Do Cathode Ray Tubes Die? - Page 1
Forum section: electronics / beginners

author: bjcuizon
author_rank: Frequent Contributor
timestamp: April 27, 2017, 02:05:59 am
content: Hi, just wondering...Do cathode ray tubes in oscilloscopes die (or stop emitting electrons)?Thanks.

author: tautech
author_rank: Super Contributor
timestamp: April 27, 2017, 02:08:13 am
content: Quote from: bjcuizon on April 27, 2017, 02:05:59 amRarely but they can get dimmer with age.In what model/brand may we ask ?

author: bjcuizon
author_rank: Frequent Contributor
timestamp: April 27, 2017, 02:13:02 am
content: It is a Toshiba 150BTB31A CRT display.

author: Brumby
author_rank: Unknown
timestamp: April 27, 2017, 02:52:19 am
content: Yes, they can die. The heater can go open circuit or you can get a vacuum failure, just like any valve - but these are more often associated with rough handling. If there has been arcing, it is possible the arc can erode the glass envelope and create a pin hole, which will let air in and kill the vacuum.The typical issues with CRTs are falling emissions and screen burn in.

author: Ian.M
author_rank: Super Contributor
timestamp: April 27, 2017, 03:15:31 am
content: Also heater-cathode shorts.

author: tautech
author_rank: Super Contributor
timestamp: April 27, 2017, 03:24:32 am
content: Quote from: bjcuizon on April 27, 2017, 02:13:02 amIn what model/brand scope ?CRO HV circuitry does fail for various reasons leaving a totally blank display that for all the world looks like a dud tube but it rarely is. The HV circuitry demands respect but it is not by any means impossible to fix but you commonly need a DMM capable of measuring 2kV which stumps many from the start.Feel free to carry this thread on for a repair or start a new one in the Repair board.

author: yada
author_rank: Frequent Contributor
timestamp: April 27, 2017, 03:52:43 am
content: I thought all tubes went bad over time. Here is a video of a guy testing one with a picture bulb tester.https://youtu.be/mReVhbWiqWoSeems filaments burn out, phosphor screen dims, and gasses escape into it.Teletype? Is this for use on a CRT?

author: Richard Crowley
author_rank: Super Contributor
timestamp: April 27, 2017, 04:13:29 am
content: Sure. It was not uncommon to find cheap CRTs that would "wear out" from cathode stripping. There were "CRT Rejuvenator" gadgets that would add a few weeks/months to the useful life of an old CRT.

author: james_s
author_rank: Super Contributor
timestamp: April 28, 2017, 06:20:44 pm
content: CRTs have a finite lifespan and a number of different failure modes. The most common way they wear out is the coating on the cathode wears and emission drops which leads to a dim and often blurry picture. You can also have a type of wear that affects cutoff, the more heavily used central portion of the cathode wears more than the edge which makes it impossible to get a nice brightness gradient. The phosphor has a half life and becomes progressively dimmer with use, screen burn is a localized effect of this but the whole face will gradually darken with use. Then there are other, often more sudden faults. Heater can go open, occasionally the connection to one of the other elements can go open. You can get a short between elements, heater to cathode is the most common but others can happen. A tube can get gassy due to a tiny leak or outgassing of internal parts.In most cases a CRT that is not abused is good for around 30,000-50,000 hours of operation before it is worn out. Operating at excessively high brightness or contrast can kill it a lot faster, double the beam current means very roughly 1/4th the life. Since nobody to my knowledge is manufacturing or rebuilding CRTs anywhere in the world at this point what is out there is what we have and I suspect they are never coming back. Take good care of the CRTs you have, they are going to become as rare as other specialized vacuum tubes.

author: helius
author_rank: Super Contributor
timestamp: April 28, 2017, 06:47:02 pm
content: To paraphrase MacArthur, Old phosphors never die. They just fade away.

author: oldway
author_rank: Super Contributor
timestamp: April 28, 2017, 09:41:40 pm
content: At the time when I was repairing CRT TVs, I found quite often weakened and dimmer tubes .... But the TV tubes had a cathode current of more than one mA ....On the other hand the tubes of oscilloscopes have a much lower current ...This probably explains why I rarely found oscilloscope cathode ray tubes that had died ....I think the oscilloscope has a better chance of dying before the cathode ray tube.

author: james_s
author_rank: Super Contributor
timestamp: April 28, 2017, 11:13:34 pm
content: A lot of oscilloscopes are probably not run 10 hours a day for 10-20 years too, some are but I don't think that's as common. I have a scope tube that came out of an old fetal vital signs monitor and it has a very tired cathode. Quite a lot of screen burn too, but the neck is completely clear making the entire electron gun visible so I can't bring myself to throw it away.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 29, 2017, 03:48:15 am
content: Cathode emission decays over time, from a variety of causes; simiarly, phosphors experience burn-in.Some of the causes (for both materials) can be considerably reduced, by careful attention to chemical purity, high vacuum, and stable operating conditions. A commercial grade vacuum tube might last 5000 hours at ratings; on the other hand, the best tubes ever made lasted decades in service, and I don't mean as special cases (survivor bias, of course your 12AX7 from half a century ago still works, all of them did, obviously), but as true statistical samples, with hundreds of units in service at a time, and no failures over decades of continuous operation.What you'll find in a real unit, in practice, varies. Most scopes were of the "commercial" to "premium quality" grade, with some exceptions here and there.For example, my Tek 475 lasted over three decades of light use, then about five years of heavy use once it got into my hands. Tek 475s are known for running the heater too hot, consequently the cathode life is unusually short. I purchased and installed a replacement CRT, and it's been fine since. (I also haven't been using it nearly as much, since I've moved on to other scopes for primary use.)Toshiba? Probably commercial quality? So, depends on how much use it's had.Tim

author: bjcuizon
author_rank: Frequent Contributor
timestamp: April 30, 2017, 02:16:42 am
content: Thanks for all your replies and advices guys! I should add that into my knowledge.

author: ebclr
author_rank: Super Contributor
timestamp: May 01, 2017, 01:28:55 am
content: For sure they die, like tubes, But many capacitors on same circuit will die first

author: bjcuizon
author_rank: Frequent Contributor
timestamp: May 01, 2017, 04:17:19 am
content: Of course caps are likely the culprit of the first failure of a device.