A Winking Colour Tube

[Mikey405]

Evenin' all.

Some people may remember but many many years ago, CPC were giving away (for the price of postage) some "seconds" colour tubes. (I think it was CPC anyway - perhaps it was HRS - or was it SEME?) Anyway, somebody was giving these tubes away basically for free.

One of these "seconds" tubes ended up in my slightly tatty looking Philips G6, but it has a really weird and slightly unsettling fault. When the set first comes on, there's a "hole" in the picture toward the bottom-right of centre - I assumed that the shadowmask holes were perhaps blocked or not etched through and that is why there is a small block of dead pixels.

But... when the set has been on for a little while, the hole starts "winking" at me and I don't like it. It's really rather disturbing and I don't know why it does it. The hole "winks" every few seconds when the set is warm. Have a look at the video and see what you think. I'd be very interested to hear what people make of this. (It's totally genuine BTW.)

http://www.oldtechnology.net/misc/CRT-Winking.mp4

Thanks all.

Kind regards.

From Mike.

[Nuvistor]

Perhaps whatever is blocking the holes, an insulator, takes a charge and that builds up to discharge to the phosphor.

Frank

[Graham G3ZVT]

That is seriously weird Mike, I don't recall ever seeing anything like it.

What is the effect of an external degassing coil, or a weakish permanent magnet on the hole?

What about when the set is on its side or upside down, any difference?

I am thinking of a "hanging chad" on the shadowmask, behaving like a gold-leaf electroscope but how that could happen with a etching process is a mystery.

[DangerMan]

I wonder whether there is a small flap or piece of aluminisation that has broken away from the phosphors.
This might allow the charge at the surface of the dots in this area to fluctuate.
So the charge on these dots goes negative during the "working" interval, it reaches a point where would-be incident electrons are returned to the mask instead. Then during the blacked out interval, the local dots charge dissipates until those phosphor dots assume a sufficient positive charge for them to once more capture the electron beam and the cycle repeats....?

Edit:
Just seen Rambo's post after I posted... along similar lines and my first thought exactly, but surely a) it would have snapped off by now? and b) once it was in contact/flat against the dots why would it move away again as it would be at the same potential as the mask and other aluminisation so no attracting force.

Does altering the brightness affect the speed of blinking?

[Andrew2]

We had something like this, but it didn't wink. It was a 28 inch Panasonic and it installed perfectly, but after a few hours the customer reported a 'black spot' on the screen. So back we went and sure enough he was right - a small black patch about 5 mm across.
So we took the TV back to the workshop and fired it up to show the boss. Lo & behold, no spot! And it stayed like that for the next 2 days. I carefully inspected the area around where the spot had been and found everything perfect. I reckoned it was a 'hanging chad' of phosphor that had re-attached itself, and I leaned the set backward and forward in an effort to provoke it but no dice. So we delivered it back and never heard a thing.

[Peter.N.]

Weird indeed, nearly all the phosphors seem to be illuminated when it 'winks'. I would be inclined to try driving round with it in the boot of the car for a bit.

Peter

[ukcol]

My time in the TV service trade included the whole of the period that the delta gun CRT was king and I never saw a CRT fault even remotely like that. I shall follow this thread with interest.

[Heatercathodeshort]

That is a really odd one Mike. I have seen some odd ones but never a winking hole.

There is a note in the final issue of the G6 manual dated 1971.

'Later receivers are fitted with a LOPT failure warning device. This consists of what appears to be a winking magic eye. The effect will cause considerable annoyance to the customer resulting in the receiver being switched off thus preventing any overheating that may become apparent due to EHT overwind failure.'

Sounds very similar Mike. Regards, John.

[Tractionist]

'Appen it fancies eey Mike? !!!!!!!!!!!!!!!!!!

You should be flattered ............................

[slidertogrid]

Is the Tube a re-gun? I used to see the odd prosper dot or sometimes a small group that failed to work on some re-gun delta tubes. Sometimes laying the set on its back and giving the affected area a clout would restore operation.
I took this to be tiny bits of crud which had got into the tube when it was reprocessed. The firm I worked for used the cheapest re-guns they could get and the quality was.. well lets say variable. They would often flash over when first installed which caused havoc in Pye hybrids. Often blowing sound and frame transistors. for some reason the IF and decoder seemed to get away with it?
I have never seen that effect though! I have only seen missing dots not ones that can't make up their mind!
Very strange..

Rich.

[PaulM]

I have seen this before, and whilst not 100% sure of the mechanism, I think that I know what's caused it - scan fail. Many years ago whilst working in R&D on 26" delta gun flight sim monitors, one of our student apprentices managed to run a monitor up with the scan coils resting on the bench yet still connected. All the protection circuits were happy but there was no deflection. This punched a neat hole in the shadowmask and burned the phosphors. However, the hole would 'heal' once things had warmed up as the tattered remnants of the mask must have fallen back into place. The phosphors were burned but still emitting. How to write-off a $2,000 CRT! The apprentice never lived it down . . .

Best regards,

Paul m

[FERNSEH]

Advancing the video frame by frame shows the phosphor dots are intact. This does tend to prove that some particle has somehow embedded itself on to the shadowmask. The "winking effect" is likely caused by an electrical charge building up on the particle then reaching a critical point when a discharge takes place.
It is certainly a creepy thing to watch. I've never seen this effect before although missing phosphor dots were a common sight.

