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Old 4th Feb 2004, 2:12 pm   #1
Roy
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Default CRT Advice : Handling + Mullard AW59-91

I have been working on a Baird model 630 (early dual-standard, 1963) which uses a Mullard AW59-91 CRT. I needed to remove the CRT for cleaning (a spider had somehow found its way between the CRT face and the tinted Perspex screen) but as I replaced it (with the set face-down on a well-padded surface), the support band with its mounting lugs came right away from the glass. At this point I left the room in a hurry, thinking that the rimband had come off, and expecting the thing to implode at any second (it’s a big tube). However a glance at the datasheet (Telefunken, found on the web) seems to suggest that the support band might be a separate item (the drawings show no sign of an external band, and no mounting lugs). The band is in two sections, tensioned using bolts at the top and bottom, with a rubber gasket between it and the glass. This rubber forms a dust-proof (but seemingly not spider-proof) seal against the inside edge of the Perspex screen. The presence of this rubber and the fact that the band was so loose suggests to me that it plays no part in implosion protection. But I never was very brave, and the door is still firmly shut …

I’m only really familiar with more modern tubes with mounting lugs integral to the rimband, so can anyone confirm how this type of tube is constructed and what type of implosion protection it employs? Does the tinted Perspex screen on this set act as an implosion shield? If the support band is separate, how should I tackle the job of re-fitting it?

Thanks.

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Old 4th Feb 2004, 2:51 pm   #2
ppppenguin
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Default Re: CRT Advice needed – Mullard AW59-91

No early tubes had integral rimbands. Mounting methods varied but a simple metal band with clamps was common. Implosion protection was (nearly) always separate from the tube. The commonest method was a sheet of heavy glass or plastic in front of the tube. I have never encountered one but a few sets used a thin and very tough plastic membrane stretched over the tube face. I have heard these are very awkward to replace without wrinkles. Finally there were the so-called bonded faceplates where a shaped layer of solid plastic was cemented to the tube by the manufacturers.

You are not in any significant danger provided you follow the usual rules for working with CRTs. These include:

Never pick it up by the neck. Use 1 hand under the face and the other to steady it. Or 1 hand at each edge. If it's really big and heavy then use 2 people.

Remember to discharge the EHT before handling. The charge can build up by itself even when the tube is disconnected so watch out. You don't want to get a belt while carrying a tube.

Never knock the tube and try not to scratch the glass.

Wear safety goggles.

The real problem tubes are the very early (1930s, 1940s) ones with lots of flaws in the glass. They can implode spontaneously or with minimal provocation. By the 1950s the glass technology was OK and you are most unlikely to have a problem unless you do something daft. I've been horrifed by seeing 24 " tubes being carried by their necks (this was at a tube regunning plant). The people there never had any problems but please don't follow their example.
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Old 4th Feb 2004, 3:28 pm   #3
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Default Re: CRT Advice needed – Mullard AW59-91


The safest tubes of that era have a border which looks black around the edge of the screen area . This practice went on right thru to the 1980s .The common name for these protected models is 'pushthru' screens as they could be pushed right thru the cabinet without any further implosion protection.

if your screen has the sorrounding border then your perspex thingy maybe some type of contrast enhancer?





 
Old 4th Feb 2004, 4:57 pm   #4
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Default Re: CRT Advice needed – Mullard AW59-91

Maybe I'm wrong but I thought that all the push-through tubes had an integral rim band. Apart from the bonded faceplate types that I mentioned in my previous post. The idea was that the rim band was highly tensioned during manufacture and pushed the edges of the tube together, thus pushing the cone and faceplate apart and relieving the external air ressure. " Together " and " apart " should not be taken as implying movement.
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Old 4th Feb 2004, 5:39 pm   #5
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Default Re: CRT Advice needed – Mullard AW59-91

Thanks for the advice so far. This tube is indeed a push-through type. I am familiar with the later push-through types with the factory fitted rimband and integral fixing lugs - and I thought this was the same until the band fell off. There is no other band underneath it - just glass.

Maybe someone with specific experience of the AW59-91 (or other sizes in this range) or the Baird 620/640 series (Trader 1733) could comment?

Incidentally, the perspex screen is a completely separate item, also " push-through " in style, and the material is about a quarter of an inch thick.
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Old 4th Feb 2004, 6:24 pm   #6
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Default Re: CRT Advice needed – Mullard AW59-91

Quote:
Originally Posted by ppppenguin
Maybe I'm wrong but I thought that all the push-through tubes had an integral rim band
yes i believe the rimband is in the black looking bit that sorrounds the tube. These tubes are as safe as houses and are very dissapointing if you enjoy an implosion or two

I suppose later on after dealing with the implosion protection they thought it was a good idea to stick the mounting lugs on as well!

Last edited by Duke_Nukem; 14th Feb 2005 at 2:25 pm.
 
Old 5th Feb 2004, 11:16 am   #7
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Default Re: CRT Advice needed – Mullard AW59-91

With the greatest of respect, I think that Sidney_Syrup is wrong in believing that a " black-looking border " implies implosion protection as such. This effect simply results from looking at the very thick glass at the CRT faceplate edge, " edge-on " . All tubes of this type, without either the bonded METAL rimband/rimguard or the laminated " twin-panel " faceplate, require a separate implosion screen to be mounted in front of the CRT itself.

These CRTs may be mounted in a " push-through " manner (so that the faceplate edges are visible to the viewer), but that's another story altogether.
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Old 5th Feb 2004, 12:14 pm   #8
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Default Re: CRT Advice needed – Mullard AW59-91

I think humorist2751 has said what I meant and said it rather better than I did. I'm not absolutely sure of what names the tube makers gave to different products but I am sure that humorist2751 is right when he says:

" All tubes of this type, without either the bonded METAL rimband/rimguard or the laminated " twin-panel " faceplate, require a separate implosion screen to be mounted in front of the CRT itself. "

I am also sure that none of us wants proof by accidental demonstration!
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Old 5th Feb 2004, 9:15 pm   #9
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Default Re: CRT Advice needed – Mullard AW59-91

Well, I put the goggles on and refitted the band, tightening it enough to stop it slipping, and the set is now reassembled and working well, with a nice clean screen. All that faffing around just because of a spider in the works! If any more get in, they can damn well stay there.

I was shocked to find how loose the band had become – the tube could have dropped out at any time. It must have been due to hardening or shrinkage of the rubber material between the band and the glass. It was a nasty moment when the tube fell out of the set, hence the earlier panic, but in the end no harm was done, and I know a bit more about CRT development than I did before.
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Old 6th Feb 2004, 6:16 pm   #10
AC/HL
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Default Re: CRT Advice needed – Mullard AW59-91

Out of interest, the " tough plastic membrane " was called a Cornehl guard. I can't remember ever having to replace one, but I believe it involves a hair dryer and lots of strong language
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