Repurposing an Old-Timey Cathode Ray Tube Television Set with Modern Hardware
In his ode to the CRT television, hobbyist Matt Evans explains how he introduced new hardware to a classic display technology
The magic of CRTs
Back in the day before the advent of modern advancements such as LED, LCD, and OLED, the world was dominated by a single type of display technology: the cathode ray tube (CRT). And for a young Matt Evans, growing up in Europe while being surrounded by them allowed for him to see how it would produce whistles, blurry images, and even electric shocks if enough static charge built up on its outer glass surface. However, the current decade has seen CRTs become all but extinct from our daily lives, as sleeker, cheaper, and safer screens have replaced them.
As a way to pay respects to this obsolete technology, Evans decided to build a compact unit that showcases the inner workings to anyone curious enough.
How they work
The name "cathode ray tube" already tells a lot about how this display works. In essence, the tube consists of a solid glass vessel that has had a vacuum pulled within its chamber. Then at the middle, a heating filament generates free electrons that are then guided by a set of metal plates at the back, thus forming the "ray" of electrons. This beam causes them to shoot forward and strike a phosphorus-coated glass plate at the front, creating a glow in a certain pattern, which is determined by a guiding electromagnet that sweeps this beam both horizontally and vertically.
Modifying the circuit
Before there existed flat, compact screens that could show previews of digital camera images, retro cameras were equipped with small CRTs that featured screens only an inch or two large. Evans' salvaged Sony HVF-2000 viewfinder is one of these, and it measures in at around five inches long by two inches wide for a diagonal screen size of 1.5". The electron gun and electromagnets are driven by a pair of PCBs that take in a display signal and use it to control the required high voltages.
Rather than disassembling the entire thing, Evans was able to swap out the video coming from the camera for a Raspberry Pi Zero instead. The Pi is responsible for playing video and sending it out over the composite header to the awaiting CRT board. The final component is a rotary encoder that gives the user freedom to change the current video being played, along with a switch to power everything off when desired.
Designing a new case
The case was designed using OpenSCAD, and it features a series of panels that are intended to be cut from transparent acrylic panels using a laser cutter and then assembled with epoxy. The resulting shell safely isolates the high voltage circuitry from mishandling and allows people to see the CRT from every angle.
Switching it on
After carefully slotting in the tube and securing the Raspberry Pi Zero, along with the supporting circuitry, to the base, Evans' small tribute to the cathode ray tube display was completed. As a test, he was able to get the Raspberry Pi to successfully output terminal text. Afterwards, he set it up to silently play TV shows from the 1980s, indefinitely.
To read more about how Evans was able to repurpose this old viewfinder, you can read his blog post here.
