ILC1-1/7 VFD Display Driving

[Richard Scales]

I have just started testing this display and I am using an A/C supply for the filament and 24V for the grid and segments.

I am seeing some interesting artefacts on the active segments and am wondering if anyone else from the group has seen similar?

I have tried different power supplies for the grid and segments, both hard wired and HV5812 driven segments and with/without series resistors on the grid and segments.

Has anyone seen anything similar?

- Richard

[Dekatron42]

Yes, I've seen it on quite a lot of these. I bought some lots of these perhaps 15-20 years ago and when I complained to the different sellers on eBay they said that they had stated that they were factory rejects due to the filaments being to far from the segments and also not equally spaced to cover each others fields linearly - I could never get that confirmed from anyone and I got a refund and got to keep them - I've been waiting for someone to come up with the true reason for this banding for all of these years, and also for someone to publish a design on how to drive them properly!

I have seen some that doesn't seem to have this banding but those photos are from to far so I couldn't really see if there is some banding in those displays.

I was experimenting with higher lower voltages/currents and also tried to "refurbish" them as you can do with the VFD-displays for Hewlett Packard instruments that use them by over-driving the filaments so they start to glow slightly orange but that didn't help either - I've successfully refurbished HP-displays that way so I know that way of refurbishing works.

I tried to have a good look to see if there was anything askew with either the filament threads or the grid  thinking that it could be a manufacturing problem with either that results in this effect but the rounded edges didn't let me have a good look of the insides so I couldn't really confirm anything.

So now I keep my fingers crossed that someone will chime in here and explain it all!

/Martin

[Instrument Resources of America]

Just a guess on my part. Have you tried driving the filament with a nice clean D.C. voltage rather than A.C.  In the early days of radio NON uni-potential cathodes (a bare heater with no separate cathode sleeve) in radio tubes caused issues, which is why cathode sleeves were soon adopted to house the heater and create a uni-potential cathode.

Ira.

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[Tom Katt]

Almost looks like the fluorescent material was unevenly applied or delaminating somehow..  Like thick / thin spots or similar thickness or distance issue.

Does the anomaly align with the cathode heater wires?

[gregebert]

I saw similar on the  smaller ILC-1/8, so I applied rectified AC on the grid and the problem went away. In my case, only the vertical filaments caused dark bands, not the mesh.

I concluded there are some electrostatic patterns that are, well, static when the grid is pure DC. With the grid varying from the rectified AC, the field changes and the phosphor averages it out. The other departure from normal vacuum-tube behavior is that the grid current can be surprisingly high, over 100mA compared to microamps for a vacuum tube, because the grid is biased positively.

If you look at aging VFD's, you can clearly darker bands closer to the filament (phosphor degradation) which I concluded were caused by electron bombardment.

[Richard Scales]

All extremely interesting - perhaps I should not worry about it so much and be grateful that they work at al!

For my project I am using a nice A/C for the filament drive (a custom designed board) then DX for the grid.

The segments are being controlled by an HV5812 VFD driver.

The filament has a 10R resistor in series to help deal with the situation where filaments are started from cold. There is also a suggestion that the grid and segments should be driven via series resistors too. I have not seen this in any other designs but am more than happy to have a go!

So far then hand wired bench testing is working nicely (apart from the patterns I noticed) so I'll be pushing ahead with an 8 digit versions (using two displays as separators/colons) though my design does allow for a 6 digit (or any multiple of 2) version that will also allow the use of an IV-26 as a colon separator (with the ability to control all 7 dots if desired!).

Essentially I have a board that accepts two ILC1-1/7's and has a single HV5812 driver. These boards will daisy chain together as desired. I also came up with a small 'link' board which has an IV-26 (there are 3 types - I propose the use of the one with all 7 dots controllable) That board has to borrow spare 'bit's from the adjacent ILC1-1/7 boards. (The HV5812 has 20 bits output - 14 needed for two displays - 6 spare so the IV-26 board has to borrow some from the left and some from the right - if you see what I mean).

For software I'll be using the same code base that I use on my panaplex clocks so it will have all the usual bells and whistles (PIR control, web gui for settings, date/temp/pressure/humidity displays as well as a definable scrolling message - albeit subject to the limitations of 7 segments!

Anyhow - it's all on the bench and I'm about to push the button on the main tube boards so I can test them more properly.

I'll keep you posted.

- Richard

[Terry Kennedy]

On Sunday, November 23, 2025 at 8:17:36 AM UTC-5 Instrument Resources of America wrote:

Just a guess on my part. Have you tried driving the filament with a nice clean D.C. voltage rather than A.C.  In the early days of radio NON uni-potential cathodes (a bare heater with no separate cathode sleeve) in radio tubes caused issues, which is why cathode sleeves were soon adopted to house the heater and create a uni-potential cathode.

These displays are so large that there's quite a large end-to-end voltage gradient between the two ends of the filament and the segments. You'll end up with a display that's brighter on one end than the other. I know because I did my initial testing with batteries driving both the filament and the segments.

[Instrument Resources of America]

I do understand the DC gradient, but if the pattern changed that would tell you something. Also when you say that you are driving the filament with A/C from "a custom designed board" does that mean that you might be able to change the 'driving frequency'. If so, then you might try to change the driving frequency and see what the result is.

Ira.

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[Richard Scales]

Yes, I can indeed alter the frequency and I am about to test that  - I shall report back asap.

- Richard