Adding an RGB input to a cheap generic NTSC TV

Featured on Hack A Day! Amazing! never thought a project of mine would make it there!

133MHz’s Junk Box proudly presents a way to convert an inexpensive generic chassis Chinese NTSC CRT TV set into a high quality RGB SCART monitor, for use with arcade game boards and home video game consoles with RGB output. Be warned that this requires non-trivial modifications to your TV set’s circuitry. CRT TV sets work with lethally high voltages, which can be present even if the set is off and unplugged, presenting a high electrocution hazard to the inexperienced. This is no puny 5-volt digital electronic project. If you don’t have any experience in TV repair, don’t attempt this modification. 133MHz’s Junk Box assumes no responsibility for your actions.

Ready to delve into the fascinating world of television circuitry?

Introduction

Ah, RGB. The additive color model in which red, green and blue light are added together in order to reproduce every color that can be seen with the human eye. It is primarily used in electronic devices as a way to capture, store and display color information in graphics, pictures, video, etc.

RGB signaling provides the cleanest, purest form of analog video you can get from any video device. No wonder why computer monitors have used analog RGB since the introduction of VGA in 1987. But television is another story. Since the dawn of color TV, we’ve been stuck with lossy encoding methods in order to cram a full-color picture in the limited bandwidth of a monochrome transmission. It really bit us in the rear when the home computer and video game system revolution started in the late 70s, clever machines that used an inexpensive home television set as a video display. The lack of a direct RGB input on TV sets meant that the signal from the computer or game device had to be encoded in a suitable broadcast TV format, only to be decoded back to RGB inside the TV set, leading to considerable losses in picture quality.

The French did it right with the introduction of the SCART connector in the late 70s. It is a 21 pin connector that provides direct RGB signaling between a TV and external video devices, among other nice additions like automatic input switching and video overlays. It was made compulsory for new TV sets sold in France starting in 1980 and then it spread throughout Europe, becoming a de-facto standard for European TVs and video devices. Having a direct RGB input offers the highest possible video quality for your DVD player or video game console, and it also simplifies internal video circuits by a great margin. Almost every video game system sold in Europe comes with an RGB SCART cable as a pack-in or as an option, for a stunning monitor-quality video display.

We got the short end of the stick

Unfortunately, RGB SCART never took off outside of Europe. Here in America it is totally unheard of, which means that we’ve been stuck with crappy composite video and S-Video inputs in our TVs for decades, while Europe has enjoyed pure, glorious RGB since pretty much forever.

Just like European gamers got the short end of the stick with their video games having horizontal black bars, crappy frame rates and in the worst case a ~16.67% slowdown in music and gameplay speed, here we had to put up with prehistoric video connections and their horrid picture quality until Component video came and saved our eyes, but that was pretty late in the game, wasn’t it? You can’t have everything, they keep saying.

RGB is Godlike

Ever noticed how crisp and clear arcade games are in comparison to your home video games? That’s because they use RGB signaling. It is the purest analog video signal you can get from the video output chip of your favorite game system directly into the electron guns of your TV set. Zero distortion, zero artifacts, blurriness and all those nasties introduced by NTSC/PAL color encoding, low quality TV decoding circuits and so on.

Don’t take my word for it. Please, see it for yourself. Especially in consoles like the Sega Genesis which have a craptastic composite video out, the difference is like night and day.

I grew up playing the SNES through the RF modulator connection, and since I discovered the SCART connector I’ve been pissed off at the complete lack of a direct RGB input on American TVs.

RGB modding a TV: Not as easy as it might seem

Over the years I’ve been trying to found a way to add a direct RGB input to a CRT TV set that wasn’t designed with such a thing in mind, without much success. From a technical and economical standpoint, it makes much more sense to pump RGB directly into the CRT electron guns, instead of implementing loads of complex and expensive circuitry to encode RGB video into a 50 year old format only to devote similar circuitry in order to decode it back to separate RGB, a few inches of wire away. It’s utterly pointless, to say the least.

