ThomasThe anatomy of a CRT television
For this project I'm using this small 5" black-and-white television. It has the brand name "Crown" stamped on it, but this design is ubiquitous and can be found under many other brand names. It is one of the last generations of small CRT televisions ever produced—you can check out this short YouTube video for an overview of this model. It is wonderfully well-suited for maker projects, primarily because of its small size and power requirements: it can operate on either AC and 12V DC power. Some of these units have a composite input, and for those that don't, it's possible to modify the board to accept an external video signal, though we won't be using such a signal for this project.
Another nice feature of this model television is that the board is exceptionally well-labeled. Not only does each component have an ID number, but many of the capacitors and resistors also have their value marked on the board, which is rare in consumer electronics. I haven't been able to find a schematic yet for this device, so knowing these values will be particularly helpful if repair becomes necessary.
The concept behind CRT technology is wonderfully elegant. An electron gun at the back of the tube shoots a stream of electrons through a vacuum towards the screen at the other end. The inside of the screen is coated in a phosphorescent layer that illuminates when hit by electrons. The direction of the stream is controlled by magnetic fields generated by horizontal and vertical deflection coils wrapped around the neck of the tube. These electromagnets move the electrons from left to right over and over again, repeating this line pattern down the height of the screen before starting over at the top. This happens 50-60 times per second, fast enough that the human eye cannot detect anything other than a solid image. (If you want to see this concept at work, check out what a CRT television screen looks like when filmed in extreme slow motion.)
Discharging the tube
Before you go poking around inside a CRT, you must discharge the tube. For this you can use a long thin screwdriver connected to ground. I use a jumper wire with alligator clips that I clip onto the ground pin of a power outlet. If the TV is plugged in and has a ground wire, you can also clip the screwdriver to the metal frame holding the tube into the chassis, as it is likely connected to this ground path. Once you've grounded the screwdriver, slip the tip under the rubber hood of the anodethat attaches to the tube. You must contact the metal conductor under the hood. You may hear a pop or see a spark, and this is good: it tells you that you have successfully discharged the tube. More often than not, however, you won't see or hear anything—that's normal, just be sure you make contact with the metal under the hood. The capacitors on the board itself may also be holding a charge, so be careful when handling it.
Desoldering the wires for the horizontal deflection coil
There are four wires that travel from the horizontal and vertical deflection coils on the tube's yoke to the TV's main board. Our goal is to disconnect the pair at the yoke that control the horizontal deflection, and in their place solder in two wires from our audio source. My TV was nice enough to label these wires on the board for me (V1, V2, H1, H2) so I would know which pair is for the horizontal deflection, but in most cases you will have to figure out on your own.
So how do we know which wires coming from the yoke control the horizontal deflection and which control the vertical? First you must use your multimeter to identify which wires are pairs. There are four solder points where the wires meet the coils on the yoke of the tube. Measuring with your multimeter in continuity mode, find which of these solder points have continuity: if two points are continuous, they make up one of the pairs.
To see which pair controls the horizontal output, you must desolder one of the pairs from the coil, then turn on the television and observe the effect. If you see a vertical line on the screen, you have succeeded in desoldering the wires for the horizontal deflection and you may proceed with the build. If you see a horizontal line, you should reattach the wires that you removed and desolder the other pair. Don't forget to discharge the tube each time after turning it on, before you go digging around inside again. At this point you should have two wires going to the yoke from the board for the vertical deflection, and two wires coming off the board for the horizontal deflection that no longer connect to the yoke. Cut the ends back to the insulation on these wires and cover them with electrical tape so that they don't accidentally come into contact with anything.
But why are we using the horizontal deflection signal for our project instead of the vertical signal? After all, don't we want the waves on the waveform to fluctuate vertically rather than horizontally, as we see on most oscilloscopes? We are using the horizontal output because the horizontal lines on the screen scan much more quickly than the vertical lines. The vertical frequency of the display (the scan rate) is 50-60Hz, depending on what part of the world you live in. The horizontal frequency, by contrast, is much higher: 15, 625 - 15, 750 Hz. What this means in practical terms is that the nature of the signal we see on the screen will be much more visually interesting (i.e., more sinusoidal) if we use the horizontal deflection. But because we want our waveform to fluctuate vertically, we need to somehow rotate the signal ninety degrees. Some televisions will allow you to loosen a screw on the yoke (where the deflection coils are located) and rotate it. If this isn't possible, or if you don't feel comfortable handling the yoke, you can also simply turn the CRT on its side.
Replacing the horizontal deflection signal with an audio signal
Now that we have disconnected the two wires from the horizontal deflection coil, we need to solder in their place the two wires that carry our audio signal. In most cases, you will need to pass your audio signal through an amplifier before it reaches the yoke of our CRT, otherwise the waveform's amplitude will be very small. I used an amp like this one because it's small, runs on 12V, and can be easily integrated into the build. Because my television can also be powered by 12V DC, this opened up the possibility of powering the entire build from one 12V power supply, which is very convenient.
If your amp uses screw terminals instead of a 3.5mm jack, you will need to cut off one of the 3.5mm connectors, strip back the insulation on the two wires, and connect them to the screw terminals on the amp.
Once the TV is connected to the amp, we can then plug the amp into an audio source (I used an iPhone) and turn on the CRT. You should see the waveform fluctuating on the display. The volume control on the amplifier will adjust the amplitude of the waves on the screen. You will not hear any sound because there is no speaker connected yet.
To hear sound while watching the display, you'll need to use a 3.5mm splitter at the audio source and connect the second line to another amp and a speaker. You could in theory use the TV's own speaker for this, but you will need to disconnect it from the board and attach it directly to your second amplifier.
Reducing the size of the build
If your plan is to incorporate this build into some sort of other project, there are further steps you can take to reduce its footprint. On most TVs, the tuning circuitry is housed in a discrete enclosure that can be removed from the board. In my case there was also a large daughter board that controls the television's radio feature. I removed both of these to reduce the height of the TV board so that it could be combined more easily with other project components. It may be tempting to also remove the large heat sink attached to the horizontal output transistor, but this will eventually lead to the demise of this transistor itself. Given that this transistor is the main power engine of the CRT display, removing it is not advisable.
Troubleshooting
Some televisions won't allow the display to power on if the horizontal deflection wires have been disconnected from their coil. If this happens to you, you can connect a high-wattage resistor across the ends of the two wires you disconnected from the coil. This should be sufficient to trick the television into powering on.
I also experienced a periodic collapse of the vertical and horizontal deflection, where the image would reduce to one large dot at the center of the screen. I don't yet understand what triggers this, but the situation usually resolves itself after a few seconds.
Wrapping up
There are endless applications for a small CRT display, especially if yours has a composite video input. My hope is that this project log inspires at least a few makers to give these displays a try for their projects. Please post links to your CRT projects in the comments, as well as any ideas for improvement (I'll try to incorporate suggestions into the project log). Many thanks for following along!