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Old 27th May 2019, 5:55 am   #1
TonyDuell
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Default CRT lost its tension band

I am not sure which section to put this in, it could also go under 'Components and Circuits', 'Test Equipment' or 'Vintage Computers'. I am putting it under TV as I suspect the people who read this part will have the most experience in this area, but if the mods feel it should be elsewhere then please move it.

I have a piece of vintage datacoms test gear, the display section of which is a 9" raster-scanned monitor.

Unfortunately before I got the instrument it was dropped (looks like from a great height). The plastic outer covers have minor damage, the chassis is twisted and the cartridge tape drive needs a lot of work. But the PCBs are all intact.

But the main problem (and the reason I am posting here) is that 9" CRT. It is intact I think. But the impact has forced it backwards and it has come out of the implosion protection tension band. It is essentially held against it by the tenson springs that earth the outer aquadag coating. Surprisingly there is no real damage to the front panel of the instrument, or to the smoked plastic screen over the front of the CRT.

It's a normal CRT arrangment with a B7G base (and the seal-off tube in the middle), an anode cap connector on the side of the flare and deflection yoke held on the neck with a screw clamp.

I feel it is not safe. My late father did a lot of work on vacuum systems and stressed the dangers of a glass system impoding. I feel I should probably remove the CRT, let the vacuum out safely and fit a replacement.

What is the safest way to do this. I want to save as much of the unit as possible (such as the deflection yoke), but I am rather worried about cracking the CRT when I try to get the yoke off.

Any suggestions/comments?
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Old 27th May 2019, 8:33 am   #2
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Default Re: CRT lost its tension band

Its hard to know what you are talking about without a photo. If you are talking about the spring metal bad that wraps around the perimeter of the faceplate, that is often partly for mounting the CRT to the front panel, but it might help a little with implosion protection. CRTs can still implode with or without a tension band.

The implosion protection on most industrial/data CRT's is simply extra toughened class that is part of the CRT's faceplate and a thick cone or bulb.

Early crt's with thin glass front faces required a separate implosion screen in front of them, but practically all magnetic deflected crts for TV computers etc post the 1970's had it integral to the CRT faceplate. One early method in the '60's was to bond an additional face over the CRT face, these tend to separate with time.

Manufacturers make the same brand & type number of CRT, with and without the tension band (mounting hardware) its an optional feature sometimes, at least for small CRT's, and it depends how the customer chooses to mount the CRT. The front face though is exposed to impacts in any case.

If you are worried about handling the CRT simply wear good safety glasses, don't scratch or score the glass with any metal edges.

Often there is a layer of tape of soft material under the the mounting band. You can probably just slip it back on. The fact it slipped off is lucky, it probably saved the CRT as it did that dissipating some of the kinetic energy.

There should be no reason to need to take the yoke off the neck, but if you have to loosen and or remove the clamp on the rear of the yoke and try to loosen the tabs which are probably embedded a little into tape on the neck, that is often there, before you attempt to rotate and or remove the yoke from the neck.

It would be a shame to ruin the CRT by opening it to air. All working CRT's are a non-renewable resource.

Last edited by Argus25; 27th May 2019 at 8:51 am.
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Old 27th May 2019, 8:44 am   #3
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Default Re: CRT lost its tension band

Well the tried and trusted method we used in the trade was to break the sealing pip at the end of the neck. How you do this is what is best for you be we used a small hammer......Sounds drastic BUT YOU GENTLY tap the sealing pip (might be easier if you remove the plastic base with the guide spigot that covers the pip). One engineer I know used a large screwdriver wedged between the pins and sealing pip and a deft twisting motion would break the pip and the gentle hiss of air would result. Generally we left the CRT in the set while we 'let it down'.

I have a small modelling hammer that I've used countless times to do this but I have resorted to a normal hammer when the small one temporarily went missing.

Most times the pip will break off cleanly, sometimes you crack the base near the pins as well.....it doesn't matter other than it goes to air quicker.

I should emphasise that care should be exercised and it's always advisable to wear protective eyewear. You could do this 1000 times and not have a problem but there is always that one time.....!

Argus 25 has a good point....make sure that it's not just the mounting band. Don't ruin an otherwise good CRT!
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Old 27th May 2019, 10:27 am   #4
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Default Re: CRT lost its tension band

The brand and type of CRT are essential to know. A few pictures would also be very handy to assess the situation. A tension band is heavily tensioned, so the CRT won't just pop out usually. Smaller instrument CRTs may not have the usual construction as the implosion force is less. Does the instrument have a separate window or is the CRT stuck through?

