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Old 19th Jun 2011, 6:35 pm   #1
chipp1968
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Default Rebuilt MW22-7 Tube in Bush TV1

We have just fitted the R A C S rebuilt tube in my TV1 . The set produces 4.5 KV eht which is low. The picture is is very good on the new tube ,not vivid in brightness ,plenty of detail and very watchable now . One problem is that Ion burn appeared almost immediately!
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Old 19th Jun 2011, 11:46 pm   #2
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Default Re: Rebuilt MW22-7 Tube in Bush TV1

Do RACS re phosphor the tube, or just a re gun? Otherwise it could be burn from previous use? Not a bad picture though.

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Old 20th Jun 2011, 6:57 am   #3
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Default Re: Rebuilt MW22-7 Tube in Bush TV1

I was under the impression they do re-gun and depending on the brief or if required a re-phosphor, that latter would add to the overall cost.
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Old 20th Jun 2011, 8:15 am   #4
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Default Re: Rebuilt MW22-7 Tube in Bush TV1

I wondered that , but was under the impression that it had a re phosphor ,if not that's the answer. I was advised against aluminium coating, due to the sets low EHT output, as it may not have been capable of driving it properly. I think this was the right choice as I mentioned above .
I found a posting on the forum that mentions a new tube used in a set that produced an ion burn straight away as well. The low EHT may not help this?

Having the set on this morning with natural light, not sunshine coming in, the picture is plenty bright enough and watchable. I have noticed it depends on what is being broadcast, as on some programmes the picture is wishy washy or too dark. But I have this problem with other sets.
The Set itself was restored prior to purchase, but does have some faults. The lopex is the wrong one, which induces a fold over on the left edge and trying to correct this with the controls just makes it unstable. But this is not a worry. The other problem is that there is a noise on sound, like a whistle wind noise, only when a signal is present. If the local distance switch is on distant there is no noise, but the picture is very contrast metallic. On local, the picture is ok, but the noise is there which is annoying. I didnt realise this with the old tube as I has it on distant and thought the metallically contrast was the tube!

Strangely ,as well as the ion burn, what appeared to happen yesterday with use was that the picture started off a blue tint but ended up with a sepia colour with blue extreme edges! This was not strong but only just noticeable. This morning I can't see that effect.

Hopefully all is well. I will run the set quite a bit this week to make sure it doesn't go to air, which is my main worry.

All in all so far good.
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Old 20th Jun 2011, 10:36 am   #5
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Default Re: Rebuilt MW22-7 Tube in Bush TV1

I Know FrankC had some issues with the phosphor on his rebuilt CRT from RACS, I wonder if others have had any issues.
A bent gun and ion trap would probably have been the best way to go!
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Old 20th Jun 2011, 6:41 pm   #6
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Default Re: Rebuilt MW22-7 Tube in Bush TV1

I didnt think of fitting a later tube .But I think I prefer it to be original ,what should be fitted in a way ,and I had that tube spare.
I was well aware that ion burn would likely appear at some point ,but I wasn't expecting it in the first 20 minutes !Surely this didn't happen when the sets were new ?
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Old 20th Jun 2011, 6:53 pm   #7
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Default Re: Rebuilt MW22-7 Tube in Bush TV1

Hi Chipp.
Indeed it took some years for it to appear and being honest it seems rather worrying. Personally after spending a fair amount of hard earned cash I would be asking some questions. I would have thought that with the light use your set is likely to have that it would have taken a long time for Ion burn to be apparent.
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Old 21st Jun 2011, 7:15 am   #8
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Default Re: Rebuilt MW22-7 Tube in Bush TV1

Hello Chipp,
Ion burns usually took a very long time to appear and in some cases never did appear even with a straight gun and no aluminising. I have a couple of Pye D16T's with the Mullard MW22-7 that are flawless even after a very heavy life.
When I worked closely with a regunner he informed me that the best screen was the original screen but at that time most of the straight gun tubes without ion trap had been retired. A company called 'ReView' used to rescreen but the pictures tended to have a green tint. Your tube may well need 'aging' and may well improve a lot after some hours use. Low EHT voltage as in your Bush TV1 will tend to make the stain more noticeable due to the beams less penetrating ability, John.
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Old 21st Jun 2011, 8:19 am   #9
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Default Re: Rebuilt MW22-7 Tube in Bush TV1

Hello John . Although I have not yet heard back ,what they have actually done re the phosphor , I have come to the conclusion that it is likely to be the original ,which in some ways is a relief as a new screen burning that quickly is quite off putting, although I thought I read something about that on the forum somewhere.
Are you saying the tube may even brighten up a bit more with use ? I like that idea ,although its quite good now considering the set its in .at least its many times better than it was . Shame about the burn though.
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Old 21st Jun 2011, 12:45 pm   #10
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Default Re: Rebuilt MW22-7 Tube in Bush TV1

Good day Gentlemen, Hi Paul,

The photographs have been sent to RACS for investigation + 2 phone calls, one Paul one to RACS to ensure some fast answers and solutions found.

Paul has agreed to take more pictures of the ion burn, if it is indeed what we are seeing.

About the rebuilding process, all B&W CRTs are cleaned down to a transparent glass vessel condition then entirely rebuilt, new screen & all.

If a customer is adamant about keeping his original phosphor, then only the gun is rebuilt. This is a very rare request, 99.xx % of customers want a new screen.

