What is the material of the glass in front of the CRT of an oscilloscope? - Page 1
Forum section: electronics / repair

author: max.wwwang
author_rank: Frequent Contributor
timestamp: April 02, 2020, 07:02:04 am
content: The glass in front of the CRT of my scope is broken. It looks like acrylic and is blueish. Is it any special material or can I just replace it with a clear (or whatever colour) acrylic?

author: wasyoungonce
author_rank: Frequent Contributor
timestamp: April 02, 2020, 07:46:57 am
content: A picture and or model number would Help.

author: Haenk
author_rank: Super Contributor
timestamp: April 02, 2020, 08:00:49 am
content: It's just colored plastic (colored to enhance contrast).

author: Electro Detective
author_rank: Super Contributor
timestamp: April 02, 2020, 08:29:06 am
content: Places like OfficeWorks sell small to larger transparent colored plastic clipboards in that close enough blue-ish color, which should do the job.Check the thickness before chopping one upDid I mention they are cheap?There used to be a seller on Ebay Australia that had the real deal ones for sale with dimensions, might be worth a shot

author: max.wwwang
author_rank: Frequent Contributor
timestamp: April 02, 2020, 09:21:51 am
content: Quote from: wasyoungonce on April 02, 2020, 07:46:57 amHitachi V-212

author: jogri
author_rank: Frequent Contributor
timestamp: April 02, 2020, 10:32:07 am
content: I have a similar Hitachi on my bench, it is in fact just blue acrylic glas (scratch test confirmed that). The dimensions are 130x98x2mm, but you need to cut two slots for the pins into the glas.

author: Gyro
author_rank: Super Contributor
timestamp: April 02, 2020, 10:49:09 am
content: Neutral grey can also work reasonably well if you can't get blue (it was an option for Tek storage scopes).Beware of using the scope without a filter, it's normally part of the implosion protection.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: April 03, 2020, 03:44:38 am
content: Quote from: Gyro on April 02, 2020, 10:49:09 amThat's exactly why I asked. I was considering any risk (implosion or radiation, etc.) of not having one.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: April 03, 2020, 03:46:36 am
content: Quote from: jogri on April 02, 2020, 10:32:07 amYes, it must be acrylic glass in general, but not very sure if it was any specialised variation or not. Theoretically, a clear one would also do but with less contrast I guess. Slots are not a problem; could be done with either a drill or hacksaw. Thanks.

author: Electro Detective
author_rank: Super Contributor
timestamp: April 03, 2020, 07:52:42 am
content: Grab a hammerand simulate a CRT POPlosion on a test piece of -suitable color and thickness candidate plastic- on soft wood or dry grass/lawn with a few good hitsIf it doesn't shatter like glass and just dents or flexes,and or unscathed and laughs at the hammer and wannabee Thor wielderI'd use it for my scopeI doubt what's fitted to CRTs is anything special,not that I've noticed any unobtanium ingredient on most of the popular brands I've taken apart to inspect/repair or dust outfwiw > uber humble for correction on that if wrong

author: max.wwwang
author_rank: Frequent Contributor
timestamp: April 04, 2020, 07:26:38 am
content: Quote from: Electro Detective on April 03, 2020, 07:52:42 amWhen I find a replacement I will have such a test.I'm sure it will not shatter like glass.

author: Ian.M
author_rank: Super Contributor
timestamp: April 04, 2020, 07:47:55 am
content: Acrylic sheet does shatter - not into lots of sharp shards like glass, but typically into a few larger pieces. If you want a clear sheet plastic that doesn't shatter, you need Polycarbonate. However a scope CRT has much smaller volume than TV CRTs and the sides are at a much greater angle to the screen, so catastrophic failure is much less likely to launch fragments forward with significant velocity, so even relatively thin Acrylic sheet will contain them.

author: gorge441
author_rank: Contributor
timestamp: April 05, 2020, 05:50:18 am
content: Would you mean Fluorescent screen?

author: max.wwwang
author_rank: Frequent Contributor
timestamp: April 06, 2020, 08:19:00 am
content: Quote from: gorge441 on April 05, 2020, 05:50:18 amIt may have such an effect, because the trace looks better with the glass. But I don't know that's why I'm asking.

author: perdrix
author_rank: Frequent Contributor
timestamp: April 06, 2020, 08:51:08 am
content: Typically there are two screens - the inner one is the implosion screen and is made of (typically) Lexan (polycarbonate). The outer one is the colour filter and is likely to be acrylic.David

author: max.wwwang
author_rank: Frequent Contributor
timestamp: April 06, 2020, 09:03:23 am
content: Quote from: perdrix on April 06, 2020, 08:51:08 amHmmmm, sounds like I was not asking a very stupid question! Maybe they are often done in one piece of glass (because I have never seen or heard of the double glass design)?

author: Electro Detective
author_rank: Super Contributor
timestamp: April 06, 2020, 10:06:12 am
content: I remember doubles on a few scopes but that was way back to remember brand and modelsand I probably still have a couple kicking around..somewhereTwo screens can't hurt, as long as the traces display properly it's a winner

author: max.wwwang
author_rank: Frequent Contributor
timestamp: April 07, 2020, 08:11:18 am
content: Quote from: Electro Detective on April 06, 2020, 10:06:12 amAbsolutely.

author: Haenk
author_rank: Super Contributor
timestamp: April 07, 2020, 01:57:46 pm
content: Quote from: perdrix on April 06, 2020, 08:51:08 amHonestly - how often do CRTs explode? Especially those small ones? I think it's been that rare, that it always appeared in the news (back in the days). I remember hanging around at my local TV and electronics shop, who had to occasionally destroy tubes (removed due to replacement). That wasn't always an easy task and certainly did not work out from the front, but rather driving a screwdriver into the side.So I'd rather take care about a good contrast filter than an explosion shield.