The best way to let off the vacuum in general would indeed be at the pip. If you feel unsafe, you could make sure the front and cone of the tube are well packed.
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Old 27th May 2019, 11:33 am   #5
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Default Re: CRT lost its tension band

You mention fitting a replacement CRT - do you have one? If not, soucing one might be harder than you think. To be honest, from years of handling CRTs, I don't think you need to be too worried about a 9" CRT, assuming it's less than, say, forty years old. Many of these small CRTs had a tensioning band as Argus 25 suggests and employed internal implosion protection.
Mind you, I'd run a mile from a pre-war Pyrex CRT!
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Old 27th May 2019, 11:36 am   #6
Lloyd 1985
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Default Re: CRT lost its tension band

It sounds more likely to me that it is simply a mounting band, if it is just a mounting band then there will probably be a tensioning screw somewhere on the band to clamp it to the CRT, if so you can just loosen it off and pop the CRT back in.

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Old 27th May 2019, 12:08 pm   #7
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Default Re: CRT lost its tension band

Quote:
Originally Posted by Lloyd 1985 View Post
pop the CRT

Regards
Lloyd

Hmmmm.....probably not the best choice of words....!!!
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Old 27th May 2019, 1:14 pm   #8
TonyDuell
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Default Re: CRT lost its tension band

It is the metal band around the edge of the screen that has come adrift. There is no clamping screw on it anywhere, and I can't sensibly push the CRT back into it. My guess is that it was originally heated, fitted and allowed to cool, probably before the CRT was evacuated.

It does clamp the mounting lugs to the CRT glass as well.

I will try to get some photos, but the instrument is back together (I feel the case would contain the glass if the CRT imploded) and I have other things on the bench at the moment.

If I am going to replace the CRT then I have to remove the yoke. My experience with doing this on 12" CRTs is that you completely remove (not just loose) the clamp and then spread the slots in the plastic yoke body that the clamp goes over with a screwdriver before trying to pull the yoke off. They stick badly otherwise. And I am a little worried about doing that to a CRT that may not have implosion protection.

As for a replacement CRT I know they are very hard to find. About 30 years ago I needed to get a similar 12" CRT for a rare computer terminal (it had the APL character set...). The easiest and cheapest way _then_ was to get a portable TV and take the CRT from it. Of course that doesn't apply now. I possibly have other less rare 9" monitors that I could raid a CRT from though.

The CRT is a 9" 90 degree deflection one. 7 pin base (so no plastic locator around the pins/'pip'). It's called a 'modified B7G' base in some databooks, the modification being the 'pip' (sealing tube) in the middle so the socket has a large hole to fit round it. My experience with similar 12" CRTs in computer terminal is that they are pretty interchangeable. A 7 pin one has a 12V-ish heater (the 8 pin B8H ones are normally 6.3V), other electrode voltages are similar enough to give something and it is the a matter of minor electrical modifications to get a good display.

I do NOT want to destroy a good CRT. But I see no way of re-mounting this one. And I certainly don't want to be showered in glass when working on the unit or when using it. If I can safely remove the CRT intact I would be happy to keep it wrapped in padding (old clothes!) in a strong box in case there is any way of restoring it in the future. But my main worry is if I can safely get it out. Removing the anode cap, and more so the deflection yoke, worries me a little. I guess I just have to work as carefully as I can.

As for cracking the 'pip', that method is suggested in some monitor/terminal service manuals as a way to safely dispose of defective CRTs. But I wonder if the safety of doing it depends on the implosion protection being intact.

Finally a bit of unitentional humour : the technical manual for one of my machines gives the following advice for working on the CRT (and this is just for adjusting presets on the monitor PCB!) :

A faceshield to BS<number> to protect the eyes
Gauntlets to BS<other number> to protect the hands
A heavy leather apron to protect those other important parts

Needless to say I do not go that far for normal monitor repairs!
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Old 27th May 2019, 1:41 pm   #9
Maarten
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Default Re: CRT lost its tension band

From your description this indeed sounds like a tension band. Especially if it's somewhat wide and possibly curved or folded, and spot welded together. The tension band being removed doesn't really affect normal handling or controlled implosion of the CRT as far as I'm aware. It mostly controls where and how far the glass goes when the cone or front glass break, pushing the energy inwards (and into the glass itself possibly) instead of letting it fly. To be safe, you should put the CRT in a padded box stuffed with padding until only part of the neck sticks out when you decide to implode it.
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Old 27th May 2019, 1:42 pm   #10
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Default Re: CRT lost its tension band

I remember dad disposing of two old CRT's in the 1950's. They were from old TVs which had separate perspex or armour plate glass explosion screens. The first one he tossed out of the back door into our concreted back yard, rapidly closing the door after him. Result, an enormous BANG and lots of tiny fragments of glass. For the second one, he first put it in a hessian sack, then put it in the dustbin and lobbed in a brick. Another, quieter, bang, but the glass was contained. After that, our neighbour, a semi-retired rag and bone man, showed him how to knock off the pip with a shovel, but I don't think he ever did any more himself.
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Old 27th May 2019, 2:04 pm   #11
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Default Re: CRT lost its tension band

Last time I dumped a tube I fitted a small drill bit to a cordless drill and went through the final anode connector.
Once it was down I put it over the bin and knocked it gently with a medium sized hammer so that the bits fell into the bin. I extracted the steel shadow mask frame and gave it to the local scrap man.
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Old 27th May 2019, 5:35 pm   #12
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Default Re: CRT lost its tension band

I was sufficiently curious that I didn't do what I intended this afternoon but instead went back to this instrument. Getting to the CRT was non-trivial (you have to get a lot of other parts out first) but once I had done that, the anode cap and pin base came off with no problems. After removing the clamp the yoke came off easily.

