let me know if you need any clarifications, etc.


EDIT: I had originally specified a SPST (on off) switch for the blanking DC to turn on RGB. This is what I used in my prototype and I've seen no issues. As viletim astutely pointed out, this leaves the RGB ON input floating and not low when turned off. This won't damage the TV or the mux board or anything, but there might be "unexpected behaviour".

Using a SPST as is might work ok, but it'd be better to use a SPDT switch to switch to ground when off will assure the input is a well defined logical low.

Another option would be to keep the on-off switch and add a 10k resistor soldered onto pins 1 and 3 of the OR gate, U3:
Image


Last edited by KnuckleheadFlow on Sat Jul 22, 2017 4:55 pm, edited 2 times in total.

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 Post subject: Re: A simple PCB for better RGB TV mods
PostPosted: Thu Jul 13, 2017 11:10 pm 


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Have you built this up on breadboard and tested this before doing a run?
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 Post subject: Re: A simple PCB for better RGB TV mods
PostPosted: Thu Jul 13, 2017 11:33 pm 



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Voultar wrote:
Have you built this up on breadboard and tested this before doing a run?


He posted info in the big TV RGB mod thread. Somewhere in the middle where he posted the schematics and showed it working on his TV. He even got feedback from others on how to reduce the size.

Is there an issue you noticed Voultar?


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 Post subject: Re: A simple PCB for better RGB TV mods
PostPosted: Fri Jul 14, 2017 12:05 am 


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It's hard to tell. I can only see the top layer, I couldn't find a schematic. Unless he's strictly going by the application data.
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 Post subject: Re: A simple PCB for better RGB TV mods
PostPosted: Fri Jul 14, 2017 12:06 am 


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It's hard to tell. I can only see the top layer, I couldn't find a schematic. Unless he's strictly going by the application data. It looks OK though. But something's bugging me about that OR gate..
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 Post subject: Re: A simple PCB for better RGB TV mods
PostPosted: Fri Jul 14, 2017 12:24 am 


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Yup, like leonk said I've had this exact circuit in my JVC for months now working like a champ. And yes, I basically built the circuits in the three ICs' datasheets and put them together. Simple. Using the OR gate to have the OSD blanking signal switch to the OSD RGB and output to the jungle IC blanking was just a guess but it worked so I've had no reason to look at other solutions. Is it weird? I had no issues with this setup, but of course I'd like to know what your concern is. Especially since your knowledge is essentially the reason I had this mod working in the first place.

Actually this circuit is pretty much unchanged from when I sketched it out having just studied the datasheets. Worked right from the get-go... almost. I first didn't have C3 physically close enough to U2 on my proto board. The datasheet said to make sure it's as close as possible and they weren't kidding. Smooth sailing once that was corrected.

And the bottom layer is just the ground plane and a single fat +5V line for the 3 ICs.


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 Post subject: Re: A simple PCB for better RGB TV mods
PostPosted: Fri Jul 14, 2017 1:44 am 



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ORIGINAL POST OF WHAT THIS DOES:

A while ago (back in October, yikes) I posted how I had made a circuit that let you keep the OSD and replaced the big multi-pole switch with a simple on-off SPST. I apologize for taking this long, but since my hastily assembled test circuit worked and the PCB software I was using started irritating me, I sort of lost the motivation. I've recently switched to kicad and, finding it a much better tool, I've set out to finish the damn thing right. I'm still not done plotting out the PCB, but here's the schematic and how it works if anyone is still interested. Message me and I can send you the schematic itself, or the PCB file once I finish it. Eventually. I promise.

I used an LT1675, a fast switching RGB multiplexer that's made for applications such as these. I wired the OSD RGB to the RGB 1 input pins (1,2 & 3) and the external console RGB to RGB 2 (6,7 & 8). When the select pin is low, it'll output RGB2 and rapidly switch to RGB1 when select is high. The OSD blanking signal is used for this.

The OSD blanking is also connected in parallel to an or gate, SN74LVC1G32DBVR, along with the RGB-on signal, 5V DC coming off a SPST switch. The output from the or gate is then sent out to the jungle IC's blanking pin. This makes it so that a) if the external RGB is off, the OSD blanking will switch to RGB1, the or gate will pass along the blanking signal and everything is as usual. b) if external RGB is on, OSD blanking can still switch to RGB1 but the output the jungle IC blanking pin is still the usual DC that blanks the whole screen.

