Rebuilt MW22-7 Tube in Bush TV1

[FRANK.C]

Hi Trevor
No it is the same. I haven't used it that much though. It appears worse at high contrast low brightness levels. The EHT is at 10 kV .

Frank

[Tazman1966]

Wow Frank! What a weird effect. It looks like a long persistence phosphor has been used. Very strange.

[chipp1968]

Hello Frank.
I am sorry to see you have had a problem with your tube . That is an odd effect! I hope you can get it sorted out soon.

[Heatercathodeshort]

Hello Frank,
That effect on a moving picture and the green afterglow gives a very similar display to the dual phosphor radar tube the RCA 5FP7. This is a small 5" electromagnetic tube without aluminising. The picture viewed from the back of the tube is a brilliant sharp blue but this is only the backing for a very long persistence muddy green phosphor that is actually viewed from the front. It is incredibly difficult to get the mix just right for television. It is a bit like adjusting the grey scale on a early colour television. The problem with building a CRT is that you have to completely rebuild the tube before you can see if you have got it spot on. I must admit I think it is a very brave man indeed to attempt to rescreen CRT's. I played around with phosphors with Central Tubes very many years ago but never managed to get a true 'white'. The results usually resulted in a greenish hue or a very course white/blue. This would not matter today of course with our vintage receivers. I think we must accept that we may not get perfection with these very old CRT's especially if the screen has had to be replaced due to vacuum loss. It should be possible to eliminate the afterglow if not obtaining a perfect screen colour. Regards, John.

[geoff999]

From post 25:

"Depositing a new P4 screen phosphor which RACS always does on any B&W CRT rebuild...."

There is description of P4 and many other phosphors on Wikipedia under "Phosphors". P4 is described as white, short persistence and used in many older black and white TVs

Geoff999

[jhalphen]

Hello to All,

Frank, this is the first time i see the long afterglow ou your 2009 rebuilt MW31-74. Your "demo" does indeed look like a P7 radar phosphor afterglow rather than an honest P4 phosphor.

I am sending the links to RACS + the captured flv files to get their opinion.

Could you be more specific in describing the colours produced: P7 uses a short persistence blue phosphor and a very long persistence green/yellow phosphor, can you distinguish clearly the initial/final colours?

I am also asking RACS if they purchase "ready-mixed" P4 or if they do the blue/white phosphor mix themselves.

Will re-post with answers.

Frank, BTW, you're not using a UV rich light source are you? - I've played around in pitch darkness with a Wood's light 8W mini-fluorescent tube (think stamp collecting and phony £ banknote detectors) and it brilliantly excites the long afterglow CRTs as in an operating radar.

Often white LEDs have some UV content, the technology being either blue initial radiation + blue to white conversion or direct UV to white conversion, in fact a solid-state fluorescent lamp.

Interestingly, P4 CRTs show a very weak white fluorescence and the same goes for colour CRTs. You can see the triads light up, but just barely. I suspect the thick faceplate glass highly attenuates the long wavelength UV energy.

http://en.wikipedia.org/wiki/Phosphor

Best Regards

jhalphen
Paris/France

[FRANK.C]

Hi Jerome

With a test card displayed to me it looks perfect. The whites are white and the blacks are black the greys likewise. there may be a slight green tint but only very slight. In normal room light if I turn off the signal to the TV a green ghost of the test card can be seen for a second or two.
The moving picture produced I would describe as being white with a green afterglow. It don't appear to produce any blue light at any time, just white with a green afterglow. Raising the brightness to above normal level helps to mask the effect but it is still there. At lower EHT (it was 9 kV initially) the effect is worse. The EHT on the movie is at the correct 10 kV.

You are right about the torch it is a LED type. I have tried the same torch on other tubes and it dose not produce that effect on any of them.

I have described the afterglow as been green but it may be green/yellow it's difficult to tell without having something to compare it to.

Thanks for all your help.
Frank

[FRANK.C]

For what it is worth I have just tried a incandescent lamp on the face of the tube and it has the same effect as the LED lamp.