Then I attacked the mouting nuts with the right spanner. After one of the ones that held the aquadag earthing spring in place was off, the CRT came right out leaving the mouting brackets and tension band behind. I removed the remaining nuts to free those and put the rest of it back together for the moment (Oh, I deliberately left an internal connector unplugged so the line output stage can't start up, since the anode connector is just flapping about I didn't want the EHT to flash over to anything if the unit is powered up).

The CRT is a 'CE678M7P4' It looks a lot smaller than I remember, it's smaller than 9" so getting a replacement is going to be very hard. The old CRT is currently in a box surrounded by old (clean) clothes and labelled as to what it is. I have not attempted to let the vacuum out.

But there's a lot to do on the rest of the instrument, and I am pretty sure I can test it with an external monitor if need be. At least it's not worth worrying about the CRT unitl I've sorted out the PSU and digital side of things.
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Old 27th May 2019, 5:53 pm   #13
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Default Re: CRT lost its tension band

Hello,
Is this the sort of 9 inch tube you have?
Yours, Richard
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Old 27th May 2019, 6:31 pm   #14
TonyDuell
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Default Re: CRT lost its tension band

Firstly I think it's a 6" CRT. It looks smaller than the 9" ones I have arounds, but I didn't
think to measure it before putting it in the box. I thought the type number would identify it but Google hasn't found anything this time.

The CRT is similar to yours but not identical. I think the mounting lugs should be on the front edge of the tension band. The anode cap is on one side of the flare (short dimension of the flare if you see what I mean) not the top or bottom.

The monitor circuitry seems to have been made by Ball Brothers (who I have heard of, they did at least one version of the monitor PCB in the DEC VT100 terminal). It is roughly what I'd expect with a 10 pin edge connector to the rest of the instrument and (as Ball Brothers often did) the line output transformer ('flyback transformer') and an inductor for the vertical output stage separately mounted on the chassis.
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Old 27th May 2019, 6:44 pm   #15
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Default Re: CRT lost its tension band

Could it be 7"?

I could only find it mentioned online in tube regenerator socket tables. Do you by any chance know which brand it is?
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Old 27th May 2019, 7:08 pm   #16
TonyDuell
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Default Re: CRT lost its tension band

It might well be 7". Next time I'm inside the unit I'll measure the positions of the mounting studs and the size of the viewing window. I'd rather not unpack the CRT if I don't have to. I think the only label on the CRT said 'Ball Brothers' but I doubt they made their own CRTs.

The CE678 (no more characters) is listed in the manual for my B&K rejuvenator. At least this confirms it's a 12V heater (I suspected it would be) and that the pinout is what I'd expect.
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Old 27th May 2019, 8:10 pm   #17
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Default Re: CRT lost its tension band

I think the only crts I've seen without a rimband are those with bonded, laminated faceplate, in smaller sizes that means oscilloscope/instrument tubes; they sometimes just have a removeable mounting clamp
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Old 28th May 2019, 9:27 am   #18
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Default Re: CRT lost its tension band

Quote:
Originally Posted by emeritus View Post
I remember dad disposing of two old CRT's in the 1950's. They were from old TVs which had separate perspex or armour plate glass explosion screens. The first one he tossed out of the back door into our concreted back yard, rapidly closing the door after him. Result, an enormous BANG and lots of tiny fragments of glass. For the second one, he first put it in a hessian sack, then put it in the dustbin and lobbed in a brick. Another, quieter, bang, but the glass was contained. After that, our neighbour, a semi-retired rag and bone man, showed him how to knock off the pip with a shovel, but I don't think he ever did any more himself.
Also in the fifties my Dad would sit them at the end of the garden and we would shoot at the necks with his air rifle.

This was before they built new houses behind ours.....
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Old 28th May 2019, 12:05 pm   #19
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Default Re: CRT lost its tension band

I can remember when I was a teenager.
We stripped an old telly for spares and valves and when we were getting rid of the old chassis we took it to a place where dodgy builders used to deposit there waste and stood the tube up with the screen facing the route of departure and opened fire with nice hard river pebbles. On dark evenings you could see the outside lights come on as the residents looked to see what the pop was in the bushes across the road.
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Old 28th May 2019, 8:37 pm   #20
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Default Re: CRT lost its tension band

Quote:
Originally Posted by Welsh Anorak View Post
Mind you, I'd run a mile from a pre-war Pyrex CRT!
An obviously wise comment. Mind you, are there actually any recorded instances amongst collectors of a pre-war tube 'going off'?


I remember once visiting a collector (he's a poster on this forum) and was astonished how he breezily waved (small) pre-war tubes around his head!


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