In theory, it should be pretty easy to add RGB to an American TV, just feed your own RGB into the Chroma processor IC (disconnecting the original RGB output from the TV tuner stage) or directly into the CRT video board, and injecting your own synchronization signal into the TV’s sync separation stage. After all, you’re doing away with the convoluted conversion steps and going straight where it’s at. But in practice, it’s nowhere as easy as it sounds. Here are some of the major hurdles that have prevented me from succeeding in my Quest for Glorious RGB in the past:

• Some TVs, especially old ones, don’t use RGB at all. They work in what it’s called Color Difference signaling, basically providing Y (the sum of the R, G and B components), R-Y and B-Y (the red and blue components with the luminance signal subtracted from them). These color difference signals reach all the way to the CRT video board, where by algebraic manipulation, the original R, G and B components are reconstructed to drive the electron guns. In other words, there’s no RGB to be found, and if you’re faced with one of these, you’re out of luck.
• Highly Integrated Chroma Processors. It’s not uncommon to find TVs where a single chip does most of the work, taking the IF signal directly from the TV tuner and outputting RGB + separate sync signals, basically combining several separate stages into one. Although you can still try to inject your RGB signal between the IC output and the CRT video board input, you’d end up having to do extensive modifications to the TV set since the chip runs practically everything and you’re effectively bypassing it, not to mention that you’ll have to build a video amplifier to get your RGB signals to adequate levels for the electron gun driving stage, and the need for a full featured sync separator circuit in order to drive the horizontal and vertical deflection stages properly. You might as well toss the TV and build your own video display from scratch.
• The infamous “Hot Chassis”. Most if not all CRT TVs until recently use what is called a “hot” or “live” power supply design, in which there’s no galvanic isolation between the secondary (output) side and the primary (household voltage input) side. This simplifies a great deal of the power supply’s design, but presents a nasty electrocution hazard if you happen to get your hands inside the TV. Basically the whole negative return side of the TV’s circuitry is directly connected to one side of the household AC line. Something so insignificant as touching any part of the circuitry could be the equivalent of sticking your finger in a wall outlet. This is the main reason why TV sets don’t have a third grounding prong and all those High Voltage warnings on the back. Hacking a direct RGB input into one of these will destroy every piece of equipment that you connect to it and also electrocute you should you ever touch the wires or connectors. The only safe way to work on such a TV would be to plug it into a cumbersome and expensive isolation transformer at all times. As a sidenote, since in a live chassis TV there must be no way for the user to come in contact with the TV’s inner workings, the antenna connection has to be isolated from the inside with small ceramic capacitors. Due to the characteristic reactance of the capacitors, they’ll block the 50/60 Hz household current from coming out the antenna terminal but they’ll allow the several hundred MHz broadcast TV signals to get in easily. Even with the caps in place, a small amount of current will always leak to the outside. If you’ve ever felt a tingle when adjusting the antenna on an old TV, now you know why.

Every single one of my several past attempts to add an RGB input to a CRT TV (especially old ones) has failed because of one or several of the above reasons, ultimately making me abandon the project for a long time.

Crappy Chinese Generic CRT TVs: A Blessing In Disguise

Recently I’ve attended a local retro gaming event, in there I saw an Atari ST computer connected to an RGB monitor and I was totally blown away by the vibrant colors and the razor sharp pixels on those low resolution graphics. Seeing that got me motivated enough to try my luck at the Quest For Glorious RGB once again.

Back in Christmas 2005 I got a 14” CRT TV + DVD player bundle from a big box store at a really good price. Both the TV and the DVD player are from Chinese origin and they’re as generic as they can possibly be. Cheap no-name brand stuff comes out of China and gets re-branded and sold as budget equipment in stores and supermarkets all around the world. Things like build quality and interface design aren’t nothing worth writing home about, but if you take well care of them they’re surprisingly good performers for their low price.

