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Old 23rd Feb 2023, 6:19 pm   #21
slidertogrid
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Default Re: How safe were re-gunned CRTs?

Same here, but most re-gun tubes were not 'tested' as far as the integrity of the rim-band and thereby implosion protection in a domestic environment. They would simply be scrapped at end of receiver life, so if the implosion protection was impaired no one would know.
The only re-gun I had smashed was the one I mentioned in post one and I am sure that didn't perform correctly as ISTR the glass had shattered and had not been held in place correctly by the rim-band. Hence my question, I wonder how safe a re-gun tube actually was (if the rim-band had not been replaced. ) Would Mullard have replaced the rim-band unnecessarily? They would have made more profit on each tube if the original could have been safely retained...
But yes that said, I too fitted loads of re-gun tubes between 1979 and the scrapping of the delta gun sets in the mid 1980s, never broke one and only had one broken.

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Last edited by slidertogrid; 23rd Feb 2023 at 6:23 pm. Reason: clarity
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Old 24th Feb 2023, 3:44 am   #22
Maarten
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Default Re: How safe were re-gunned CRTs?

As said, it's probably a myth that a rim band will always "hold the glass correctly in place". Slightly depending on the definition of correctly, of course.

The main goal of the rimband is making sure parts don't come flying out of the front at high speeds. In this particular case, parts didn't come flying out of the front anyway, but whether that was because of the rimband or because of the floor stopping those parts of glass, is not really known as far as I can tell.

Still, a failure mode has been brought up in which the rimband could have failed to do its jobe correctly, so it can indeed go both ways.
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Old 24th Feb 2023, 7:10 pm   #23
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Default Re: How safe were re-gunned CRTs?

The shows picture what a rim-band should do, that is to keep the faceplate under compression so as it cracks stays in place while the air enters through the hole. No Implosion.
The tube I had go "went Bang" there was no or very little glass left on the rim-band, yes most of the glass did stay inside the set as it landed on the screen but I hate to think how far it would have been ejected into the room if the tube had been smashed with the set upright....
That said I think this was an example of one where the implosion protection probably failed I am not saying all re-gunned tubes without a new rim-band were unsafe, we would need to smash more than one to prove that...
But it does make me wonder how many were compromised and who would have been liable if someone was injured? This was before the compensation culture in this country of course, so maybe no-one!
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Old 26th Feb 2023, 10:14 pm   #24
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Default Re: How safe were re-gunned CRTs?

That picture shows an almost ideal failure, just as the round hole that was described elsewhere in this thread.

Just for clarity, I'm not saying it didn't happen as you wrote, since there is clearly a possible failure mode with a similar result in which a compromised tension band would have been involved.

My hypothesis though, is that this same failure could have happened with a factory original tension band as well, so there's no way to tell for sure without some experiments to recreate the failure.

I will be clearing out some old CRT sets with (duff) factory original CRTs in them later this year, so there's a chance I'll try and recreate - if I decide to go through, instead of dismantling tubes the safe way, I'll post the results.
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Old 26th Feb 2023, 11:16 pm   #25
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Default Re: How safe were re-gunned CRTs?

I like this vintage film on Youtube where some Tektronix engineers decided to see what would happen during the implosion of various oscilloscope and monitor CRT'S

https://www.youtube.com/watch?v=DEPo2SvOHPA

It is interesting to see the violence that erupts when the tube collapses, especially of a 11" rectangular tube near to the end of the film with no rimband protection. It almost jumps off the operating table and disappears!! The entire room floor is covered in broken glass.

What strikes me is the speed and acceleration of broken glass particles when the tube collapses as the steel ball hits the faceplate. You can see in a number of tests that the broken faceplate gets sucked into the CRT cone so quickly that it smashes out the rear of the cone.
A couple of tests are done with the mu-metal screen in place which drastically improves the spread of broken glass when the tube shatters.

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Old 28th Mar 2023, 10:47 pm   #26
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Default Re: How safe were re-gunned CRTs?

I managed to find a regunned A66/54AX for £20.00 and I collected it. The name of the company who had reg-gunned it was Varlectron. It lasted for 18 months and then the colours faded with loss of contrast and focus.

