My Most Obsolete Skill: Delta-Gun Convergence

In a lifetime of working with electronics we see a lot of technologies arrive, become mighty, then disappear as though they had never been. The germanium transistor for instance, thermionic valves (“tubes”), helical-scan video tape, or the CRT display. Along the way we pick up a trove of general knowledge and special skills associated with working on the devices, which become redundant once the world has moved on, and are suitable only reminiscing about times gone by.

When I think about my now-redundant special skills, there is one that comes to the fore through both the complexity and skill required, and its complete irrelevance today. I’m talking about convergence of the delta-gun shadow mask colour CRTs that were the height of television technology until the 1970s, and which were still readily available for tinkering purposes by a teenager in the 1980s.
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Adventures in Small Screen Video

[Kevin] wanted to make something using a small CRT, maybe an oscilloscope clock or something similar. He thought he scored big with a portable black and white TV that someone threw away, but it wouldn’t power on. Once opened, he thought he found the culprit—a couple of crusty, popped capacitors. [Kevin] ordered some new ones and played with the Arduino TVout code while he waited.

The caps arrived, but the little TV still wouldn’t chooch. Closer inspection revealed that someone had been there before him and ripped out some JST-connected components. Undaunted, [Kevin] went looking for a new CRT and found a vintage JVC camcorder viewfinder on the electronic bay with a 1-1/8″ screen.

At this point, he knew he wanted to display the time, date, and temperature. He figured out how the viewfinder CRT is wired, correctly assuming that the lone shielded wire is meant for composite video. It worked, but the image was backwards and off-center. No problem, just a matter of tracing out the horizontal and vertical deflection wires, swapping the horizontal ones, and nudging a few pixels in the code. Now he just has to spin a PCB, build an enclosure, and roll his own font.

[Kevin]’s CRT is pretty small, but it’s got to be easier on the eyes than the tiniest video game system.

VGA In Memoriam

The reports of the death of the VGA connector are greatly exaggerated. Rumors of the demise of the VGA connector has been going around for a decade now, but VGA has been remarkably resiliant in the face of its impending doom; this post was written on a nine-month old laptop connected to an external monitor through the very familiar thick cable with two blue ends. VGA is a port that can still be found on the back of millions of TVs and monitors that will be shipped this year.

This year is, however, the year that VGA finally dies. After 30 years, after being depreciated by several technologies, and after it became easy to put a VGA output on everything from an eight-pin microcontroller to a Raspberry Pi, VGA has died. It’s not supported by the latest Intel chips, and it’s hard to find a motherboard with the very familiar VGA connector.

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Replacing the CRT in a Vectrex

The Vectrex is a rare beast in the world of retro video games. Introduced in 1982, this was the only video game system to put a monitor right in the console, and it did so for good reason. This was a games system with vector graphics and rotating 3D objects, something that just couldn’t happen on the TV in the family room. A while ago, [John] dug his old Vectrex out of his basement and replaced a faulty logic board. The CRT was still broken, but with a little bit of research and a not-so-ugly kludge, he managed to replace the CRT in a Vectrex.

[John] found someone willing to part with an old CRT online, and after whipping out his credit card, the tube was on his way to his front door. This new tube wasn’t a direct drop in; The original Vectrex had small ears around the edges of the screen that served as mounting points. The new tube had no such ears. Now, a bit of plastic strapping holds the CRT in the chassis. It’s a bit of a kludge, but at least now [John] has a source of Vectrex CRTs.

While the rest of [John]’s repair work didn’t go as well – the Vectrex in question still has all the logic board problems it had when it was taken out of storage. This Vectrex does have a new CRT, and with a bit more work on rehabbing this old machine, it should keep on working for another thirty years.

Whenever you come cross an interesting CRT, make sure you snatch it up. Here’s another offering that uses a tiny screen for some classic MAME action.

Homebrew Analog Scope Project Log

[GK] had some old CRTs lying around, so naturally he decided to build an old school analog scope with one of them. Lucky for us, he’s been documenting his progress. Since it was a big project to tackle, he started out with Spice modeling to work out all the right values.

