Oscilloscope LCD screens versus CRT screens - Page 1
Forum section: electronics / general

author: Computeruser
author_rank: Regular Contributor
timestamp: September 02, 2011, 10:31:43 pm
content: My general purpose analogue oscilloscopes have P31 phosphor in the CRT. Still, I do not relish any kind of burn in. So if I am working in my shop (say with my trains) and have my scopes on, I will typically ground the input (GND position) and press the NORM trigerring button. This turns off the free running baseline that occurs with AUTO triggering. On my bench scopes, I turn off the readout as well. So the instruments are running and warm, but no danger of burn in.Question: Will my TDS2024C exhibit any burn in over time (the screen cannot be turned off), or I am just on the wrong track? ... C

author: tesla500
author_rank: Regular Contributor
timestamp: September 02, 2011, 10:52:28 pm
content: LCDs don't burn in, but the backlight may wear out if it's CCFL. It should last for tens of thousands of hours though.

author: ejeffrey
author_rank: Super Contributor
timestamp: September 02, 2011, 10:56:54 pm
content: LCDs are basically immune to burn in.For that matter, modern CRT displays are rather resistant to burn-in as long as the beam intensity is not too high. For instance, a DSO with a CRT will be not very prone to burn in (although not immune: menu text will often get burned in), as the screen is always retraced quickly at the appropriate drive level. The problem with analog scopes is that the optimal beam intensity depends on the sweep speed and the waveform. It is easy to overdrive the phosphors by leaving it with no signal on a slow sweep or in x-y mode.

author: Computeruser
author_rank: Regular Contributor
timestamp: September 02, 2011, 11:14:50 pm
content: Thank you both. This was helpful to know. My Thinkpad T61p screen turns off whenever I am not using it, and likewise the NEC Monitor on my Thinkcentre desktop machine turns off into power saver mode. So I am just used to seeing screens turn off and I am used to that (more than to seeing my TEK scope screen not turn off).>>> It is easy to overdrive the phosphors by leaving it with no signal on a slow sweep or in x-y mode <-- I don't use X-Y mode a lot, and I typically leave my horizonatal channels at a default of 1 Mhz, so when I switch to NORM (no triggering), I don't think I am causing any issue. ... C

author: Bored@Work
author_rank: Super Contributor
timestamp: September 03, 2011, 06:32:42 am
content: When it comes to LCDs, yes, burn-in exists, it is just not called burn-in, butimage persistence. It is often non-permanent, but can be permanent, too. I once had to chuck an LCD monitor because of this, were the usual cure (display a white image over a long period of time), didn't help any more.The most prominent case of image persistence was a a series of Apple LCDs. Here is Apple's support note about the issuehttp://support.apple.com/kb/HT2807

author: rf-loop
author_rank: Super Contributor
timestamp: September 03, 2011, 07:00:06 am
content: I have not seen >30 years old TFT/LCD screens and do not know how they wear. But most old have also really bad "picture" freezing problems. (specially if driving vas bad done. (bias problem)) and persist problems etc etc. Today TFT.s are better but who have seen 20-30 years old lot of used TFT?Why need think "picture burn-in" problem if product lifetime is designed for 3-4 years.But yes, one short time misuse can burn scope vector CRT. raster CRT can also burn but normally it need long time keep fixed picture on the screen... example in industrial 24/7 production test use.But, I have seen hundreds of this age (mostly >10 - >30years) CRT vector and raster monitor in good condition and also some wear condition.Short time ago I look Tektronix analog oscilloscope with vector monitor and power on hours "only" just <70000h. It works well and also tube was not just perfect but still good. (need some adjustments and some light burn-ins)I have myself used Tektronix vector CRT small digital oscilloscope outside < -45 celsius temperature, it works well exept probe cables "littlebit" not so flexible. I do not want try this with LCD. Mil equipments need work also in cold, not only on lab table.I have seen very low amount really bad or failed CRT's but what I have seen lot of and littlebit more -- failed LCD/TFT's. Example one Fluke scopemeter model is nearly as garbage, these I have seen more failed than good (series 190). (oh but this can not tell - after then there are thousend flukefanboys over me)But products lifetime cycle is today so short that maybe LCD/TFT is not problem - it is only waste recycling problem.I'm many times laughing as manufacturers tell that product have lifetime warranty, but many peoples do not realize what it really means.I have TFT scopes, raster CRT and vector CRT. Most I like use amazing good "old" tektronix vector monitors. Even if it is digital oscilloscope. But today I need also use these today digitals and mainly only becouse they have easy wfm and pic saving to usb. Well if compare cheap Rigol type TFT or old Tektronix - there is no competition. Or maybe you can compare matchbox radio and high end sound studio. But in this case. Both price is around same (becouse this Tek is maybe more than 20y old and still it beats 10-1 this kind of soap boxes. And its longevity is nearly sure still better than these 3year boxes.I can easy believe that my some 20-30 years old Tektronix and HP have still rest 10-20years lifetime but I can not believe same if I look example some UNIToySiglentAttenRigolTenma things (or also today made new Tek or Agilent .. they have designed dead time but this is not LCD/TFT.. it is more "clever" done.)

