Adding RGB To A CRT

There was a time when all TVs came with only an antenna socket on their backs, and bringing any form of video input to them meant dicing with live-chassis power supplies. Then sets with switch-mode supplies made delving into a CRT TV much safer, and we could bodge in composite video and even RGB sockets by tapping into their circuitry. For Europeans the arrival of the SCART socket gave us ready-made connectivity, but in the rest of the world there was still a need to break out the soldering iron for an RGB input. [Jacques Gagnon] is in Canada, and has treated us to a bit of old-school TV input hacking as he put an RGB socket on his JVC CRT set.

Earlier hacks had inventive incursions into discrete analogue circuitry, but on later sets such as this one the trick was to take advantage of the on-screen-display features. The signal processing chip would usually have an RGB input with a blanking input to turn of the picture during the OSD chip’s output. These could be readily hijacked to provide an RGB input, and this is the course taken here. We see a VGA socket on the rear panel going to a resistor network on a piece of protoboard stuck in a vacant space on the PCB, from which a set of lines then go to the signal processing chip. The result is a CRT gaming monitor for retro consoles, of the highest quality.

For those of us who cut our teeth on CRT TVs it’s always good to see a bit of TV hacking. It’s a mod we’ve seen before, too.

20 thoughts on “Adding RGB To A CRT

  1. I came here to see how to modify a monochrome CRT to display color, and I have to say I was a bit… disappointed when I discovered that “CRT” vs. “CRT TV” is apparently the new “transistor” vs. “transistor radio”.

    1. Come on. Virtually all CRT tv sets tossed in the last ten or twenty years were color. So even aside from nitpicking, there’s no lack of color sets if you want/need that. But they mostly lack video input.

      Color wheels were obsolete when ntsc came along.

  2. For retrocomputing, it’s probably easier to do this than find a CGA monitor.

    BTW delving into a “headrest LCD” for in car DVD player, I found RGB pins called out on one of the internal connectors. Will take that further sometime.

    Most TVs you get the RGB coming up to the neck board, but unless there’s much semiconductors on it, you can bet they are electron gun drive levels not 1V. Anyway you could look into your own single transistor amps to inject there if the rest of the boards are an incomprehensible mess.

  3. after the demodulator you always have separate RGB at usable levels, the trick is isolation(older tvs had live chassis) and getting the sync in.
    Once fried a few VGA ISA cards because the tv I was hacking had a wonky grounding setup, worked fine until I turned it off while connected to the computer, the vga was fried instantly.

  4. Reading here it sounds like doing this to the older, pre-menu sets was really hard but in my experience it was anything but.

    As a kid I bought an old tv at a garage sale. I’m guessing it was from the late 70s. It was color but from a time when that was still something to advertise. Anyway, I wanted to connect a VCR and a game system. The VCR had an RF modulator of course but the video quality was always better using composite.

    So I cracked open the case. Inside I found that the VHF and UHF tuners were separate modules mounted in their own shielded boxes that hung from the face of the TV by their dials. The UHF tuner was not connected to the main board, it was connected to the VHF tuner. That made sense since one chose UHF by turning the VHF knob to ‘U’. It just fed through. The VHF tuner had a few solder posts on the back with wires soldered on leading down to the main board. It also had a plain old RCA jack with a coax jumper plugged in also leading down to the mainboard.

    I guessed that the RCA connection was video. And I was right! I just unplugged it, soldered up an extension cable with a female RCA socket and ran that out the back through a vent hole. That’s all it took to get video. I didn’t even solder anything to the TV. I could just pull out my extension, plug the tuner back in and it would be like nothing happened.

    I imagine audio must have been on one of those soldered wires. It wouldn’t have been too hard to find it. I was using an external amp and speakers wired into the vcr for sound so it wasn’t necessary. I just kept the volume turned all the way down and used it as a monitor.

    Was the shield of that connector hot? Had I plugged the VCR and TV into separate outlets that turned out to be on opposite phases would I have had a fire? I don’t know. Who does that?

    1. That makes no sense.

      Tuners had radio output, just lower than tv frequencies. The signal still had to be amp!ified, and detected.

      Even if it worked by some odd chance, it wasn’t RGB.

      1. Dependent on the degree of integration I would think. I have come across modules in VCRs that went from UHF to video, haven’t found one like that in a TV but don’t get inside that many TVs personally.

      2. Yes no maybe

        Yes there is another IF stage after the tuner.

        If the signal level injected is higher than the original signal level then it may pass though the attenuation from the next IF that will be cause by baseband or composite frequencies being much lower than normal.

        After that there will be AGC that will bring the signal level back up to what is expected and at that point it is the correct frequencies. And in color.

        The filter specs in old sets were poor and not strongly selective.

        So I can see that this may work on some TVs but not others.

        It’s not the right way to do this, it’s s real kludge or hack but I can see it working in an unintended way.

        1. Huh. Well.. maybe that is how it worked. That is an interesting thought. I’m pretty surprised though because as I remember it the picture quality was just as good as anything I had ever seen by plugging into an actual factory-provided composite input.

      1. No, I suppose not. Sorry, I wasn’t thinking about that, I was only thinking about getting a baseband input. I suppose the author was right, you would need to get down to the high voltage stuff to get actual separate R, G and B inputs on one of those old sets.

        I never worried about that back then because the accessories I had to connect to it, an Atari, a ColecoVision and a lower end VCR never had separate R, G and B outputs either. I suppose there’s a hack for that.. But it would have been beyond me in those days.

        Still, I can’t imagine that old CRT could have produced any better of a picture if it did have an RGB input. That composite input looked really nice! I guess that was more my point. You could get a high quality input w/o going so deep.

  5. These were great CRTs, my family had the 32″ D-series version after a 27″ version of almost exactly the one in the linked article was moved downstairs to upgrade an ancient Hitachi with no AV inputs.

    The D-series had an incredible picture on component video for watching DVDs… once you found how to open the service menu and disable the NTSC “red boost” or whatever it was called.

    1. Hitachi did their own thing with AV for a while, had a 9 pin din with RGB and sync I think, inputs. On models that didn’t have it exposed, there was often the unfilled solder pads on the edge of the board and the blank space on the back cover for it.

    2. Oof, Sony WEGAs had that red boost BS. In NTSC regions (or perhaps just USA…?) it was called “AXNT” in the service menu and is best left at zero. And, oh yeah… “__NT”! I think it was “AXPL” in PAL regions :)

  6. The Heathkit GR-2000 “Solid State” TV was great for hacking. I had three of them (or parts thereof). With the complete schematic and theory of operation, right down to the internals of each IC, you could have lots of fun. I put a switch on the video input so we could pipe an Apple ][c into it.

  7. In Europe, TV manufacturers often included the venerable TDA3561 chip in its output stages for Teletext. Great little chip that accepted RGB 0.7v p-p signals, fast blanking and slow blanking signals so you could overlay text on TV pictures. I used that interface on a Grundig TV (much to the then wife’s displeasure) to act as a colour text / chunky graphics monitor for a 6809-based system I was building. The TDA3561 appeared before SCART, so TVs had an internal connector for a Teletext option. Fortunately they were isolated with a transformer, so safe to stick the computer straight in.

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