Uneven VFD Vacuum Fluorescent Display - Page 1
Forum section: electronics / repair

author: GaFord
author_rank: Newbie
timestamp: October 15, 2018, 04:37:11 pm
content: The display on my oven is a VFD - Vacuum Fluorescent Display. The display is uneven. It is brightest in the horizontal center of the display, but vertically above or below the center the display is dim. Attached is a picture. So far, I have changed all the caps with new low ESR caps and tested all the diodes, the problem remains. The horizontal lines (cathode) all appear to work and I can see faint red lines (there are 6 cathode lines). I also tested the cathode with a 9v volt battery for 1 second and all cathode lines turned red. The display is about 20 years old. I am trying to determine if the display is worn out and I should throw in the towel, or if there is an issue that can be fixed. Thanks for any thoughts.

author: PKTKS
author_rank: Super Contributor
timestamp: October 15, 2018, 04:57:35 pm
content: Usually all the same segments on each number are driven bythe same line of the IC chip controller.**IF** the same segments all fail as equal probably thatcontrol line is failing. VFDs are high voltage devices witha very tricky grid controlCheck if the same "segments" are failing equally and probablythat line is failing to generate the proper control voltage ...In such case of a single IC controller would be to replace the chipPaul

author: GaFord
author_rank: Newbie
timestamp: October 15, 2018, 05:02:01 pm
content: Thanks for the quick reply. It looks like the same segment is both bright and dim, and other segments are only dim. Would a faulty controller chip result in the same segment being both bright and dim? Thanks.

author: Photon939
author_rank: Regular Contributor
timestamp: October 15, 2018, 08:15:25 pm
content: That looks like how they usually fail due to age - there is a great thread in the test equipment section regarding rejuvenating VFDs by temporarily overvolting the filaments to hopefully restore some electron emission. I am not sure of the name of the thread off hand but you should be able to find it with a search or two.

author: GaFord
author_rank: Newbie
timestamp: October 15, 2018, 08:35:23 pm
content: Thanks, I will look for that thread.

author: james_s
author_rank: Super Contributor
timestamp: October 16, 2018, 01:02:37 am
content: That looks like some of the cathodes (filaments) are completely worn out, there isn't much else it could be. Sounds like you already tried boosting the voltage to bake off the cathodes so at this point it's probably just worn out.You may be able to find a replacement display that can be made to work, depending on what if any custom segments it has. Electrically most VFDs are about the same.

author: macboy
author_rank: Super Contributor
timestamp: October 16, 2018, 01:05:57 pm
content: I tend to agree with james_s, it is worn out. I would have suggested the rejuvenation process of heating the cathodes to clean off buildup, and/or boosting filament current a little to improve emission. Since you can see the faint red glow of the cathodes, they have been run hot all their life and are probably simply worn out. The oxide coatings which emit electrons don't last forever. On a healthy VFD, the cathodes usually do not run so hot as to visibly glow, even in dark conditions.As a temporary fix, I'd suggest boosting filament current until the display is usable. This will speed up the aging process so don't expect to double its life or anything.

author: Gyro
author_rank: Super Contributor
timestamp: October 17, 2018, 05:57:55 pm
content: I know it's not going to help you but I was once told by Noritake rep that I should severely limit the use of red phosphors in VFD specs because they had a life reducing 'poisoning' effect. I wish now that I had probed further to understand whether it speeded up the degradation of filament emission or the main phosphor (my guess would be filament though).The only reason I mention it is that the display shown in the OP has the highest proportion of red of any VFD I remember seeing. Maybe a case of designer misjudgement - of course all VFDs fade eventually but that one looks particularly nasty.