Strange VFD display phenomenon - uneven brightness - Page 1
Forum section: electronics / repair

author: compet17
author_rank: Regular Contributor
timestamp: March 21, 2017, 07:20:12 pm
content: From my experience with VFD displays I know that they tend to wear out when they get old. Segments used more frequently are getting darker. So far so good....I have a Toshiba Aurex FM tuner that has the opposite problem. It was tuned to one station and displayed the frequency (92.3 MHz) for a long time. Last week I noticed that when I change the station, the 92.3 segments are much brighter than the others. The display is working correctly but is hardly readable because the other segments are really dim.I first suspected the VFD driver chips but that's not possible. The segments are multiplexed and a bad driver would let the segment of all digits fail. This is not the case... only 9 2 3....It seems like the segments have burnt in but in an opposite way.Has someone ever experienced that? Is there something I can do against it?

author: edpalmer42
author_rank: Super Contributor
timestamp: March 21, 2017, 07:56:54 pm
content: I have an old microwave oven with a VFD display. If I forget to clear the timer, the digits tend to 'burn in' like your tuner. I've found that if I key in 88:88 (yes, it lets me do that) and leave it sitting like that for a while, the segments even out.Ed

author: compet17
author_rank: Regular Contributor
timestamp: March 22, 2017, 05:42:49 pm
content: That's right.... I fixed it almost the same way. I disconnected the antenna and set the tuner for automatic station search. It counted from 108 down to 88 and back. Next morning everything was ok.I didn't know that VFDs can burn in the opposite way.

author: james_s
author_rank: Super Contributor
timestamp: March 22, 2017, 06:06:51 pm
content: I doubt it's burn, more likely it has something to do with charge building up somewhere. Some VFDs are straight multiplexed like LEDs, while others have an array of control grids that divide up the display into sections.

author: SoundTech-LG
author_rank: Frequent Contributor
timestamp: March 22, 2017, 09:00:10 pm
content: VFD in my Lexicon MC-12 was left on accidentally for many months, it got real dim. Leaving it off for a few days "cured" it...

author: tooki
author_rank: Super Contributor
timestamp: March 26, 2017, 11:24:29 am
content: Quote from: james_s on March 22, 2017, 06:06:51 pmYeah, burn-in necessarily means dimming. If it's brighter, it has to be a charge. It's the same in plasma TVs: they suffer from (permanent) burn-in causing local dimming if a static image is left on for years, but also from temporary image persistence causing local brightness. The latter takes only a few minutes, but also goes away quickly. For example, if I watch an old program in 4:3 aspect ratio on my plasma, and then (in full darkness) turn the TV to a full black frame, I can see the 4:3 area as a slight afterglow. But it's not phosphor persistence, it's a stored charge preventing the pixels from fully turning off. It will dissipate in an hour or so.

author: edpalmer42
author_rank: Super Contributor
timestamp: March 26, 2017, 04:42:41 pm
content: On my microwave oven, the problem won't go away by itself. I can go for days with uneven brightness on the VFD. The only way to even the segments out is to key in 88:88 and let it soak for a while. It's been a long time since I've had to do it so I can't even remember how long 'a while' is.Ed

author: helius
author_rank: Super Contributor
timestamp: March 26, 2017, 05:21:22 pm
content: the VFD could be slightly gassy or have passivation built up on inactive anodes

author: David Hess
author_rank: Super Contributor
timestamp: March 27, 2017, 01:10:05 am
content: This sounds like the "sputtering" which occurs on nixie type displays.

author: james_s
author_rank: Super Contributor
timestamp: March 27, 2017, 07:49:10 am
content: I don't think so. Sputtering occurs in glow discharge devices, and is the result of metal atoms being blasted off of the cathodes by what is a rather violent action on a microscopic scale. In a VFD the cathode does not glow, it is a simple filament emitting a spray of electrons which are drawn toward phosphor coated anodes. VFDs are essentially flood gun CRTs.

author: David Hess
author_rank: Super Contributor
timestamp: March 27, 2017, 05:55:30 pm
content: Maybe more like cathode interference then? I know somewhere I read a thorough article about VFD rejuvenation.

author: james_s
author_rank: Super Contributor
timestamp: March 27, 2017, 06:13:13 pm
content: There's a post somewhere here on the forum about it, the process is very similar to rejuvenating a tired CRT. You run the filaments much hotter than usual to boil off any contamination and expose a fresh layer of the oxide coating that emits electrons. There's only so much emissive coating on the cathodes though so you can't just keep doing this forever.

author: VK5RC
author_rank: Unknown
timestamp: March 27, 2017, 11:22:13 pm
content: I recall Dave mentioned something about old VFD in a video a year or so ago. Below references not vfd but may be helpful?http://www.ke5fx.com/crt.htmlhttp://www.tube-tester.com/sites/nixie/different/cathode%20poisoning/cathode-poisoning.htm

author: helius
author_rank: Super Contributor
timestamp: March 27, 2017, 11:46:35 pm
content: The analogy of VFDs to CRTs may be misleading in cases like this. Electrons in a CRT are ballistic: the accelerating voltage is many kilovolts, and the electrons shoot straight out of the gun striking the screen, even though the anodes are to the side, not in the screen. The accelerating voltage of a VFD is a comparatively tiny 12 to 50 volts, and electrons travel slowly toward the anodes much like in a triode. The structure of the cathode is also much different. In a CRT, the cathode is compact, and its emission is a single parameter which makes the entire screen dimmer or brighter. In a VFD, the cathodes are directly heated filaments (again more like a triode) which can have uneven emission along their length, affecting brightness unevenly. This difference is important because CRT rejuvenation is designed to affect emission; when emission returns to a normal level the tube is rejuvenated. Trying the same procedure on a VFD may affect brightness unevenly because the filament's emission is still not uniform.

author: bitseeker
author_rank: Super Contributor
timestamp: March 28, 2017, 12:53:04 am
content: Quote from: David Hess on March 27, 2017, 05:55:30 pmThe thread below talked about bit about rejuvenation with success on old test gear.https://www.eevblog.com/forum/chat/vacuum-fluorescent-display-rejuvenation/