Prototyping the power supply took some custom transformer winding, but when done, the power supply did the job. Although he’s still wiring up the Z (intensity) axis, the scope is already capable of displaying signals and even text characters using a character generator he built earlier (see video below).

[GK] spends most of the time so far talking about the high voltage power supply design. For the particular tubes he had on hand he needed +200V, -400V, -550V, and 6.3VAC for the CRT heater. This is certainly not the typical Arduino-based digital scope that everyone builds at least once.

We love analog scopes for art projects, logic analyzer conversions, and gaming. Of course, if you don’t have an old CRT in your parts bin, you might consider trying a laser.

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Headphone Amp Features A Tiny CRT

[ErikaFluff] needed an amp for his Grado open cans. Rather than build yet another boring black box, he built what may be the most awesome headphone amp ever. [ErikaFluff] added a tiny CRT to the project, which displays the current audio waveform passing through the amp. He packaged all this up in a customized Hammond box which makes it look like it just rolled off the line from some audiophile studio.

The amplifier in this case is based upon the CMoy, a common headphone amp design. [ErikaFluff] added a MOSFET on the output to drive his relatively low impedance (32 ohm) Grado headphones with reasonable volume. The CRT is from an old video camera viewfinder. Before LCDs were advanced and cheap enough to include in video cameras, CRTs were the only show in town. These tiny black and white screens use high voltage to scan an electron beam across a phosphor screen just like their bigger brethren.

In action! - ImgurSince he was going with an oscilloscope style vector scan rather than the raster scan the screen electronics were originally designed for, [ErikaFluff] had to create his own horizontal and vertical deflection circuits. Horizontal scan is created by a 555 timer generating a sawtooth wave at 75 Hz. Vertical deflection is via an LM386 driving a hand wound impedance matching transformer. The high voltage flyback transformer and its associated driver circuit were kept from the original CRT, though repackaged to make them as small as possible.

You might think that having a few thousand volts next to a sensitive audio amplifier would cause some noise issues. We also worried a bit about shorts causing unexpected shock treatments through the wearer’s ears. [ErikaFluff] says there is no need to for concern. The signal is fed to the CRT circuit through optocouplers. The audio circuit is also electrically split from the CRT and runs on a virtual ground. Judicious amounts of shielding tape keeps the two circuits isolated.

This may not be the most practical project, but we think it’s pretty darn cool. The response over on Reddit’s electronics subreddit seems to be positive as well. We hope [ErikaFluff] is sitting down when this post gets published!

A Revolutionary Input Device, 30 Years Too Late

Way before you kids had touch screens and mice, we had to walk uphill both ways to tell a computer where we were pointing at on the screen. I speak, of course, of light pens. When these photodiodes in a pen were pointed at a CRT, the display driver would tell the computer where the pen was pointing. It’s a pretty incredible video hack today, and these things were around in the 1970s. You could, of course, use a light pen with most of the old 8-bit home computers, including the Commodore 64.

[Jan] has a soft spot for the light pen on the C64. So much so he made a new input device using this tech. It’s great, and if this existed in 1985, all the cool kids would have known about it.

The build is called the LightHammer. It’s a light pen, inside the head of a plastic hammer, with a few springs, nuts, and washers to tell the computer to read the light pen input. The light pen itself is just a photodiode with a few transistors; it was a simple circuit in the 80s, and it’s a simple circuit today.

A new input device isn’t worth anything without an app to show off the tech, and [Jan] is about three steps ahead of us here. He wrote a game for this LightHammer – a digital version of Whac-A-Mole and Simon. They’re exactly what you think they are: the classic ‘repeat the computer’ and ‘murder rodents’ games.

If that’s not enough, [Jan] also built an arcade cabinet for his C64 setup, with the monitor, joysticks, a 1541, and a TV mounted in a cabinet that would look great in a bar. You can check out a video of that and the games using the LightHammer below.

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