Finally, the LT1675 requires negative voltage, for which I used an LM2776. Perhaps some TVs may have a -5V rail you can use.

Image

The schematic is for a simple JVC setup that has 0.7Vp-p video signals, 5ish V blanking and 75 Ω impedance, but it can be adapted to any TV that can be modded.

E: One more thing; the LM2776 datasheet states that the charge pump capacitor, C3 in the schematic above, needs to physically be as close to its C+ and C- pins as possible. It's not kidding. At first I had it about a centimetre away and... 0V on the -5V output. Just a heads up.


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 Post subject: Re: A simple PCB for better RGB TV mods
PostPosted: Fri Jul 14, 2017 4:29 am 


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Okay, this makes MUCH more sense to me.

This all looks okay. But a few things.
Image

The MUX chip you selected is 6dB. Are you assuming that the OSD transmission lines are always .7vPP? If you don't source terminate the OSD transmission lines, the amplitude could exceed 2V which will saturate and clip/distort on that I/C and will destroy any clamping ability it may have. Nonetheless, it certainly violates the input specifications. But I see you're terminating the EXT. RGB properly, that's good. Maybe consider that as an option for OSD transmission lines, too? I've seen OSD lines output 1.2vPP as they coupled into a Jungle I/C's RGB inputs. Just something to give consideration.



On the RGB outputs, where are you DC restoring the video? It's important that your black levels reference 0VDC. Otherwise your output is going to be less than ideal. The Jungle I/C is 99% of the clamper circuit, you just need to put couple-caps on the ends of this thing. Are you just piggybacking off of the couple-caps that are (probably) on the inputs of the Jungle I/C?

Image

The datasheet says to not add capacitance to the outputs. But just for good measure, I would pull out my big, massive scope.. And I would output black just to verify that it lives @ 0VDC.

I understand how the OR gate is operating, now. It's keeping the OSD blanking and RGB blanking from fighting with each other in a match of Mortal Kombat. This is quite similar to what's found in a lot of PAL sets that need to multiplex or MUX the RGB input with OSD stuff.

Other than that, not bad at all!
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 Post subject: Re: A simple PCB for better RGB TV mods
PostPosted: Fri Jul 14, 2017 1:48 pm 



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Lord Voultar.. does it matter what the OSD transmission lines are?

If the Jungle IC expects 0.7v pp, and we tap the OSD right where it enters the jungle IC, can't we assume that the TV chassis designer correctly attenuated the OSD IC output?


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 Post subject: Re: A simple PCB for better RGB TV mods
PostPosted: Fri Jul 14, 2017 3:16 pm 


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Thanks Voultar, it's nice to know it's all good.

I designed this based on the TVs I saw modded in the thread and the ones I've modded myself, presuming that these would represent a good cross section of most of the TVs out there; that is, most jungle ICs' RGB input will expect 0.5 or 0.7 Vp-p. Ideally I wanted the circuit to output the OSD as it was inputted. I am, of course, open to making revisions based on peoples' experiences and I'm looking forward to feedback on how this first batch works out.

I did consider including pads for the decoupling caps on this, but a) on the TVs I've done, I've just used pads on the TV's board b) I suspected there might be a reason the decoupling caps are so close to the jungle IC's RGB inputs. I wanted people to have the option to mount this wherever convenient, which might be far enough from those inputs to cause a problem. Given the pads would take up space, I decided to exclude it. It wouldn't look as nice, but someone could put ceramic disk through hole caps in the output holes if need be.

It's cool to hear that using an OR gate is actually done on consumer sets. I had everything else figured out, I knew I wanted the OSD blanking to switch the RGB and still have it blank normally. Thinking if:
- RGB is off, send OSD blanking to the jungle IC
- RGB is on, send DC to the jungle IC
- if the jungle IC blanking is getting DC, then the OSD blanking is superfluous, but still needed for RGB switching without any Mortal Kombat as you said lol.

