Help please: VFD rejuvenation - I am on the right track? - Page 1
Forum section: electronics / repair

author: EL Kabong
author_rank: Newbie
timestamp: June 11, 2017, 07:38:23 pm
content: I have a pair of late 90's desktop microcomponents from JVC, together they were sold as "JVC CA-EX70". Nothing too special about them but the amplifier is quite good for my home office as it can handle 6 ohm speakers and has a subwoofer output and the CD changer still works. These were used on and off by various members of my family then were left to sit for a long time (perhaps two years).So the VFD is very dim on both of these, unreadable now in daylight (can really only be seen in a dark room). I've plgged them in just now to leave them on and see if it improves but I know they were quite dim before being stored.I found the service manual on electrotanya (https://elektrotanya.com/jvc_ca-ex70_ca-ex70r.pdf/download.html) but I'm a novice at electronics (I've built a couple of projects around Arduino and some simple kits but I am still learning). So I thought I should post here and ask before I blow something up, as the displays for these are listed as "special order" at around $70 USD each and I can get a nice old used integrated amp or receiver with similar power for not much more than the pair of the displays (though it would be a full size component and not have a remote).I do have access to a benchtop power supply that can do constant current/constant voltage and a benchtop meter.After searching and reading posts here I think what I need to do is to follow the instructions posted here:https://xdevs.com/fix/kei2000/#vfd_rejuvThat is, to remove the VFD (or isolate the filament pins) and then to start at a low constant voltage (around 4-5v DC) and low current (140mA) then increase current to ~180mA while observing the filaments and not allowing them to get too hot (watch for red glow), and leaving the display to "cook" the filament for a few moments.Does that sound like a reasonable plan? Or do I need to start somewhere else?

author: macboy
author_rank: Super Contributor
timestamp: June 12, 2017, 02:15:55 pm
content: DO NOT use constant current mode of the power supply. The filaments have a positive temperature coefficient, meaning that they increase in resistance with temperature, and they increase in temperature a lot. If using constant current, you can get into a thermal runaway situation where the power goes up (due to resistance going up), which increase temperature, which increases resistance, which increases power & temperature more... you get the idea. Adjust the voltage instead. This really is very important.The idea is to use heat clear away stuff (whatever it is) that has coated the filaments and is preventing them from emitting electrons efficiently. It may or may not work. Also note that, like all other types of vacuum tubes, the filaments in a VFD have a limited lifetime.Also effective is simply increasing the power used to heat the filaments in normal operation. This will increase electron emission, increasing display brightness. It will also reduce filament lifetime, maybe drastically. As long as you don't exceed the point where the filaments just barely glow visibly (in a dark room), then you are probably OK, at least for a while. If it means getting more useful life from a display that is unusable today, then it may be worth it.

author: TiN
author_rank: Super Contributor
timestamp: June 12, 2017, 05:39:07 pm
content: Also too high temperature of the filament can cause "needed stuff" to go/die causing even sorta working display to go into trash.Proceed as is. 70$ ain't that bad for the VFD

author: Gyro
author_rank: Super Contributor
timestamp: June 12, 2017, 06:37:15 pm
content: CRT Rejuvinators used to work by increasing the filament voltage by about 20% while applying a positive voltage to one of the electrodes close to the cathode, probably the grid (which is an electrode with a hole in it rather than a grid in a CRT). The aim was to 'strip' some of the cathode coating surface to reveal fresh emissive coating underneath. Apparently this used to be quite successful in a fair percentage of tubes and was quite lucrative for TV service engineers. IRRC, they used to monitor the 'anode' (grid) current which would hopefully increase and then level out, stopping the treatment as soon as it levelled to avoid removing excess coating.The alternative solution was to install a filament booster transformer in the back of the TV, that had tappings at +5%, +10%, +15% etc. which extended the life of a tired CRT for a while (much as macboy suggests). The engineer would just keep coming back every few months and increase the tapping point as emission continued to fall, presumably until it failed.The above is all remembered from an old valve era TV servicing book.I wonder if something similar might be possible for a VFD. Instead of just cooking the filaments, tie all the digit grids (not segments) together and apply a positive voltage (+50V 'ish?) to the to them, while over-running the filaments at +20% (this must have been a compromise figure in CRT days between risk of failure against having enough temporary emission to pull enough current from the cathode surface).The VFD filament drive needs to be AC of course so that emmision is equalized across the full length. I don't know if anyone has ever tried this on a VFD but it could be interesting. Obviously it would have to be isolated / removed from the driver circuit.On the other hand, this treatment could completely trash the VDF of course!P.S. The other thing to check first is the condition of the electrolytic capacitors on the VFD supply rails!P.P.S. You probably want to check out this thread!https://www.eevblog.com/forum/chat/vacuum-fluorescent-display-rejuvenation/

author: Cerebus
author_rank: Super Contributor
timestamp: June 12, 2017, 09:20:20 pm
content: I've seen several 'VFD rejuvenation' threads over the years and I've yet to see a clear authoritative 'this is how to do it' explanation.I, for one, would be very grateful to anybody who can demonstrate:How to do thisTheir authoritativeness i.e. That they, unambitiously, know what they are talking about - as opposed to 'my best guess' type answers.I'd be grateful because I've got a Sony 5.1 AV receiver that is otherwise in great shape but the VFD has reached the point of invisibility in any conditions other than total darkness and I'd love to fix it, if it is fixable.

author: stj
author_rank: Super Contributor
timestamp: June 13, 2017, 02:27:15 am
content: before you risk the glass, check the drive circuits.most vfd applications use electrolytics for things like voltage conversion or dc blocking and the caps dry out.i used to see this a lot when i repaired vcr's

author: floobydust
author_rank: Super Contributor
timestamp: June 14, 2017, 04:37:04 am
content: Yes - first check the filament and anode supply voltages are not to blame.It can be the VFD phosphor that is aged or burnt. You can tell if some (rarely on) segments/digits are really bright compared to others. This says it's not the VFD's emission that is low, but rather dead phosphor causing the low brightness.I found a bunch of CRT rejuvenator manuals and schematics here:Antelope Arcade File ArchiveThey jack up the filament voltage ~30% AND apply +ve voltage to the control grid. Not possible in say an AV receiver, unless the filaments are floating and driven -ve with control grids grounded. You need to pump current through the VFD somehow, to form new oxides on the surface of the cathode.

author: Cerebus
author_rank: Super Contributor
timestamp: June 14, 2017, 01:52:37 pm
content: Quote from: floobydust on June 14, 2017, 04:37:04 amThat sounds like a very useful diagnostic, worth remembering and also suggests that the VFD I want to revive might be in the salvageable category.

author: andy2000
author_rank: Frequent Contributor
timestamp: June 15, 2017, 07:06:30 am
content: Assuming all of the supply voltages are correct, you might be able to rejuvenate the display. I have a hp function generator which had a very dim display. Running the heaters at an increased voltage (so they lit up yellow orange for a few seconds) improved its brightness noticeably. It's not like new, but it's easily readable in room lighting several years later.. The heaters are fairly tough as far as burning out is concerned, but be careful not to vaporize the cathode coating.