NEX 14NT01. Also found under a billion different names.

The TV in question is a NEX 14NT01, which doesn’t really mean anything. You won’t find a bit of information on the web by that model number. My first thought is that there must be a plain brand-name motherboard inside, or at least some kind of chassis identification I could use to find more information about it. Turns out I was partially right.

Something that has always bugged me about this TV is that it only works with its original remote control. It won’t work at all with a universal remote. No amount of name-brand codes will allow you to control any remote function. That always struck me as odd. Could such a craptastic TV have a completely generic chassis that doesn’t conform to any brand-name TV remote standards?

Taking that sucker apart confirmed my suspicions. This is the only bit of information I could find:

The only bit of identification: 3Y03.

It is indeed a completely generic Chinese chassis manufactured solely for ultra-budget TV sets, and it goes by the name 3Y03. There is no manufacturer identification to be found, these are probably churned out by several sweatshop type factories by the millions. It uses no custom parts and offers a great deal of flexibility for the chassis builders, with optional components like multiple AV inputs or Stereo/MTS selectable via software. It even allows for driving of picture tubes from 14” to 21”. Basically you can fit this anywhere and have a working TV with as little or as many options as you’d like, for pennies on the dollar. No wonder why China is taking over the world!

Googling for 3Y03 reveals a bunch of schematic diagrams and service procedures from several different manufacturers. It also reveals that 3Y03 chassis TVs go for a great number of different “brands” and “models”, complete with different case designs, but the same no-name insides. Here are some of them:

• NEX 14NT01
• Sanyo TVS2132A
• IRT CTK-2182
• Groven 21TN
• Simtron TV-2122
• Akira CT-21LS9M
• Fisher PC-R20R2
• Emerson ETV-2091
• Emerson ETV21F1
• Bluesky 14″

And I wouldn’t doubt for a second that there might be tons more. This is the kind of TV you find at a K-Mart special, no wonder it’s been sold all over the world under a zillion names and designs.

Back to our Quest for Glorious RGB, what’s so special about a no-name generic Chinese TV? Well, being a generic and highly modular design, the potential for hacking is enormous. Unlike a rigidly and well designed TV, a generic piece of crap with lots of empty board space (for the features you didn’t pay for) and plenty of cheap, fully documented ICs is a hardware hacker’s dream. It might be possible to RGB mod a TV after all! Let’s tackle the advantages that are relevant to our interests:

• Being a generic design intended to be sold all over the world, where only small changes are made to accommodate for different international markets, such as varying domestic voltages, local TV standards and antenna/video connectors, there’s a great probability that the chassis already has provisions for a direct RGB input that simply wasn’t fitted for the American market, and adding the missing components (and possibly enabling them via software) would accomplish our goal. In my case, this turned out to be partially correct.
• Because it is a microprocessor driven TV with 100% off-the-shelf parts for the microprocessor, memory and Jungle ICs and with fully digital on-screen controls, the built-in On Screen Display (OSD) character generator provides a great opportunity for hacking our own RGB input. The video signal for the On Screen Display is generated inside the microprocessor and it’s encoded into digital or analog RGB, this is fed into the chroma processor, which then mixes it with the RGB signal from the video sources in order to produce a video overlay, superimposing the on screen menus on top of the TV display. If we can inject our own RGB signal into the lines used for the On Screen Display and the overlay processing, then we might get our Glorious RGB! In my case, this turned out to be the key to success.
• Finally, since the chassis is intended for a global market with several voltage, frequency and plug standards, a “universal” or “auto-sensing” power supply design is used. Such a power supply automatically adjusts itself to work on a wide variety of power conditions, even compensating for voltage transients during operation. It’s the greatest thing that has come of modern switching power supply design. Modern power supplies are galvanically isolated by design, so that they comply with all sorts of safety regulations imposed around the world. In this modern age of intelligent and safe appliances, live chassis designs have gone the way of the dodo, and with good reason. 3Y03 based TVs come with a universal type power supply, so we can safely tinker with them to our heart’s content.