author: Computeruser
author_rank: Regular Contributor
timestamp: September 03, 2011, 12:06:07 pm
content: >>> Why need think "picture burn-in" problem if product lifetime is designed for 3-4 years <--- I usually have things for much longer than that and do not have any significant problem. ..... C

author: rf-loop
author_rank: Super Contributor
timestamp: September 03, 2011, 01:04:59 pm
content: Quote from: Computeruser on September 03, 2011, 12:06:07 pmYes of course. How long time some quipment can run without fail is littlebit different as product lifetime.And also, what we _now_ have running moe than 3-4 years...Some months ago I have R&S <10 years old special test equipment. New this product price have been around >100k$Today this product lifetime is end. There is no professional need anymore. Product is "obsolete".R&S put LCD (spectrum display, some other things etc..) to these machines. well.. I have seen around 20-30 these equipments and half of them suffer more or less failed LCD screen (and fail was not CCFL) . Why they care LCD longevity... 100k$ equipment is in waste recycling after some years active professional use time. So what if LCD die.If product is still alive two years after warranty time what we can think. Design team have failed to design reduced longevity.Product need be as good as possible over warranty time. Soon after then it need fail or some other way go out from useability.I remember well time when HP (and many others) try do as good as possible without saving materials, selecting good components, thinking many things related to reliability and longevity also with accuracy. These was "golden years". They do not come back. Today it is perhaps most difficult "science" to design good short time reliability and then decay fast and soon go to waste recycling. This is shame but this is busines.

author: Zero999
author_rank: Super Contributor
timestamp: September 03, 2011, 03:42:57 pm
content: Quote from: rf-loop on September 03, 2011, 07:00:06 amNo, TFT LCD technology is under 20 years old and only be came common on laptop displays in the mid 90s. Before then the response time was pitiful: moving the mouse cursor across the screen would result in trails and 3D computer games of the day were unplayable on laptops. I remember noticing how bad Doom was on a 1992 486 laptop, apart from the trails hardware scaling wasn't advanced enough to properly scale the 320x200 MCGA resolution used by Doom up to the native resolution of 640x480 so it scaled it up to 640x400 which resulted in vertically shrunk display.Later I played Quake on a 1996 P200 laptop. It had a TFT LCD which was much more responsive. The hardware scaling sill wasn't good enough to scale to 800x600 but the game had an option to user 400x300 which scaled perfectly. It was still not perfect, the LCD had a maximum colour depth of 16-bit even though the graphics card could do 24-bit and the viewing angle and colour weren't as good as a modern TFT monitor.

author: tekfan
author_rank: Frequent Contributor
timestamp: September 03, 2011, 05:55:28 pm
content: Quote from: Computeruser on September 02, 2011, 10:31:43 pmWARNING: Running an analog CRT oscilloscope with the beam constatntly blanked out or intensity all the way down can actually decrease cathode emission. It is actually better to let the beam scan the screen constantly at a normal level or you can position the trace off the screen.From the 70's and onward almost every oscilloscope CRT had a fine mesh behind the phosphor screen and these CRTs are almost impossible to burn (unless you're stupid enough to turn the intensity all the way up and wait a few hours).The mesh behind the screen was added because it greatly increased the deflection sensitivity of the CRT. A few 10 volts at the deflection plates are enough to move the beam off the screen.60's scopes didn't have the mesh because they didn't need ot since they were all tube based and had deflection plate voltages of over 300 volts. These older CRTs are much more prone to burn in.Analog oscilloscopes with a MCP microchannel plate accelerator are also very sensitive to burn in (Tektrtonix 7104, 2467)Burned screen of a Tek 7104

