Burn-in? Nope!

Many folks have asked whether screen burn-in, or phosphor burn, is not a problem. They are concerned by what was a frequent occurrence in the CRT monitors and oscilloscopes of yesteryear: a permanent scar prominently visible on the screen…

Phosphor burn – this old spectrum analyser looks ‘on’ even when it’s off!

To understand why this occurs, first think of an iron burn. If you deliver too much heat for too long into the same spot, your nice new Oscilloclock brand T-shirt will feature a prominent (and permanent) mark as shown below.

Iron burn – this shirt’s fibres have been literally scorched!

(I could push for another analogy, and describe livestock branding – but I think you get the message.)

In a CRT, a beam of fast-moving electrons bombards the phosphor coating on the screen to produce an image. If the beam is too intense, or it is allowed to trace the same route on the screen over a long period of time, the phosphor compound may degrade and lose its luminance. The end result is:

  • The screen won’t light up well in those spots any longer.
  • The damaged areas may appear dark even with the power off – a ‘ghost image’.

Interestingly, this damage does not actually shorten the working life of the CRT! (It does not affect the longevity of the heater, or the amount of gas permeating the vacuum.) However, it is certainly not attractive, and is most definitely NOT an effect you wish to observe on your fancy custom-crafted Oscilloclock…

Keeping the ghosts at bay

Happily, screen burn-in is not much a problem with the Oscilloclock. Let’s see why.

1. CRT selection

Some CRT types and brands are more susceptible to screen burn-in than others. There are a number of factors for this, and all of these are considered during CRT selection to minimize the risk of burn-in:

First, there is the phosphor compound used. Some phosphors, just by their chemical makeup, degrade faster than others. More significant, though, is the fact that some phosphors require more energy (electron beam intensity) to produce the same level of visible light output as others.

For example, a long-persistence blue P7 phosphor, such as used in the Model 1-S and the Prototype, is by its nature ‘darker’; it requires a higher beam intensity than the crisp green P1 or P31 phosphors used in many other models. The higher beam does make the P7 more vulnerable to burn-in.

Different phosphors need different intensities to appear ‘bright’ – so some will burn faster

Fortunately, the simple protection mechanisms in place in the Oscilloclock (we’ll get to these later) will avoid burn-in even on sensitive phosphors. The customer need not be concerned about this risk factor, and can select any of the available phosphors.

The second factor is the thickness of the phosphor coating. The thicker the phosphor, the less burn-in for the same beam intensity. Some CRTs are infamous for having ridiculously thin phosphor coatings, making them extremely susceptible to burn-in. Sadly, some CRTs that are most readily available today fall into this category, and their data sheets even specify an incredibly short maximum longevity of 1000 hours. That’s less than 2 months of continuous use!

Beware CRTs with short lifetime ratings – they may have ridiculously thin phosphors!

Most CRT manufacturers did not publish lifetime ratings, nor did they publish specifications of phosphor thickness. In the Oscilloclock lab, I rely mainly on my and others’ experiences with the manufacturer, and pick and choose only the highest-quality CRTs. Expensive – but definitely worth it!

The third factor is the use of any additional technology in the CRT that would allow for reduced beam intensities. The most common example is the aluminized screen, an additional coating on the rear of the phosphor. This coating reflects the light that would normally emanate from the phosphor towards the rear of the CRT, back into the phosphor (and the front of the screen). A much more efficient use of energy!

However, this technology was a later development, so many CRTs with an aluminized screen tend to be rectangular and have an in-built graticule. These may not be as visually pleasing in a standard Oscilloclock as non-aluminized CRTs.

2. Software (Firmware) protection mechanisms

Remember the phrase “screen saver”? In the pre-LCD monitor days, most computers employed some form of software that would stop the same image being displayed for too long, to avoid screen burn-in.

My favourite screensaver – Flying Toasters!
(Image used under Fair Use terms)

While there is nothing as fancy as flying toasters, the Oscilloclock has several mechanisms in place.

  1. Hourly XY Bump screen saver
    This feature simply shifts the image by a small amount in the X and Y directions every hour. The shift pattern repeats every 31 hours (a prime number), to ensure that every hour numeral will be placed in every screen position.

  2. Auto screen switch
    This feature simply cycles through the screens (clock faces) at regular intervals, configurable from 0 (off) to 90 seconds. This is by far the most commonly enabled feature, as it allows one to enjoy all the Oscilloclock screens without touching the control!

  3. Auto power off
    Strongly recommended by Oscilloclock labs, this feature simply turns the Oscilloclock off after a period of non-activity (not touching the control), configurable from 0 (off) to 90 minutes.

    This may sound counter-intuitive, but in practice, nearly all Oscilloclock owners are comfortable to turn their unit on just when they intend to enjoy it, and allow it to switch itself off. The exceptions are clocks that are permanent fixtures in offices and restaurants, in which case the owners manually turn their clocks on and off together with other appliances in the premises.

These features are of course highlighted in the Operation Guide that accompanies every Oscilloclock.

Summing it up

So there we have it – there’s not so much to be concerned about after all. While CRTs do have a delicate phosphor coating, by selecting a decent CRT in the first place and looking after it in use, the risk of screen burn-in is drastically reduced. In fact, in 7 years of constructing Oscilloclocks, as of today not a single unit has come back for a CRT replacement!