author: Electro Detective
author_rank: Super Contributor
timestamp: April 08, 2020, 02:20:55 am
content: Why not have either or both fitted if the CRT traces are sharp and crisp ?A bit of light loss, if any, with a new or DIY screen can be compensated for with a slight tweak of the brightness/focus knobsLight blue or green screens usually work well, but IIRC I had some older CROs and displays that had red-ish or orange?I don't know what the actual likelihood of an aging, faulty, weakened, or previously 'dropped and hushed up' CRT popping in ones face is,so I'd rather gamble it pops into some colored plastic sheet instead,rather than banging my head in 'assumption regret' lateragainst a wall I may no longer be able to see

author: james_s
author_rank: Super Contributor
timestamp: April 08, 2020, 06:44:56 am
content: Quote from: max.wwwang on April 03, 2020, 03:44:38 amNo there isn't. CRTs made since the 1960s all have integral implosion protection and the glass of the CRT face is leaded to stop any xrays that are produced, this is a legal requirement, it cannot rely on a separate removable shield for safety. That piddly little plastic filter would shatter into a million pieces if the tube actually suffered an uncontained implosion, acrylic is very brittle and not impact resistant.The plastic filter on your scope is cosmetic only, there is no risk of using the instrument without it.

author: Gyro
author_rank: Super Contributor
timestamp: April 08, 2020, 10:33:32 am
content: Quote from: james_s on April 08, 2020, 06:44:56 amWhat you say applies to TV CRTs not scope ones! Flat faced Scope CRTs lack the metal implosion protection band which protects the curved screen of a TV CRT from collapse (implosion from impact) and are therefore much more fragile. An implosion band only works with a curved screen, where it can constrain the base of the 'conedome' or curve from expanding.Of course the piddly little plastic filter wouldn't withstand an uncontained implosion. It's purpose is to protect the glass face from direct metallic impacts and scratches which would weaken it.If the filter was cosmetic only, then why would Tektronix include a clear filter, for use when screen tinting is not required, and instruct that one filter must always be fitted. Note that they also describe the filters as "plastic implosion shields". I have attached the relevant section of the Tek 475A manual for reference.Please speak of what youknowto be true, rather than stating your opinion as fact when safety (and particularly eye safety) is involved!

author: Electro Detective
author_rank: Super Contributor
timestamp: April 08, 2020, 10:45:37 pm
content: Whatever the deal is, you only get one set of eyes, and one face free of shrapnel at birth,you don't need to be playing two-up coin toss games with either or bothas opposed to lots of opportunities to buy replacement CRT scopes if any pop badly or just stop workingI've always had a piece of clear perspex sitting in front of a CRT screen when up close,just in case of contact fluid spray or solder spatterHandy too when using a scope upright on the floor, to keep any falling objects from hitting itI have two perspex pieces, one covers most CRT screens,the other fits the entire front panel of most oscilloscopes, so nothing can -easily- make it's way to the controls or screenIt appears that I've been sort of unawares protecting the owner/operator over the years as well as the CRT oscilloscopesand TBH I'd rather cop a stiff piece of flying perspex knocking me out and bruised up for weeks,than trashed out by a CRT screen that's gone intoh@nd gren@demode

author: james_s
author_rank: Super Contributor
timestamp: April 09, 2020, 04:15:14 am
content: I'd still replace the filter, I mean as mentioned above it does serve to protect the tube from scratches or other damage.As for implosion protection that may be true, although I've never, ever even heard of a scope CRT imploding, you're probably more likely to get stabbed in the eye with a probe. The only CRTs that make me nervous are the really old round ones made of thin glass. I have a 10" radar tube that I think is WWII surplus and other than lighting it up once I've kept it stored in the original carton, it needs to be in some kind of enclosure. The square face scope CRTs have quite a thick faceplate, I think you'd really have to put some effort into getting it to implode, I've certainly never worried about it.

author: Gyro
author_rank: Super Contributor
timestamp: April 09, 2020, 04:30:33 pm
content: Quote from: Electro Detective on April 08, 2020, 10:45:37 pm^ This is one of the big issues, you're unlikely to try to put a Tek7000 series mainframe on its back but most CRT scopes are designed to be portable and useable vertical.Just a quick thought... Atmospheric pressure puts just over 1kg per cm2on the surface of the CRT. That means a distributed 80kg on the average 10x8cm screen, obviously the level of stress increases towards the centre of the screen rather than the supported edges but that's a lot. The screen is the only flat piece of glass I can think of in readily available a vacuum tube, all other surfaces tend to be either thicker or curved outwards to some extent which significantly improves strength.I remember reading than the overall pressure on a portable TV CRT was something of the order of 13 Tonnes (I haven't done the sums myself but I can believe it).I have an old Maltese Cross and a Paddle Wheel Crookes tubes at home. They are clearly hand blown from very thin looking glass, complete with bubbles and even a partial fingerprint (ouch!). These get treated with great respect and caution!

author: max.wwwang
author_rank: Frequent Contributor
timestamp: April 11, 2020, 10:09:56 pm
content: Quote from: Gyro on April 08, 2020, 10:33:32 amThanks so I take this as the verdict. I have respect for Tektronix at least (I've been longing for one).No doubt the chance of implosion isverylow. But this does not prevent from requiring one protection shield for safety reasons.Helpful and healthy debate! Thanks all.