I do recall back in I think 1992 there was a shop selling brand new and re-gunned tubes in Battersea Rise. I purchased two brand new green label Mullard A66/540AX tubes and if my memory is correct I paid £160.00. I had two Stereotext Ferguson TX10's and I had already got a few sets that had decent emission tubes but had failed LOPTX's, so as the cost of repair was turned down I was asked to dispose of the sets in question. I did dispose of them but kept the tubes for my own sets.

I have a Bang & Oulfsen 8902, which does work but the tube is failing and I have 1 decent A66/540AX to use, but I will continue to look for old sets that may have a decent tube.
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Old 29th Mar 2023, 2:19 pm   #27
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Default Re: How safe were re-gunned CRTs?

Interesting...

I never really did much with TVs, but I was wondering if anyone ever came across regunned VDU/computer-monitor CRTs? To my way of thinking, given their hard use [8 or 10 hours a day 5 or 6 days a week?] a computer monitor in a commercial/office environment would surely be more susceptible to wearing out its gun than a TV.

Yet I never heard of a computer monitor with a 'weak' tube. Screen-burn was more often an issue, with secondhand monitors/VDUs, you could sometimes easily read the burned-in logos and names of the company who previously used them.
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Old 29th Mar 2023, 3:10 pm   #28
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Default Re: How safe were re-gunned CRTs?

I repaired a few monitors back in the 90's As you say a low tube was very uncommon probably as small screen tubes tended to last longer. The 20" PIL tubes in for instance the KT3 lasted 5/8 years yet the 14" which used the same gun, lasted forever. This could be down to portables being used less as second sets.
I also repaired gaming machines, from mono screen stuff like Space invaders and Asteroids to the later colour ones, the larger colour tubes in those tended to go low after a few years. The mono ones were more prone to screen burn.
Luckily a lot of the larger screen monitors used conventional tubes, some even had a TV chassis. TX9 was the one I saw the most of.
I never fitted a re-gun though as the repairs were always very budget. A second hand tube from a scrap TV would be used.
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Old 29th Mar 2023, 4:10 pm   #29
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Default Re: How safe were re-gunned CRTs?

I've had a few weak computer monitor CRTs -- and one that wasn't...

The first was as Apple Mac+ with a dull and somewhat smeary picture. After checking the CRT voltages (looked fine) and doing battle with the video amplifer (everything was fine), I checked the CRT. It was low emission.

I've worked on serveral Moniterm mono monitors, the ones that were used with the landscape version of the PERQ2T2 (and 2T4...). Most of the time dull/no image is down to the EHT generator module (this is independant of the line output stage, it's a unit that takes in 24V DC and gives out around +1000V and -150V for the electron gun electrodes andabout 18kV EHT to the final anode). But I have had the odd one where the EHT is fine and the CRT is low emission.

The one that wasn't? A DEC VT100 with a dull picture. Electrode voltages were fine, video amplifier was working properly. CRT emission was also fine. Anyone want to guess what the 'fault' was? (FWIW I felt a right idiot when I found it)
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Old 29th Mar 2023, 4:36 pm   #30
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Default Re: How safe were re-gunned CRTs?

Quote:
Originally Posted by TonyDuell View Post
The one that wasn't? A DEC VT100 with a dull picture. Electrode voltages were fine, video amplifier was working properly. CRT emission was also fine. Anyone want to guess what the 'fault' was? (FWIW I felt a right idiot when I found it)
Was it that the thing was being told to display characters in reduced-brightness? Can't remember if the VT100 had per-character variable intensity as an option but certainly the later VT2xx/3xx series could modify each on-screen character-location with a second-byte of data that gave things like blinking, half-intensity, reverse-video.
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Old 29th Mar 2023, 4:46 pm   #31
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Default Re: How safe were re-gunned CRTs?

I think you need the AVO (Advanced Video Option, nothing to do with multimeters) to get half-intensity, etc, on the VT100. But no, it was nothing like that. The fault was visible in setup mode, and therefore doesn't depend on what the host was sending to the terminal.

Hint, curing it involved the use of some propan-2-ol.
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Old 29th Mar 2023, 5:04 pm   #32
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Default Re: How safe were re-gunned CRTs?