author: Computeruser
author_rank: Regular Contributor
timestamp: September 03, 2011, 06:53:55 pm
content: I have a 7104 (microchannel) and a 7704 with standard P31 phosphor.To your points:1. The Readout on both can be turned off (not just down, but off). I should think (a) it will not harm the tube in any way to turn off the Readout and (b) turning off the Readout wiill prevent the problem you show.2. When the sccope is not triggered, the beam is turned off, I think. Is that correct?The 7104 cannot be run very bright or the limit mechanism will turn it off.So then help me out please with the NORM triggering function and no input signal.Thank you, .... C

author: tekfan
author_rank: Frequent Contributor
timestamp: September 03, 2011, 10:08:42 pm
content: Quote from: Computeruser on September 03, 2011, 06:53:55 pmYes that is correct. The beam is blanked out in the same way as if you would turn the intensity to minimum.The heater inside the electron gun is still on but no electrons are accelerated towards the screen. This can in long term decrease cathode emission and thus trace brightness.The only way to extend the life of the CRT is to turn everything off (HV supply, heater...)In the case of the 7104 it might be better to use it only when you need it and use the 7704 normally as you would and just reduce the beam and readout intensity (not all the way down) when you're not in the lab.I've got a 7904 (same tube as 7704) that I turn on with normal trace and readout intensity (triggering in auto mode) and then leave the scope for a few hours. I can't see any visible burn marks at all on the screen. If it didn't get any burns in 35 years of misuse and neglect I don't think it will any time soon.The only way you might burn the CRT is with the intensity all the way up for a few hours. Microchannel and storage CRTs are much more susceptible to burn in. Normal CRTs not nearly as much.

author: Computeruser
author_rank: Regular Contributor
timestamp: September 03, 2011, 10:14:02 pm
content: >>> In the case of the 7104 it might be better to use it only when you need it and use the 7704 normallyThat is pretty much what I do. I would not normally run the 7704 for more than a couple of hours (because time in my shop is somewhat limited). So when I am done, I turn things off.Overall, then, based on your discussion and the discussions above, I do not think I am hurting or abusing my instruments.... C

author: IanB
author_rank: Super Contributor
timestamp: September 04, 2011, 01:02:05 am
content: Quote from: Computeruser on September 02, 2011, 11:14:50 pmThe screens turning off is more about power saving than about extending the life of the screen. Modern digital oscilloscopes with LCD displays ought to turn the screen off when not in use like computer monitors do, for the same reason. I don't really see why CRT oscilloscopes couldn't go into power saving mode either. CRT televisions had a perfectly usable standby mode and it only took them a few seconds to come back on again. Why couldn't oscilloscope CRTs be powered down when not in use the same way?

author: Computeruser
author_rank: Regular Contributor
timestamp: September 04, 2011, 01:06:56 am
content: >>>> Why couldn't oscilloscope CRTs be powered down when not in use the same way?I absolutely agree with that. Push AutoSet and all would come back to life immediately. ... C

author: tekfan
author_rank: Frequent Contributor
timestamp: September 04, 2011, 02:30:51 am
content: Quote from: Computeruser on September 04, 2011, 01:06:56 amIt kind of defeats the purpouse of having an oscilloscope, especially an analog one. If there's no picture on the CRT it's as good as just turned off completely or not having a scope at all.It could be a feature on a DSO for when you're sampling waveforms at long time intervals but does the screen really need to be off? The screen doesn't consume more than a few watts of power.My Tek 7904 uses about 300 watts. 90% or even more of this power is just for the amplifiers. The rest is just the CRT circuit.And since when have you seen an autoset button on a true analog scope?

author: Computeruser
author_rank: Regular Contributor
timestamp: September 04, 2011, 02:54:22 am
content: I don't completely agree.I might have my oscilloscope on but be dealing with some decoder logic in a train. I am just not using it all the time. In the same way, I don't use my computer all the time, and when I don't the screen goes off.You have nicely explained why this strategy doesn't work for a CRT analogue oscilloscope and I agree with that. I had been turning off the beam to "save" the CRT and as you note, for a P31 phosphor and normal use, I needn't have bothered.But it would be nice for the LCD screen in my DSO scope to go out in the same way that an LCD screen on a computer goes out. It turns the backlight off which would be useful.... C