On the heels of our post on retro-Soviet transistor teardowns and die-shots, [nikitas] wrote in to tell us about a huge thread on rare vacuum devices of all varieties: oddball cathode-ray tubes, obscure Nixies, and strange Soviet valves. We thought the other forum post was overwhelming at just over 110 pages, but how about 391 pages (and counting) of blown-glass electronics?
If you read through the decaptholon, we mentioned that a particularly enthusiastic poster, [lalka], looked to be cataloguing every Soviet oscillator circuit. It turns out that he’s also the one behind this incredible (random) compendium of everything that’s had the air sucked out of it.
The modern office has become a sea of LCD monitors. It’s hard to believe that only a few years ago we were sitting behind Cathode Ray Tubes (CRTs). People have already forgotten the heat, the dust, and the lovely high frequency squeal from their flyback transformers.
There was one feature of those old monitors which seems to be poorly understood. The lowly degauss button. On some monitors it was a physical button. On others, it was a magnet icon on the On Screen Display (OSD). Pressing it rewarded the user with around 5 seconds of a wavy display accompanied by a loud hum.
But what exactly did this button do? It seems that many never knew the purpose of that silly little button, beyond the light-and-sound show. The truth is that degaussing is rather important. Not only to CRTs, but in many other electronic and industrial applications.
Of Shadow Masks and Aperture Grilles
A CRT has quite a few components. There are three electron guns as well as steering and convergence coils at the rear (yoke) of the tube. The front of the tube has a phosphor-coated glass plate which forms the screen. Just behind that glass is a metal grid called the shadow mask. If you had enough money for a Sony screen, the shadow mask was replaced by the famous Trinitron aperture grille, a fine mesh of wires which performed a similar function. The shadow mask or aperture grille’s job is to ensure that the right beams of electrons hit the red, green, or blue phosphor coatings on the front of the screen.
This all required a very precise alignment. Any stray magnetic fields imprinted on the mask would cause the electron beams to bend as they flew through the tube. Too strong a magnetic field, and your TV or monitor would start showing rainbows like something out of a 1960’s acid trip movie. Even the Earth’s own magnetic field could become imprinted on the shadow mask. Simply turning a TV from North to East could cause problems. The official term for it was “Color Purity”.
These issues were well known from the early days of color TV sets. To combat this, manufacturers added a degaussing coil to their sets. A coil of wire wrapped around the front of the tube, just behind the bezel of the set. When the set was powered on, the coil would be fed with mains voltage. This is the well-known ‘fwoomp and buzz’ those old TV sets and monitors would make when you first turned them on. The 50 Hz or 60 Hz AC would create a strong moving magnetic field. This field would effectively erase the imprinted magnetic fields on the shadow mask or aperture grille.
Running high current through the thin degaussing coil would quickly lead to a fire. Sets avoided this by using a Positive Temperature Coefficient (PTC) thermistor in-line with the coil. The current itself (or a small heating coil) would heat up the PTC, causing resistance to increase, and current through the coil to drop. After about 5 seconds, the coil was completely shut down, and the screen was (hopefully) degaussed.
As time went on monitors became embedded systems. The PTC devices were replaced by transistors controlled by the monitor’s main microcontroller. Monitor manufacturers knew that their sets were higher resolution than the average TV set, and thus even more sensitive to magnetic fields. Users are also more likely to move a monitor while using it. This lead the manufacturers to add a degauss button to the front of their sets. A push of the button would energize the coil for a few seconds under software control. Some monitors would also limit the number of times a user could push the button, ensuring the coil didn’t get too hot.
Holding a magnet near the front of a black and white (or a monochrome ‘green screen’) CRT created visible distortion, but no lasting damage. Mid-century hackers who tried the same trick with their first color TV quickly learned that the rainbow effect stayed long after the magnet was moved away. In extreme cases like these, the internal degaussing coil wouldn’t be strong enough to clear the shadow mask.
When all else failed, a handheld degaussing coil or wand could be used. Literally waving the magic wand in front of the screen would usually clear things up. It was of course possible to permanently damage the shadow mask. Back in 2007, I was working for a radar company which had been slow to switch to LCD monitors. Being a radar shop, we had a few strong magnetron magnets lying around. One of these magnets was passed around among the engineers. Leaving the magnet under your monitor overnight would guarantee rainbows in the morning, and a shiny new LCD within a few days.
CRTs aren’t the only devices which use degaussing coils. The term was originally coined in 1945 by Charles F. Goodeve of the Royal Canadian Naval Volunteer Reserve (RCNVR). German mines were capable of detecting the magnetic fields in a naval ship’s steel hull. Coils were used to mask this field. The Queen Mary is one of the more famous ships fitted with a degaussing coil to avoid the deadly mines.
Even mechanical wristwatches can benefit from a bit of degaussing. A watch which has been magnetized will typically run fast. Typically this is due to the steel balance spring becoming a weak magnet. The coils of the spring stick together as the balance wheel winds and unwinds each second. A degaussing coil (or in this case, more properly a demagnetizer) can quickly eliminate the problem.
A story on degaussing wouldn’t be complete without mentioning magnetic media. Handheld or tabletop degaussing coils can be used to bulk erase floppy disks, magnetic tape, even hard disks. One has to wonder if the degaussing coils in monitors were responsible for floppy disks becoming corrupted back in the old days.
So there you have it. The magic degaussing button demystified!
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.
[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.
[GK] has a bit of a fetish for old oscilloscopes, and since he’s using an old ‘scope tube, the design was rather simple for him; there aren’t any schematics here, just what he could put together off the top of his head.
Still, some of [GK]’s earlier projects helped him along the way in turning this CRT into a monitor. The high voltage came from a variable output PSU he had originally designed for photomultiplier tubes. Since this is a monochrome display, the chrominance was discarded with an old Sony Y/C module found in a part drawer.
It’s a great piece of work that, in the words of someone we highly respect is, “worth more than a gazillion lame Hackaday posts where someone connected an Arduino to something, or left a breadboard in a supposedly “finished” project.” Love ya, [Mike].