Frank

[FRANK.C]

I have uploaded another video which can be seen here. It shows the test card displayed and then the signal been removed, first in darkness and then with the room light on.

Frank

[hans]

The camera is helping to exaggerate effect.

[chipp1968]

I dont know about exaggerate , but none of my tvs do that at all

[FRANK.C]

Hi hans9001
The camera dose not exaggerate the effect at all. The human eye can see the effect for longer that the camera is able record it. Here is a video for comparison of a BUSH TV24C taken with the same camera. Note also the bars that roll across the picture.

Frank

[Heatercathodeshort]

Early this morning I cobbled together a RCA 5FP7 dual phosphor radar tube and a very old television chassis. The afterglow effect can clearly be seen and with this particular tube it is quite extended. The rear 'activating' phosphor is bright blue not shown very well by the digital camera but can be seen as a blue 'ray' in the pictures. The overall effect is very similar to your tube Frank and I feel it is all down to a slight unbalance in the screen mix. Regards, John. [Spooky!]

[Panrock]

This all looks very similar to the effect produced by the CV2810 - a 6-inch ex-WD VCR97-shaped tube I once fitted to my Argus.

This had a long'ish persistence light green phosphor on the screen backed with a short persistence blue/violet phosphor behind. I assumed only the rear violet layer was directly energised by the electron beam. The green layer was fluorescing as a result of the violet light coming from the layer behind. The end result was 'black and white' but with green smears on moving images, when used for television.

If two separate phosphors have been used here, then there's a simple test to find out. If Frank looks at the picture from behind, through the tube wall, then he will see a deep blue picture with hardly any green smearing visible.

[FRANK.C]

Hi John
Thanks for your experiments it must have taken a lot of effort to rig that up.
It has made it clear to me what is meant by short persistence blue phosphor. Can this blue colour be seen on the screen or is it just visible from the rear?. There is no blue visible on the screen of my tube just white with green afterglow.

Hi Steve
The picture you describe on your CV2810 tube is very similar to what I am seeing. I don't think it is possible to see the rear of the phosphor as it has a aluminium coating and the bowl has a conductive coating.

Frank

[Heatercathodeshort]

Hello Frank,
The blue visible from the back of the 5FP7 does not appear from the viewing angle. I think the blue phosphor activates the long persistence phosphor rather like a luminous watch face of old. I guess yours has a slight mix imbalance producing a combination of the two. Your tube certainly reproduces an excellent test card of apparently good white balance. 'Re-View' [see 1950's adverts in P.T.] used to rescreen all their tubes but they always had a slightly greenish hue. Customers soon got used to this and did not seem to worry about it as the pictures were bright and well focused. It was only slight similar to an incorrect grey scale with colour.
Cheers, John.

[Panrock]

Since there is no obvious colour cast on stationary images, the mix between the phosphors cannot be far off already. Yet there is still green lag.

It seems more likely that the material used to provide the green component was simply incorrect. Or, if it was in fact correct, better results would have been obtained with the mix adjusted to give a bluer picture.

[FRANK.C]

The test card that it displays I cannot fault in anyway. It's bright well focused and good grey scale.
The videos shows it at it's worse increasing the brightness helps to make it less obvious but even then someone running down a street appears ghostlike.

I have found a VCR517C. I have no way of firing it up but shining the lamp across it's screen produces a similar effect.
The glow off my tube is quite yellow when compared to the VCR517C. The VCR517C glows a lot brighter in the dark after the lights are switched off.

I have also found a CV1097 tube( I believe it's equivalent to a VCR97) shining the lamp across it's screen produces no effect at all.

Frank

[chipp1968]

It does seam that the wrong phosphor has been applied to the tube .Hopefully this can be sorted out .

[valvekits]

I had a look at your interesting video Frank. Its almost as though when the phosphor releases light, some of the light is transmitted through the face of the tube but some of the energy is reflected back from the glass to re-energise the phosphor again, albeit at a lower dosage.

Eddie