DFWB.

[Lloyd 1985]

Very odd! It would be interesting to see what it does with a blank raster, and maybe wipe a bit of anti-static foam cleaner over it and see if it affects it.

The strangest CRT phenomenon I ever saw was on a 12" round CRM-122 fitted to my Ekco TU 211, I could deflect most of the picture by lightly slapping the palm of my hand onto the implosion screen! The tube was well-duff, probably had air in it, the picture was so dim that I couldn't work on the set until it was dark outside!

I've also got a 28" widescreen TV (Bush branded, probably Vestel..) with a dead pixel on it, the CRT is a Thomson labelled one. I guessed it was something stuck in the shadow mask, as the phosphor looks fine.

Regards,
Lloyd.

[hans]

To me it looks like the coating the phophor dots are printed on have come loose from the glass and the coating (with dots) is flapping back and forth due to static charge.

[PaulM]

In our case (post #11) there was no 'oscillation'. At first switch on the image had a black hole similar in shape and size to the one in the video. Over a period of perhaps 5 minutes the hole would 'heal' and the only clue was reduced efficiency from the phosphor dots. We had a low power microscope for inspecting beam landing accuracy and using that on the damaged area as it warmed up showed a couple of 'flaps' gradually closing. At the time, there was much speculation (as here) as to the mechanism and opinion was split between electrostatic action of some form or the mask warming leading to movement.

I'm not sure what cost the most - the tube write-off or the man-hours in looking at the aftermath! In the end it was useful as a gash test tube and the 'closing doors' effect something to show people. On some images it was barely noticeable but from cold there was always a gaping black hole. I recall that displaying a white field hastened the closure. Perhaps this one varies with beam current and APL too?

Best regards,

Paul M

[Freya]

The shadow mask could have contaminated impurity in the area, forcing the directed beam away. Or the impurity could have been on the glass prior to the phosphors being applied.

[Heatercathodeshort]

I have experienced some very weird defects with colour CRTs.

A Samsung 14" portable was brought in for repair with the complaint that it had a 'ghost' on the picture. It certainly was strange. About 2 seconds after switching on a low brightness slightly out of focus monochrome picture appeared on the screen followed a few seconds later by the normal colour picture slightly displaced, hence the ghost.
A replacement tube cured the problem. I can only think it was some form of secondary emission but it certainly was a strange one.

Blocked phosphor dots and sometimes larger areas were caused by minute particles of dust and debris that entered the tube during the rebuilding process. Sometimes they were on the gun itself, the screen or on the inside of the neck. I'm talking TINY here.

John had a rubber 'cosh' that he would aim at the face of the tube and bring it down smartly and evenly across the face while it was powered up. This usually cleared any impurities and after a soak test all was well.

Impurities also cause flash over when the CRT is operated above 20kv. John [Central Tubes] used to picture test all his tubes in a ITT CVC5 chassis. I was always replacing the AGC control transistor blown to bits when a tube flashed over.

Have you ever wondered why the excellent 9" Mullard tubes of the MW22-7- MW22-14C mostly appear to be 100% after 70 years?
There are a number of reasons for this including the evacuation process. These tubes are side evacuated on the bulb itself. The seal off tube being covered with a black rubber hat.

When tubes are evacuated during original manufacture, any trapped particles or polluted air are drawn out via the side bulb point and not over the cathode assembly as is normal with the later pump tube mounted in the base of the neck below the gun assembly.

There is an incredible amount of scientific skills required to reliably rebuild a CRT.
I learnt something on every visit to John's works and that was at least three times a week!

As far as your winking admirer goes Mike, just stick a bit of insulating tape over it.. Regards, John.

[Graham G3ZVT]

Originally Posted by Heatercathodeshort

A Samsung 14" portable was brought in for repair with the complaint that it had a 'ghost' on the picture. It certainly was strange. About 2 seconds after switching on a low brightness slightly out of focus monochrome picture appeared on the screen followed a few seconds later by the normal colour picture slightly displaced, hence the ghost.
A replacement tube cured the problem. I can only think it was some form of secondary emission but it certainly was a strange one.

I've seen something like that, notably on a 22inch tube I salvaged from a written off Salora, and put in my own Decca "Bradford"
When the CRT was well and truly cut-off and the A1s turned down a faint raster could still be seen if you turned the room lights off.
I rather think it was a gassy tube and a cold-cathode effect, but I don't recall disconnecting the heaters to prove this.
If my theory is correct, it's a wonder why the effect wasn't commonly seen.
<EDIT>
Come to think about it, I seem to recall seeing the same thing on a G8, and the ghost raster appeared immediately at switch-on.

[Heatercathodeshort]

Originally Posted by rambo1152

Come to think about it, I seem to recall seeing the same thing on a G8, and the ghost raster appeared immediately at switch-on.

Yes thinking about it the strange mono picture did appear on switch on followed a few seconds later by the colour one.
As you say a 'cold cathode' effect was the probable cause.
It was a Samsung manufactured tube with a dark blue label.

Once the EHT exceeds about 18kv things start to go bump in the night. J.

[dseymo1]

I always understood that it was bad practice to operate a camera tube face-down, due to the possibility of loose material contaminating the target. If that's the case, clearly the same could happen to a receiver tube.
Would it be worth a try running the tube face-up, and vibrating the shadowmask locally with, say, a tape head demagnetiser?