Naturally I took my 3Y03 based TV apart in order to look for signs that would lead me to my goal, looking for missing components, looking up the datasheets for the ICs in order to see what kind of inputs they have, and of course, studying the schematic diagram of the TV itself.

I found a bunch of missing components, mainly from stereo audio and MTS support, but nothing related to RGB SCART input. However, I did find a sign of RGB SCART – on the back panel!

This is the innocent looking back panel, sporting basic composite video input and output and mono audio:

Plain TV back panel

They look kind of lonely don’t they? When I removed it and looked at the back, the large amount of pre-made holes for various connectors caught my attention, especially that large-ish hole in the bottom left corner. That one’s for a SCART plug!

Reminds me of Swiss cheese, but with a SCART input.

So this was meant to have an RGB SCART input, at least in some way or another.

That back panel cover is a perfect example of the great modularity of these Chinese designs. Manufacture one universal board and fill up as little or as many holes as features you want. One design to rule them all, and nothing goes to waste. Also great for us hard hackers, so we can fill up with the features we didn’t pay for, or with our own creations. Want more AV inputs? Install them yourself!

So there are no electrical provisions for a direct RGB input, however there is a perfectly good On Screen Display system I could experiment with. Studying the schematics and datasheets, I found that it would be perfect for my needs.

Hacking the On Screen Display

First let’s expand on the basic notions on how the On Screen Display system works. Here’s a simplified diagram I made for the purpose:

Something.

The microprocessor controls the TV in its entirety. It’s got a Serial EEPROM in which system and user settings are stored, and by means of a bunch of analog and digital inputs and outputs, it talks to the rest of the devices such as the TV tuner, Jungle IC, deflection systems, etc. Microcontrollers in modern TVs are pretty powerful, rivaling early 8-bit home computers in terms of processing power and graphic capabilities.

Among the countless other functions, the microprocessor contains an integrated video display generator for the On Screen Display function, usually being able to generate low resolution text and rudimentary graphics in monochrome or basic colors. This system also generates the Closed Caption subtitles in the TV sets fitted with a Closed Caption decoder.

The 3Y03 chassis uses the Sanyo LC8633224C microcontroller. Its OSD circuitry generates a 36 x 16 character display and it has the ability of drawing simple graphics with 16 simultaneous colors. The software running on it ultimately decides the looks of the on-screen menus.

Plain Vanilla TV On Screen Display (OSD)

The framebuffer for the on screen display is encoded as a digital or analog RGB signal in order to leave the microcontroller and it is also synchronized to the TV’s horizontal and vertical deflection timings. The Jungle IC, which handles all of the chroma processing and conversion stages, takes the input signal either from the TV tuner or the AV inputs, extracts the horizontal and vertical sync information, decodes the video information back into RGB signaling and feeds the individual R, G and B components into the CRT’s electron guns, to create a full color video display. As stated before, it also contains a dedicated RGB input for the On Screen Display video coming from the microprocessor. When instructed to do so, the Jungle IC mixes the OSD video with the regular video coming from the tuner or AV input, seamlessly superimposing the on-screen menus on top of the regular TV material.

Since the Jungle IC needs to know when to display the OSD video instead of the regular video, an input called the Fast Blanking Input is provided. Depending on the voltage level applied to this input, the Jungle IC selects between the OSD and the regular video to be displayed. A corresponding Fast Blanking Output is provided on the microcontroller, in order to drive the Jungle IC and display the menus at the correct time.

Since the microcontroller’s OSD generator is synchronized to the TV’s deflection timings in order to generate a correct and steady on screen display, by carefully manipulating the status of the Fast Blanking pin it is possible to draw boxes of various sizes or “floating” text/graphics without disturbing the original TV picture as a whole, by activating the Fast Blanking during the exact sweep period when something is to be overlaid, and quickly disabling it when it’s not needed. As you can see, the Fast Blanking acts pretty much like an Enable pin of a digital device like a ROM chip or a tri-state buffer. This is why the Fast Blanking input is represented as controlling a two-way switch, selecting between regular and OSD video.