Dirty screen?

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Old 29th Mar 2023, 5:05 pm   #33
TonyDuell
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Default Re: How safe were re-gunned CRTs?

You got it. Cleaned the CRT screen with propan-2-ol and the display was fine.

I did feel a right idiot that I'd traced signals through the video amplifier, etc, before thinking to check that!
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Old 29th Mar 2023, 5:06 pm   #34
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Default Re: How safe were re-gunned CRTs?

Quote:
Originally Posted by TonyDuell View Post
I think you need the AVO (Advanced Video Option, nothing to do with multimeters) to get half-intensity, etc, on the VT100. But no, it was nothing like that. The fault was visible in setup mode, and therefore doesn't depend on what the host was sending to the terminal.

Hint, curing it involved the use of some propan-2-ol.
Screen covered with a semi-opaque film of something-disgusting?

I had a few like that; they came from places like foundries and sewage-works.

The ex-sewage-farm keyboards were the worst.
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Old 30th Mar 2023, 5:26 pm   #35
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Default Re: How safe were re-gunned CRTs?

Some early Samsung plasma screens weren't immune - dirt could (and did) get between a glass faceplate and the screen. Stripping it down to clean was not or the faint-hearted.
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Old 31st Mar 2023, 5:43 pm   #36
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Default Re: How safe were re-gunned CRTs?

Quote:
Originally Posted by Welsh Anorak View Post
Some early Samsung plasma screens weren't immune - dirt could (and did) get between a glass faceplate and the screen. Stripping it down to clean was not or the faint-hearted.
I remember there was a 1990s VDU/terminal which had a 'flatter squarer tube' style CRT with an early form of 'touch-screen' implemented by a glass/conductive-plastic-film faceplate overlay.

We had a few hundred of these.

One summer was particularly hot, which triggered a plague of the tiny black 'thunder-flies'. These little monsters get _everywhere_ and yes they managed to get between the CRT faceplates and the touch-screen overlays. Causing loads of service-calls. Normally I wouldn't have let our field-service guys go into client sites wearing T-shirts, shorts and trainers... but when night-time temperatures are still in the low-30s and you're also having to offer them triple-overtime/night-rate/24*7 working to keep on top of the calls and keep the clients happy, well, sometimes sartorial standards have to be allowed to slip....

I hate thunder-flies.
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Old 31st Mar 2023, 9:40 pm   #37
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Default Re: How safe were re-gunned CRTs?

I've seen plenty of low emission computer monitors, usually quite high end ones. Most memorable were a DEC VT340 terminal (the colour one - might have been a VT240) which was, in that workplace, very desirable because everyone else had monochrome VT220s and VT320s. But the image had all the hallmarks of a low emission CRT. It was dimmer than it ought to have been, and rapidly defocused if you tried to turn the brightness up.

The Trinitron monitors supplied with Sun and HP workstations also wore out their CRTs. I had one in one job which was distinctly pink and went fuzzy if I tried to get any reasonable brightness out of it. All the workstations there were on a maintenance contract so it got replaced. The HP 9000 series workstations at my university also had monitors in various states of disrepair. Some had obvious timebsse faults, but there were definitely signs of low emission tubes too.

I doubt any coliur monitor CRTs after about 1980 ever got regunned. They were all PIL or Trinitron tubes and very high resolution, so getting good static convergence or purity after re gunning would have been really hard. And convergence and purity had to be perfect for computer use.

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Old 15th Apr 2023, 9:20 am   #38
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Default Re: How safe were re-gunned CRTs?

Very interesting video on the CRT implosion, l wouldn't like to see a large CRT blow apart when l think as a kid l used to mess about with TV's and take the CRT's out.

l carried a 20" CRT home on the bus once, having removed it from a scrap TV in a derelict house.
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Old 15th Apr 2023, 9:30 pm   #39
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Default Re: How safe were re-gunned CRTs?

Back in the 70’s I used to move old TV sets on my push bike by strapping them on to the back rack!! Not console sets though.
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Old 18th Apr 2023, 8:14 pm   #40
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Default Re: How safe were re-gunned CRTs?

Haven't we all done that at some point in time? I did it mostly in the 90's.
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