I realized this looks like OR logic, so I tried using an OR gate. Didn't know if just splitting that OSD blanking signal between the OR gate and the LT1675 RGB select pin would work. I think the OR gate's datasheet said the input current it needs for high is like 5 µA though, so I figured it'd be all right.

While I have your attention, are there any books on CRTs that you could recommend? I bought myself the Art of Electronics 3rd ed. and it actually has a nice little section on analog TV! It's probably a leftover from the 2nd ed, but I'm glad they've kept it.


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 Post subject: Re: A simple PCB for better RGB TV mods
PostPosted: Wed Jul 19, 2017 4:09 pm 



Joined: 07 Aug 2006
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KnuckleheadFlow,

Assorted notes about your design.

It would be better if you placed some through-hole pads for coupling capacitor on your board. This way you could simply remove the coupling capacitors from the TV chassis and place them on your board.

You forgot the pull down resistor connecting to U3's switch input. When the switch is open the input is floating an undetermined or oscillating. As well as fixing this, you could improve this part by adding a couple of transistors which detect the RGB switch signal on SCART pin 16 and switch over to RGB automatically.

As the board will be hand populated, the layout of the board could be improved by aligning all the passive components in the same direction (you'll realise this out when you assemble it). C4 is too close to the pins of U1. It makes it hard to hand solder U1 if C4 is already fitted. Best to leave a gap of 2mm from fine pitch IC pins.


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 Post subject: Re: A simple PCB for better RGB TV mods
PostPosted: Thu Jul 20, 2017 6:29 pm 


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viletim wrote:
KnuckleheadFlow,

Assorted notes about your design.

It would be better if you placed some through-hole pads for coupling capacitor on your board. This way you could simply remove the coupling capacitors from the TV chassis and place them on your board.

You forgot the pull down resistor connecting to U3's switch input. When the switch is open the input is floating an undetermined or oscillating. As well as fixing this, you could improve this part by adding a couple of transistors which detect the RGB switch signal on SCART pin 16 and switch over to RGB automatically.

As the board will be hand populated, the layout of the board could be improved by aligning all the passive components in the same direction (you'll realise this out when you assemble it). C4 is too close to the pins of U1. It makes it hard to hand solder U1 if C4 is already fitted. Best to leave a gap of 2mm from fine pitch IC pins.


Thank you for the input, Tim. I really do appreciate feedback and advice from those more experienced and I've already started making some improvements for the next revision. At least none of the issues you brought up are a show stopper.

Pull down resistor! Knew I’d forget something. I had no issues with the floating input in my prototype, so it never came to mind. It’s the proper way though, so I’ll correct that in the next revision. I think 10kΩ should be fine. Making do with the 40 boards I already ordered (should arrive today) however, I would rather not need a resistor bodged on there. I’m thinking that using a SPDT switch, instead of a SPST, and switching between 5V and ground should do the trick. It’d have to be break before make of course. I think the momentary floating during actuation wouldn’t be a big deal I imagine (given the proto’s floating input being ok).

Interesting to have SCART switch on the RGB. I hadn’t considered that since I don’t actually have anything SCART (I find the BNC click satisfying). I want to avoid feature creep, but if there’s enough desire for it…

And another vote for putting the decoupling caps on the board, hmm? Maybe there’s a desire for it after all. There’s room to expand the board’s length 5.56mm and keep it under 25mm x 50mm, minimizing cost per board. Speaking of which, making this as small as possible was why I have some of the components rotated and packed so close. With more length, I can rearrange them of course. I’m also considering moving some of the passives, like terminating resistors and bypass caps, to the bottom of the board. It’s not like they’re charging by the via, so why not.


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 Post subject: Re: A simple PCB for better RGB TV mods
PostPosted: Fri Jul 21, 2017 12:06 am 


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Look good! I'll populate it tomorrow or Saturday and put it through it's paces.

Image
Image

Small enough?
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 Post subject: Re: A simple PCB for better RGB TV mods
PostPosted: Fri Jul 21, 2017 10:14 am 


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Really cool, would love to use this in my 2 Wega's.


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 Post subject: Re: A simple PCB for better RGB TV mods
PostPosted: Sat Jul 22, 2017 3:42 pm 


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Are they 4:3 Wegas and moddable using the standard method people have been using in the mod thread?