According to the microcontroller’s datasheet, the OSD RGB output can be either analog or digital, depending on the status of a specific bit. Digital would be of no use to me, unless I wanted to emulate a CGA monitor or something. Fortunately it was set to analog, which means it’s a potential candidate for our hacking purposes.

Theory of Operation

In theory, by disconnecting the OSD RGB output of the microcontroller from the OSD RGB input of the Jungle IC and injecting our own RGB signal directly into the OSD input the TV can be used to display RGB video. In practice this turns out to be correct but there are some hurdles to overcome.

As you may already have guessed, by doing this you lose the on-screen display when in RGB mode. I think it’s a small price to pay for Glorious RGB, and frankly, I couldn’t care less. A switch will have to be fitted in order to select between normal TV and RGB mode.

The impedance of the RGB inputs might need to be matched or adjusted in some way in order to get a correct video display. This is easily done using resistors.

Getting our picture to sync would be an issue. There is a SYNC pin on the Jungle IC but further analysis reveals that it is an output instead of an input – the OSD syncs to the TV’s deflection, not the other way around. My elegant way of solving this problem is by using a composite video input as the sync input for the RGB signal, and selecting said AV input on the TV so that it syncs up correctly. SCART uses the same approach of extracting the sync information from the composite video signal, after all.

Last but not least, the Fast Blanking input will need to be manually controlled. If it’s left connected to the microcontroller, our RGB video would only “show through” the dialogs and boxes of the on-screen menus! I’m going to assume that the Fast Blanking pin is a TTL compatible digital input, and thus pulling it permanently to +5V would blank the entire screen. Pulling it to ground would disable the OSD entirely.

With these constraints in mind, I designed a simple way to switch between regular TV with on-screen menus to an external RGB input using a four pole double throw switch. This will be the basis for the full RGB modification.

Basic RGB switching device

When the OSD RGB output is connected to the OSD RGB input, the Fast Blanking pin is connected as usual. When the switch is flipped to the Aux. RGB input, the Fast Blanking pin is pulled high in order to blank the entire display.

Before going full swing with modifying the TV set, let’s mess around with the on screen display to see these concepts in action.

Messing with the On Screen Display for Fun and Profit

Now that we understand the inner workings of the On Screen Display system, let’s have some fun with it by trying all those crazy theories that were derived from my assumptions.

Here’s the plain vanilla OSD in case you forgot how it looks like:

Plain Vanilla TV On Screen Display (OSD)

First, if we disconnect the Fast Blanking input and connect it to ground (according to my digital input hypothesis), the on screen display would be disabled entirely, making it invisible, since the Jungle IC would never switch to it.

Now, what if we disconnect the OSD RGB output from the microcontroller and connect the OSD RGB inputs of the Jungle IC to ground (forcing a permanent zero volts in all three inputs), without messing with the Fast Blanking pin? The OSD will still get drawn on the screen, but there would be no RGB information to display. Since the RGB pins are tied to zero volts, the menus should be completely black!

Black holes sucking up information!

Black voids appear where the OSD information should’ve been. If you didn’t quite grasp the concept with my earlier explanations, these pictures should allow you to visualize it quite clearly.

Now let’s get some RGB video signal into those pins, say from a Sega Genesis video game console with its convenient RGB output, and like before, not touching the Fast Blanking pin. The composite sync signal from the Genesis is being fed into one of the composite video inputs of the TV, and the TV is set up to display said composite video input. The composite sync signal of the Genesis is simply a composite video signal but without any video information at all, just the sync pulses, so by feeding it into a composite video input, you should get a solid black screen. Now I’ll press the MENU button on the remote control to bring up the On Screen Display…

ZOMG On Screen Sonic!

