Magic Eye mystery

[Radio Wrangler]

A curiosity:

We all know magic eyes get dimmer and dimmer then invisible. The emissive surfaces on cathodes easily last longer, millions of valves prove this. So, it must be the phosphor.

But CRTs show phosphors in-vacuo can last for ever unless too intense a beam burns them away. Magic eyes don't run that sort of intensity.

So what is the wear-out mechanism in magic eyes, when it looks like we long had the technology to make them far longer-lived?

David

[60136 Alcazar]

Can't answer that question - but I have revived the display on some way back in the 1980s. I no longer remember exactly how, but it may have involved using an AVO VCM Mk4 set to over-run the heater for a limited period? Is this how TV tube guns were resurrected as well?
Perhaps there is some coating which accumulates on the cathode (or target) which can be 'burnt off' at higher temperature? Maybe proximity has an effect in magic eyes which is countered by lack thereof in CRTs?

[Trigon.]

A comprehensive explanation, too long to repeat here, which discusses how the low anode voltage as compared to a CRT affects phosphor life:-

https://www.radiomuseum.org/forum/lighting_up_dark_eye_tubes.html

Cheers

[cmjones01]

I thought it was something to do with ions damaging the phosphor, rather as they used to do before ion-trap magnets and thence aluminised CRTs were introduced. Hence the difference in longevity between the types with a phosphor-coated anode (EM34 et al) and the ones with the phosphor on the glass (EM84 and so on). My hypothesis is that the ions don't make it round the bend to the glass, but I have no evidence for this. Presumably the low anode voltage is why an aluminised phosphor wouldn't work.

I'd heard that heating the phosphor can get it working again. I tried putting the unusably dim Y63/VI103 from my R1155 in a hot oven (200C, I think) for a while but it made precisely no difference other than making the phenolic base a bit bubbly. I'd be curious to try one of those induction heaters which have become popular in the last few years for undoing rusty nuts. That could be a neat way of heating an internal electrode.

Chris

[vinrads]

I notice the phosphor coating goes darker when it goes low brightness. Mick.

[kalee20]

I've assumed it is thermal degredation.

Even a target current of 1mA equates to 250mW, in a small piece of metal in a vacuum. It's going to get warm.

By contrast, a CRT has its phosphor on the inside of the glass envelope, which is kept cool.

I've noticed that the EM87 sort of magic eye does seem to retain its brightness, and this is a magic eye with the phosphor on the glass.

[G6Tanuki]

A fascinating thread, particularly the radiomuseum link.

Here I have only one radio - a modified WWII-era Pye PCR - that's fitted with a magic eye. In this case it is an ex-RAF VI103 (same as a Y63) and to prolong its life I have added a push button (using an old Burgess microswitch assembly) that only puts HT on the glow target when I press the button.

I wonder if the lack of emission from the cathode when it's spending many hours heated but without having to emit electrons will result in cathode poisoning?

Equally, I will probably be in my grave before this becomes an issue.

I have heard of people trying to reactivate magic eyes by blasting them in a microwave oven.

[turretslug]

That Radiomuseum discussion was interesting. I was aware of people using HT voltage doublers to squeeze a bit more go out of faded eyes, but regarded that as a sort of last ditch, nothing to lose, even brutish resort. However, the article suggests that it does actually have reason behind it- though, as kalee20 highlights, putting more energy into the phosphor coating is a bit worrying. There's also the question of needing a greater voltage swing to drive the pattern change with higher HT.

[paulsherwin]

The indicators with metal targets like the EM34 and EM81 seem to have much shorter lives than valves with the target on the glass like EM84s.

[Radio Wrangler]

Very interesting!

In a set with a proper mains transformer, it would be possible to add a Cockroft-Walton voltage multiplier from one arm of the HT winding to realise the higher target voltage at the cost of a couple of high voltage diodes and capacitors.

The idea of surface contamination of the phosphor and the deeper penetration of more energetic electrons from higher accelerating voltage seems to fit with the disproportionately improved brightness from higher voltage. Also interesting is the thought that the phosphor surface can become significantly charged thereby repelling further electrons, while faster electrons penetrate further and better connect to the target metal, not making so much local charge and potential. Also the charge potential on the phosphor surface being less significant compared to the accelerating voltage.

It seemed odd that no-one had had a good dig into this area, especially considering the expense of replacement tubes. I don't have anything with a magic eye myself at the moment, but I was seeing other people suffering.

Thanks, Trigon, that's a good reference.

It could easily have turned out that there was nothing that could be done without having to get inside the glassware, but a route to a bit of electronics work outside is the best outcome. Maybe a little snippet of PCB in a plastic box for an HT multiplier (fusible resistor protecting the input?) could be a handy mod.

David

[60136 Alcazar]

Looking at this again, Google AI came up with: "Temporary Brightness: Applying extra heater voltage for a short period can temporarily restore electron emission and fluorescence."
It seems this could be a lift from Jozef Bogin, a Czeck engineer, from 2018 -
https://boginjr.com/electronics/lv/magic-eye/

His article ends with:
"An obnoxious problem with these vacuum tubes is their degradation in brightness. This can be caused by cathode poisoning, which reduces the emission of electrons. If this is the case, you can “rejuvenate” the tube by applying the heater up to its nominal voltage, and then, slowly and gradually, you increase the filament voltage up to two (or even two and a half) times rated voltage – and keep it there for a few minutes. Anode supply stays off the whole time. Note that this is just a temporary solution, plus if you don’t do it slowly, you risk rupturing the filament, rendering the tube useless.
A second case is the phosphor being just flat worn out, due to burn in (pictured above), or that it had been subjected to visible and UV light for too long, which degrades its luminiscence. Just like a CRT screen. This cannot be helped – the only thing you can do is to raise the filament voltage, which also increases cathode emission. While you’re at it, you can also ramp up the plate voltage, even up to 500 volts if you wish. Of course, this shortens the lifespan of the tube rapidly, but it will be eventually rendered useless after some time, one way or another."

This seems to agree with the posts in this thread.

[kalee20]

It's easy to see if it is cathode degredation - just measure the target current.

As likely as not, it will be pretty much as it should be, thus exonerating the cathode - but without much light being produced.

I'm sure I started a similar thread to this, some years go - but I can't find it! However, I did find this from 13 years ago in which I had put forward a cogent argument against the phosphor fade being heat related!

[Leon Crampin]

A difficulty which may arise if the target voltage is increased, is a reduction of the sensitivity of the eye.

As electrons have finite mass, an increased acceleration and hence velocity near the deflection electrode will result in the electron stream being under the influence of the deflection field for less time. This gives effectively, a lower sensitivity.

A perusal of some eye data sheets confirms that lowering the target voltage to, say, 150V as may occur in an AC/DC set, increases the eye's sensitivity. Presumably, the converse applies.

Leon.