A Humpty Dumpty CRT

Humpty Dumpty sat on a wall…
Humpty Dumpty had a great fall…

…and so the great nursery rhyme goes! But here at Oscilloclock labs, we’re not talking about an egg (which, one theory goes, represented the defeated King Richard III). We’re talking about a beautiful old CRT, savagely shaken and shattered during international shipping. What a waste. But oh, what a great chance to see the insides close-up!

This broken CRT missed its chance to live again in a nice VectorClock

This broken CRT missed its chance to live again in a nice VectorClock

Looking down the barrel. Imagine you are a phosphor molecule, with projectiles from this gun hitting you at the speed of light!

Looking down the barrel. Imagine you are a phosphor molecule, with projectiles from this gun hitting you at the speed of light!

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Robbie’s Place

What do you do on a mundane business trip to London?

Why, shopping, of course! But if you were the humble proprietor of Oscilloclock.com, you would do much more than that… You would seek to expand your vintage electronic empire!

And so it was that I found myself hunting old electronic devices on Portobello Road one fine Saturday morning. Unfortunately, the game there was far and few between; only two relatively mediocre valve radios, in even more mediocre shape, at far more than mediocre prices…

Fortunately, my colleague was going to save the day. “Pop out to Cambridge for a visit – I’ve seen a few antique shops here,” said he. And after the hour train journey and a wee but of walking, we stumbled onto a veritable gold mine – Robbie’s.

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Tek 520A VectorClock!

Television broadcasting has switched from analog to digital – and if you’ve got a nice HD TV, you’ll be loving it!

But with that transition came the death of an entire breed of equipment – the Vectorscope.

Tektronix 1420 Vectorscope

Just to be clear, these are not monitors for playing ancient video games using vector graphics!!  No, the Vectorscope is (was) used to give a delightful view of the ‘vectors’ inside an NTSC or PAL video signal, describing the color components of the signal.

If you were lucky enough to be a TV broadcast technician, you’d use your Vectorscope all the time to check your vectors’ amplitudes and phase. You would even give your vectors names like ‘Jack’ and ‘Jill’, and check up on their relationships daily, just as any responsible guardian would!

But above all, you would marvel every single day at the beautiful hardware you were using, and the complex circuitry involved. Take a look at my Tektronix 526 Vectorscope, which has oodles of delicious tubes to heat my shop on a nice winter’s day:

Tektronix 526 Vectorscope

Well, it all went digital and there is no longer any need for analog color signal analysis. But dry your tears… There is something even better:

Announcing the Tek 520A VectorClock

This lovely Oscilloclock reincarnation of a Tektronix 520A, sold at Maker Faire Tokyo 2013, allows its new owner to forever relive the magic of NTSC, PAL and SECAM analog color.

Tektronix 520A VectorClock - brilliant blend of the old and new!

Tektronix 520A VectorClock – brilliant blend of the old and new!


See more related videos on my YouTube channel

The Tektronix 520A has a stunning built-in array of lights for illuminating the CRT graticules. By simply removing the bezel and external graticule, the Tek 520A morphs into a deliciously moody timepiece!

Tek 520A VectorClock - Glorious Glow

Normally, I shun CRTs with built-in graticules. Their lines detract dreadfully from an Oscilloclock image. But here! The Tek 520A’s internal vectorscope graticule is round! What better way to accentuate a Circle Graphics driven display?

Silky smooth Circle Graphics on steroids!

Silky smooth Circle Graphics on steroids!

Under the Cover…

The Tek 520A is solid-state. It can be left on 24 hours a day and not fail for many years. This makes it a perfect match for my Maximum Re-use + Minimum Invasion policy: nearly all existing circuits – HV power supply, deflection amplifiers, blanking – are put to use, with just a few (reversible) tweaks.

Tek 520A VectorClock - Maximum re-use, Minimum invasion

The Oscilloclock Power Board is mounted neatly next to its own dedicated low voltage supply. A small relay board can be seen below, for controlling the Tek’s main power unit. All cabling is HV-tolerant and neatly fastened with high-temperature cable ties.

Tek 520A VectorClock - Control Board mount and cabling

Of the more interesting reversible ‘tweaks’ needed for this retrofit, here we see a delightful little trimpot pretending to be a transistor. Quite an act, I would say!

Tek 520A VectorClock - an unorthodox transistor replacement

Like what you see?

If you love big, looming Vectorscopes and need to have one put to good use in your living room, Contact me. And be sure to subscribe from the front page, to track all the other exotic experiments and unique timepieces targeted for 2014!


Credits to [Quinn] in Canada, for providing the initial inspiration for the Tek 520A VectorClock project!

BOO!

Oscilloclock.com proudly presents a new feature – Seasonal Treats !

This month, the ghosts come out of the attic and merge with the electron stream, leaving their telltale prints in the phosphor… but if you get too scared? Just push the button and blow them up!

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The Invisible … Beam?

One of the most exciting things you can do with a Scope Clock is to simply look at it from behind! In many CRTs, the anode coating (a conductive black surface sprayed onto the inside of the glass) doesn’t extend all the way to the screen – leaving a nice gap of clear glass from which to observe the beautiful electron beam.

Ha – you can put down your magnifying glasses now.

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Oscilloclock.com site is open!

This site is dedicated to showcasing my Scope Clock designs, and to sharing technical information openly to anyone interested in these vintage electronic timepieces.

What is an “(Oscillo)scope Clock”?

A unique style of clock that displays the time (and other fun things) on a CRT taken from an old oscilloscope. These clocks operate on a fusion of old and new electronics, and are often housed in beautifully crafted cases of wood, metal, or acrylic.

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