Now that’s awesome sauce! The TV’s microcontroller is still under control of the Fast Blanking signal, so that the text boxes still get drawn in their respective places, but instead of the usual TV settings, Sonic the Hedgehog 2 shows up through them.

How cool is that?

If you zoom in on those pictures you’ll notice that some of the original OSD text is readable on top of the Sonic 2 video. This is most likely because the Fast Blanking pin is not a strict digital input as I supposed, but actually an analog input that allows to vary the amount of “mixing” between the OSD video and regular video, probably to control the amount of intensity of the OSD colors, or to create some sort of transparency effect.

This also means I cheated a bit – the OSD boxes are never completely black when tying the RGB inputs to ground because of this. It actually looks like this:

At least I didn't hide the truth. Oh well.

Black indeed, but the text is still readable due to the different “shades” of black created by the variable amount of “blanking” used.

Now it should be pretty clear to see why forcing the Fast Blanking pin to enabled at all times would wipe out the entire screen and basically provide us with a free RGB input to tinker with.

Modifying the 3Y03 Chassis for Direct RGB Input

Let’s start by analyzing the relevant portion of the circuit, in this case the OSD RGB lines between the microcontroller and the Jungle IC.

Clipping from the 3Y03 Schematic Diagram. Click to zoom in.

Bingo! There we have our OSD RGB and blanking signals, complete with conveniently placed resistors, waiting to be hacked. There’s a nifty little array of diodes and resistors on the RGB outputs of the microcontroller, probably to bring the output signal to adequate levels suitable for the Jungle IC’s inputs.

Impedance matching used in the 3Y03 Chassis

I might have to build something similar in nature for my external RGB input, or do some other impedance matching sorcery to get the color levels right.

By removing resistors R029, R030, R031 and soldering wires to their pad holes, I can conveniently place my RGB switching device without cutting any tracks or desoldering chips off the board. As for the Fast Blanking pin, it seems that I can solder a resistor connected to +5V directly to it without having to break its path from the circuit, because there’s a diode in series with the microcontroller output preventing current from flowing back into it.

I found out that the composite sync voltage level is a bit too high for a standard composite video input, so by putting a 1.8kΩ resistor in series between the composite sync output and the composite video input I was able to lower its voltage enough to make the TV happy.

As for the particular RGB levels, since my oscilloscope is broken I had to resort to trial and error in order to find the combination of resistors that would give me a correct picture, taking some hints from the original OSD circuitry. Something quirky I found about the Jungle’s OSD RGB inputs is that they seem to have internal pull-up resistors. If left floating, displayed video will be white instead of black. If an RGB video source is directly connected to the OSD RGB inputs, a color picture will appear but the dark areas will be white instead of black, leading me to believe that these are some fairly strong pull-ups. That certainly explains the presence of those 5.1kΩ resistors from each of the video lines to ground, without them you get no black level.

After lots of experimentation using a pair of Human Eyes™ as a measuring device, I managed to find the right resistance combination to get the RGB signals to the correct levels, and therefore correct, vibrant colors on screen. Here it is:

My own impedance matching for Aux. RGB Input

Now that I got it all sorted out, it was a matter of putting it to the test. I took a spare Sega Genesis system that I have lying around (not using my main one in case I blow it up because of a dumb mistake), stripped it to the bare motherboard, soldered wires to the RGB & CSYNC outputs and hooked those up to the TV following every single one of my previous guidelines. 

What an ungodly mess of wires and dangerously high voltages. I love it!

The result? Glorious, Glorious RGB!

Sonic has never looked so amazing on an American TV

Finally I got to experience pure RGB from a video game console, something that you Europeans take for granted, but for us is quite an experience. Now that it works, it’s time to make it into something presentable, with the hope that I still end up with a functional TV at the end of it.