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 Post subject: Re: A simple PCB for better RGB TV mods
PostPosted: Sat Jul 22, 2017 5:32 pm 


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Yes.


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 Post subject: Re: A simple PCB for better RGB TV mods
PostPosted: Sat Jul 22, 2017 8:46 pm 



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if you could put some solder pads on here for a panel mount scart socket, I think installations might go quicker because you wouldnt have to mount the board somewhere it would already be attached to the back of the scart socket.

Or not, if people didnt want to use that way. nice option though.


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 Post subject: Re: A simple PCB for better RGB TV mods
PostPosted: Sat Jul 22, 2017 10:05 pm 



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KnuckleheadFlow wrote:

Pull down resistor! Knew I’d forget something. I had no issues with the floating input in my prototype, so it never came to mind. It’s the proper way though, so I’ll correct that in the next revision. I think 10kΩ should be fine. Making do with the 40 boards I already ordered (should arrive today) however, I would rather not need a resistor bodged on there. I’m thinking that using a SPDT switch, instead of a SPST, and switching between 5V and ground should do the trick. It’d have to be break before make of course. I think the momentary floating during actuation wouldn’t be a big deal I imagine (given the proto’s floating input being ok).


eh, having it temporarily floating is unlikely to do any harm, it just may switch to the wrong input briefly.

Are you selling any of the first batch of boards?


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 Post subject: Re: A simple PCB for better RGB TV mods
PostPosted: Sun Jul 23, 2017 1:52 am 


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Pasky wrote:
Yes.

Ah, well then... Good :)

mvsfan wrote:
if you could put some solder pads on here for a panel mount scart socket, I think installations might go quicker because you wouldnt have to mount the board somewhere it would already be attached to the back of the scart socket.

Or not, if people didnt want to use that way. nice option though.


I actually like this idea. Maybe not as a part of the actual board, but an adapter? Hmmm...

adcurtin wrote:
eh, having it temporarily floating is unlikely to do any harm, it just may switch to the wrong input briefly.


Yeah, I know it's more of an issue with more critical or complicated logic/buses. Still, I'm all about best practices and all that.

adcurtin wrote:
Are you selling any of the first batch of boards?


Right, I probably should've made it clearer. Yes I am. I ordered 20 boards with two units on each, so 40. Elecrow, the PCB fabricator I went with, sent 80! I'm planning to keep 10, I've got another 25 set aside. Plenty left up for grabs still. PM me.

My first one's all done, just need to put it in a TV. It should be easy with one that was already modded with a switch earlier. I thought i had a ton of 75Ω 1206 resistors. Nope, only 5. Got lots of 1210 (the wide ones). Oops. Still fit, kinda.
Image

No problems soldering it all up, but I do have a microscope for the fine pitch stuff.
Image
(Looks a bit foggy, optics probably could use a cleaning)

I gotta say, it's very satisfying soldering a board you designed yourself. Guys, if any of you have made a little circuit for something or you've got an idea hovering around in your head, go for it! Put the idea out there, look for guidance, figure out how to make it, see what happens. It feels good!


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 Post subject: Re: A simple PCB for better RGB TV mods
PostPosted: Mon Jul 24, 2017 5:05 pm 


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Will you be selling these already populated? There's no way I could solder that tiny stuff :D


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 Post subject: Re: A simple PCB for better RGB TV mods
PostPosted: Tue Jul 25, 2017 4:08 pm 


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I am considering it. There are probably a lot of people that could solder this into a TV but couldn't solder the fine pitch components. I'm not sure what a fair price to charge for it would be and if it'd be worth my while.


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 Post subject: Re: A simple PCB for better RGB TV mods
PostPosted: Tue Jul 25, 2017 5:01 pm 



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If you can hit 35-40 USD per pcb fully soldered I think would be a good target.


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 Post subject: Re: A simple PCB for better RGB TV mods
PostPosted: Tue Jul 25, 2017 6:27 pm 


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That sounds very reasonable.


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 Post subject: Re: A simple PCB for better RGB TV mods
PostPosted: Wed Jul 26, 2017 12:25 am 



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This is really cool. I might be interested in one some day. I have a 4:3 Wega 37" i could use this on iirc. How difficult would it be you think (I know thats a very broad question).