If the set had a very specific tinge (greenish, yellowish, even pink) as can be seen on some pre-war sets before the P4 phosphor was standardized, RACS can supply a colour match of new phosphor whith the same colour temperature as the original.

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Old 21st Jun 2011, 7:11 pm   #11
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Default Re: Rebuilt MW22-7 Tube in Bush TV1

It may well 'bed in' and improve considerably. This is a well established state and slightly under aging at the processing stage is not a bad thing. Over aging can result in a shortened life but we are talking about years of use and not just a few hours a week that we might subject the tube to today. See how it goes. Regards, John.
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Old 21st Jun 2011, 7:19 pm   #12
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Default Re: Rebuilt MW22-7 Tube in Bush TV1

Are you sure it is ion burn Paul? You say the EHT is low. I have a TV which will give all the symptoms of a burn but only if I reduce the EHT. When I wind the EHT back up it totally goes.
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Old 21st Jun 2011, 11:24 pm   #13
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Default Re: Rebuilt MW22-7 Tube in Bush TV1

I dont know what it is , only that its just like Ion Burn and is identical to what the set had on the tube before , if slightly less pronounced im sure the low eht doesn't help.
Dave when you say the set you have when eht is reduced has the symptom , maybe that is Ion burn ? Is it a straight gun tube?
I need to test whether the EHT cap is working properly somehow!
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Old 22nd Jun 2011, 12:01 am   #14
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Default Re: Rebuilt MW22-7 Tube in Bush TV1

Hi this could be bulb charge which is caused by low eht. if it is then i would try it with a bit more eht from another set if you have one. I was restoring a baird portable and it had bad ion burn effects but once the eht was up to scratch its almost disapered. Danny
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Old 22nd Jun 2011, 8:15 am   #15
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Default Re: Rebuilt MW22-7 Tube in Bush TV1

There is some change on the outside of the tube .If I touch the bare screen with my finger the picture distorts. The tube is not a coated type
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Old 22nd Jun 2011, 1:05 pm   #16
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Default Re: Rebuilt MW22-7 Tube in Bush TV1

Hello to All,

RACS staff will look at your pictures today - was out of the office.

From the over the phone description i gave him this morning, Philippe said it sounded like borderline marginal EHT (too low).

Paul, do you have a KV meter or a probe for a VOM? if you could measure the actual EHT it would be valuable information.

Thanks! in advance,

Best Regards

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Old 22nd Jun 2011, 1:30 pm   #17
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Default Re: Rebuilt MW22-7 Tube in Bush TV1

Quote:
Originally Posted by chipp1968 View Post
Dave when you say the set you have when eht is reduced has the symptom , maybe that is Ion burn ? Is it a straight gun tube?
Hi Paul. Mine is a 22MW tube with a bent gun. When I first restored the set I was quite upset as it appeared to have a fairly bad ion burn but as I say I sorted the EHT out and it is now not only fine but reducing the EHT no longer shows the burn. It sounds daft but the burn seems to have cured itself almost certainly due to the tube being run under its correct operating conditions.

It will be interesting what RACS have to say about this.
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Old 22nd Jun 2011, 7:15 pm   #18
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Default Re: Rebuilt MW22-7 Tube in Bush TV1

Hello J , Unfortunately I have no way of testing this at the moment ,which is annoying !
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Old 22nd Jun 2011, 7:33 pm   #19
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Default Re: Rebuilt MW22-7 Tube in Bush TV1

Reduced EHT will allow an ion burn to be observed much more easily. This is due to the beam being less punchy and able to activate the contaminated centre area of the phosphor. An electrostatic charge on the bulb will show blacked out areas around the edges of the tube similar to water slowly levelling out on a flat surface. The picture fills the screen within about a minute. This usually occurs with tubes that are near the end of their life, usually low emission. Placing a finger on the tube face can create local charges that deflect the beam. This is not the sign of a faulty tube. John.
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Old 22nd Jun 2011, 7:59 pm   #20
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Default Re: Rebuilt MW22-7 Tube in Bush TV1

Quote:
Originally Posted by Boom View Post
Hi Paul. Mine is a 22MW tube with a bent gun. When I first restored the set I was quite upset as it appeared to have a fairly bad ion burn but as I say I sorted the EHT out and it is now not only fine but reducing the EHT no longer shows the burn. It sounds daft but the burn seems to have cured itself almost certainly due to the tube being run under its correct operating conditions.
How can a bent gun tube ever develop an ion burn? The ions get shot into the side of the tube by default. The ion trap changes the path back to the screen, but only for the lightweight electrons. Or at least that's how I understood it. However, a misplaced ion trap magnet could indeed produce various shading effects on the picture. Could this be what you saw?

When there is a true ion burn present, more EHT can certainly help matters. I've never understood why. Perhaps it is something to do with the luminosity characteristic of the burned area having a sort of 'knee', which more EHT overcomes...

In general RACS tubes are great but occasionally I have seen some issues with them. These have mostly been mild shading effects on the screen. Also, when a tube has been aluminised, I suspect the deposited aluminium coating may not always be absolutely even in depth. A central shading effect, generally very mild, can result and when a low'ish EHT is used by the set in question, this might make it more noticeable.

It must be a very difficult and precise process to successfully refurbish vintage tubes on an individual basis. It's incredible that RACS has achieved the level of performance they have in doing this. Most of their tubes are fine, and occasionally when there's a problem they always do their best to put it right. Without them, where would we be?

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