Hiding the ugly mess

While I could just wire in a four pole double throw switch as a quick & dirty way to switch between regular TV with OSD and SCART RGB mode, the difficulty of finding such a large switch and the desire to do things the right way (I can’t believe I’m saying this), besides having to manually select the proper AV input in order to get the picture to sync up correctly, and also because I’ve already studied the TV’s schematic in great detail, I decided to modify the TV’s circuitry in such a way that one of the AV channels would be permanently converted into an RGB input channel, eliminating the need for ugly switches and having to line up things by hand, just push a button on the remote and there you go.

The 3Y03 chassis uses the common and readily available HEF4052 dual 4 channel analog multiplexer IC in order to select between its internal TV tuner and its external AV inputs. There are two ICs on board, one for audio and the other for video, both controlled by specific pins from the microcontroller, with their address inputs driven by transistors.

3Y03 TV/AV multiplexing circuitry. Click to zoom in.

I figured, since the microcontroller is already driving common multiplexers to do its internal AV switching, I could piggyback my own multiplexers from the same control lines, in such a way that selecting a particular AV channel would produce an automatic switching of the OSD RGB and blanking inputs into my Ext. RGB input. No fuss, no messing with switches and stuff.

Two of these 4052 ICs would give me exactly four channels to do my bidding, three for doing the actual RGB line switching and the extra one for altering the status of the Fast Blanking pin. I just need to figure out the right combination to make them switch when a particular AV input is selected on the TV and that AV input will be automagically converted into a nice RGB input channel. I thought it would be a good idea to spare the AV2 (rear) input for this purpose, since it’s the second one and I never use the rear AV input anyways.

Pins 9 and 10 correspond to the address inputs of the multiplexers, MSB and LSB respectively. These are driven by two microcontroller lines conveniently labeled AV/TV SW and AV1/2 SW. The state of these two lines determines which input is seen by the Jungle IC and therefore displayed on the screen. I hooked up my logic analyzer (actually, just a bunch of LEDs) to these address lines in order to see what’s the combination that selects AV2. Here’s the truth table:

10 binary (2 decimal) is the winning combo here, therefore I must hook up my external RGB input lines into the third inputs of the multiplexer, and the internal OSD RGB lines into the first, second and fourth inputs, so that normal OSD is maintained in all modes except for AV2. Same with the Fast Blanking pin, +5V is connected through a 680 Ω resistor into the third input in order to pull the blanking line high when AV2 is selected; the other inputs are simply left floating to allow the original blanking circuit to do its job in TV and AV1 modes.

The multiplexers feed themselves through the +9V line, while the Fast Blanking pin is pulled up to +5V, must keep that difference in mind. Also since the multiplexers aren’t perfect switching devices, they present a non-zero resistance in the ON position which must be accounted for by altering the resistor values in my original RGB impedance matcher. Just use 75 Ω for each of the OSD and external RGB inputs instead of 330 Ω and keep the 680 Ω resistors to ground in the multiplexer’s outputs.

You might be wondering why I’m not providing a schematic for this.

Short answer: Laziness.

Long answer: If you know your digital logic and your way inside a TV set, then you don’t really need a schematic for this. The information above is more than enough to come up with a successful implementation of the automatic switching circuit (and you can always install a big ugly switch). If you have an inquiring mind, don’t hesitate to ask.

Anyway, I’m too lazy to design a PCB for a circuit so simple that I’ll probably build only once in my life so I used a bit of perfboard, and it doesn’t look that bad!

Fully Automatic RGB Switching Circuit

Of course I still need to solder a few billion wires to the board if I want it to do something. Mounting homebrew boards inside a TV set can be tricky to say the least, most TVs are already an ungodly mess of wires and with high voltage high power circuitry everywhere you look you have to be pretty careful with your component placement. Fortunately the only things I had to remove from the TV were the three 330 Ω resistors for the RGB lines, for the rest of the stuff I managed to find empty solder pads from the missing components which allowed for a convenient way to tap into all the required signals needed for my automatic switching circuit, the ones I couldn’t find convenient pads for were simply routed through holes on the motherboard.