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 Post subject: Re: A simple PCB for better RGB TV mods
PostPosted: Fri Jul 28, 2017 2:20 am 


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nakedarthur wrote:
That sounds very reasonable.


Ok, I can do it for that. Let me get my shit in order and I'll let people know here once I'm able to prepare some populated boards. Any money I make will just be going into my retro gaming hobby, so I guess now's the time to mention; since I'm not really expecting big bucks with this, in lieu of payment, interesting trades considered.

BTW, you've had a look through the mod thread, I presume, to make sure your TV is moddable?

GeekMan1222 wrote:
This is really cool. I might be interested in one some day. I have a 4:3 Wega 37" i could use this on iirc. How difficult would it be you think (I know thats a very broad question).


Yeah, it depends on your understanding of electronics, ability to read circuitry and how handy you are soldering. Depending on the Wega it can be doable. What model is it? Have a look through that thread, maybe someone has done one already. If not, try to find a service manual for it and using the info in the mod thread see if it's got the right OSD lines for it.
Personally I found the mod (without the board) pretty easy. But Toshibas and JVCs are pretty much perfect for it. That's not to say Sonys aren't of course. If you're TV is moddable, all you'd have to do is figure out where to splice in board. It may involve desoldering about 4 (RGB and blank) resistors, capacitors (making sure to replace them in the circuit) or jumper wires on the TV's PCB. Or even lifting 4 pins on the jungle IC. Really depends on the TV.


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 Post subject: Re: A simple PCB for better RGB TV mods
PostPosted: Fri Jul 28, 2017 2:26 am 


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KnuckleheadFlow wrote:
nakedarthur wrote:
That sounds very reasonable.


Ok, I can do it for that. Let me get my shit in order and I'll let people know here once I'm able to prepare some populated boards. Any money I make will just be going into my retro gaming hobby, so I guess now's the time to mention; since I'm not really expecting big bucks with this, in lieu of payment, interesting trades considered.

BTW, you've had a look through the mod thread, I presume, to make sure your TV is moddable?


Awesome, will keep an eye out. And yea, I have a KV-27S42 like the OP on the RGB thread used.


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 Post subject: Re: A simple PCB for better RGB TV mods
PostPosted: Tue Aug 01, 2017 3:10 am 


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I can't wait to get my (HAND) on this.


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 Post subject: Re: A simple PCB for better RGB TV mods
PostPosted: Wed Aug 02, 2017 6:44 am 



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I want to get a few of these. I know you are entertaining the idea of selling populated boards and that is great. I don't want to throw too many options out there, but I'd be interested in boards and parts and I can assemble myself. If only to coalesce shipping and streamline sourcing. I've got a handful of TVs that need this. Should I PM you?


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 Post subject: Re: A simple PCB for better RGB TV mods
PostPosted: Wed Aug 02, 2017 2:39 pm 


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Sure man, go ahead.

I've hit a bit of a snag with my install. I'm pretty sure it's a problem with the TV and not the board because I get the same issue using the old prototype board that's worked fine for months.
Sorry for the delay to everyone. I sincerely doubt there's a problem with the board itself, but I need to be certain everything is fine before I send anyone anything.


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 Post subject: Re: A simple PCB for better RGB TV mods
PostPosted: Sat Aug 05, 2017 7:20 pm 



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Awesome stuff so far. Glad you pointed this thread out as I wasn't aware you made one just for this project. Wish I knew electronics well enough to design and make circuits lol

I see someone mentioned using the signal from SCART cable to switch/trigger RGB which is something I was wondering about when I was looking at the SCART pinout (Pin 8?) If I understood my friend correctly, that's what its intended function is?

That'd probably be the only feature I can think of that'd be nice. Any way to eliminate the external toggle switch seems like a plus to me. Or any way that I can wire this thing in while maintaining the function of the original inputs.

I guess having pads for the relevant SCART socket inputs could make things easier? I'd imagine having the socket directly attached to the MUX board would only work if your socket installation location allows it, but at least having the pads means you could still wire from SCART to the board if needed.

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?


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