I found a nice screw hole on a nearby heatsink which was perfect for mounting my tiny switching board. Hurray!

Seems like it was meant to be there. No, really.

Wires everywhere! Still, it doesn’t look half bad compared to the rest of the TV. It sort of reminds me of a chipped PlayStation console.

A modded TV set. Does it receive pirate TV broadcasts now?

Now that the switching issue has been solved, it’s time to find a suitable connector for interfacing to the outside world.

Installing a nice plug

If I had the means to, I’d get a female SCART connector, install it in the conveniently provided hole on the back panel, wire it up and call it a day. Not only it would be really awesome, it could even pass for a legit European TV set. Unfortunately there’s no way for me to get one of those. Male to male SCART cables are plentiful and cheap online, but the female connector used in TVs and VCRs doesn’t follow this trend. My only real option to get one would be to have someone in Europe take apart an old TV or VCR, remove the connector from the motherboard and ship it to me, and that’s just way out of my reach.

Back to the real world, I’d have to settle for a standard connector, like the ubiquitous DE-9 or the popular HD-15 (VGA) connector. Both are pretty good choices but they would require me to drill holes into the plastic casing to mount them, something I’m not very fond of doing, especially with all those pre-made holes in the plastic back panel. I found out that the bigger holes on it can perfectly accommodate a DIN connector, so I settled for that. I had a C-shaped female DIN-8 connector and suitable cables at hand, I could use those and get a perfect fit without spending a single penny, so I did.

It was just a matter of using an X-acto knife to cut down the flimsy adhesive cover and fitting the DIN jack through the hole. Even screw holes were provided. Couldn’t be easier and the results look amazingly clean.

Fits like a glove, but have you ever seen a DIN plug on a TV?

This very same connector is used in the SNK Neo Geo video game console as a video output, which shares its pinout with the Sega Genesis (albeit with a slightly different plug), so as a natural choice I went with the SNK/Sega pinout for my custom RGB connector for ease of use and maximum compatibility.

Needless to say I’m more than satisfied with the results:

I wouldn't be able to get a cleaner look if I tried.

It looks like it was meant to be there in the first place, and since DIN connectors are plentiful and cheap I can just build cables for each of my RGB enabled consoles, an adapter cable with a VGA connector or even a SCART plug if the need for it ever arises.

Final Thoughts

Warning: Razor Sharp Pixels. Keep away from children and pets. Click to zoom in. Please.

I couldn’t be happier with this. I’ve always wanted my own RGB SCART monitor but I was never able to get one due to their scarcity and high price. I did have an opportunity to get a Commodore monitor once but it was really expensive and the picture tube was pretty worn out, something you can’t avoid with these old, heavily (ab)used pieces of computer equipment. Unless you’re prepared to pay premium price for a seldom or never used monitor, be prepared for a mediocre experience because of a tired picture tube or dried up electrolytic capacitors.

Chinese TVs like the 3Y03 based ones are abundant all around the world and exceptionally cheap, and taking into account that most of the world is switching to digital TV in the coming years, they’re going to be even cheaper. Most importantly, these are still being made, so you can get a brand new TV for your old school gaming/hacking with a pristine picture tube for the best possible picture quality. Having access to hacker friendly devices for amazingly low prices is one of the most awesome things of the 21^st century.

I have turned a cheap Chinese crap TV set into something that can be considered a scarce premium around these parts of the world. I can now use it to test and play arcade boards, display video game systems with their best picture quality, or even as an RGB color monitor for computers like the Atari ST or the Commodore Amiga. I have increased its usefulness beyond its original capabilities, and without the need for expensive electronic components. I’ve also fulfilled one of my dreams, one I’ve been trying to accomplish for years without success.

An old school analog RGB monitor is a nice and useful device for anyone who is into retro gaming, retro computing or even electronics. If you know your way around a TV set, you can have your own too.

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