Vacuum fluorescent display rejuvenation...SUCCESS

[Signal Simon]

I have a home cinema amplifier, Sony STR-DB985D with a very tired display.
It's a unit I acquired for repair and to use as it seems to be the only model I could find with integrated DAB tuner.

This doesn't seem to be just a case of worn phosphors, as the rarely used segments are also very dim, just a little less so.
The heaters do seem very bright compared to other VFD displays I have seen. I have not measured the voltage as yet.

I suspected a low negative rail, but it is at -18.7V as per manual (although labelled -28V).
I am going to try and source a replacement display, the same part number seems to be used in a number of models.

Could the display have been damaged by the filaments being over-run from new (production issue)?

Simon.

[james_s]

It's possible, or the filaments may be contaminated, you could try heating them up to a yellow glow for a few seconds and then return to the rated voltage and see if it makes any improvement.

[Signal Simon]

Cheers, I will give it a go.
I did get another Sony AV amp, but the display was different.
In that case, there was no display because the negative supply was missing.
The negative voltage was -33V (again as per manual) once restored.
The display on that unit is good as new and there is no visible glow from the cathode.
Interestingly, the driver IC is the same, albeit one is NEC uPD16315 and the other is Princeton PT6315

I've attached a photo of the display on the DB895D, you can see how bright the cathode wires are glowing.
The display looks readable on the photo, but when you consider the relative visibility of the unlit segments, it isn't bright at all.

It will be another week or so before I have another go.

Simon.
 

[Jamieson]

Just a quick thanks for all who contributed to this useful thread!

I used this technique to bring back the VFD on a Sony Mini Disc deck (JE500, circa 1996). In my case the filaments started to glow dim orange around 200mA. So i ramped my bench supply from 0 to 250mA, held it there for a few seconds. Repeated a couple of times. Now the VFD is uniformly bright, looks like new, whereas before it had splotchy dim areas and dim horizontal streaks.

Edit: I should add that I first checked the negative rail voltage going to the VFD driver chip and it was bang on at -32VDC per the service manual.

[elecdonia]

I have a CD player with a vacuum fluorescent display where 2 digits intermittently get dimmer. This issue is with one digit or with the other digit. They don't both get dimmer at the same time.

All external connections have been checked and resoldered. The MCU which drives the display works perfectly. All of the digits and icons in the display are nice and bright except for the 2 which occasionally get dimmer. Note: This display has 12 "anodes" and 6 "grids."

Lightly tapping on the VFD glass itself causes the problem to appear or disappear. When I am tapping it I can sometimes see something vibrating or moving slightly inside the display itself. I suspect one of the internal filament wires might not have sufficient tension. Perhaps it occasionally touches one of the grids or anodes, thus causing this fault? Or perhaps there is a tiny bit of foreign material loose inside the display which is electrically conductive?

If I tap and shake the display for a while I can usually get it into a state where all of it works.

I'm wondering if there is any chance of being able to apply just enough volts and mA to the affected filament and grid in a manner which might "burn out" the intermittent short circuit?

-E

[james_s]

I suspect it isn't the filament at all but a connection to one of the grids, or perhaps the grid is shorting to something else. If you look closely with a magnifier while tapping on the display you might be able to see what's loose, but it's unlikely that you'll be able to fix it. If it's a short to an adjacent grid you *might* be able to blow out the short, I fixed a nixie tube one time that had been dropped by doing that, but there is a risk of blowing out the wrong part and making things worse.

[floobydust]

Always first check for fractured solder joints at the VFD's pins. Being made of Kovar, they don't solder well. It's the number one problem with intermittents and VFD's. Look at the driver IC's pins as well.

Tapping will naturally make everything vibrate. If a filament wire breaks or is loose it will short to a grid and light up like a light bulb. If a grid shorts, it will smoke the driver IC. It's not like things are moving around in there without consequences.

Number two for causing (entire) dim displays is bad electrolytic capacitors.
Number three for displays cutting out is bad filament soldering connections, these are the pins at each end of the tube.
Number four for dim digits and segments is phosphor burns from aging, this you cannot fix at all.
Number five is contaminated filament wires due to aging, the entire display will seem dim. This thread was about "burning off the oxides" by slight overvoltaging of the filament for a short time.

[stojke]

Hi, please excuse me, but in the following example, where would you apply higher voltage that is on which pins?:

[EDIT]
And what makes the display brighter or dimmer? -VKK (-30V DC) ? Or is it the AC power (3.5V)?

[floobydust]

The filament pins are at each of the far ends of the tube, where AC power is fed in.
What I find works is:
1. Disconnect one end of the filament (to stop damage of the transformer or surrounding circuitry etc.) either unplug a connector or lift the tube's filament pins out of the PC board.
2. Gently over-voltage the tube filament using a DC power supply at least 300mA. The DC power supply should be floating/isolated from everything else and with a series resistor around 5-10ohms 5W. Start at low voltage and slowly ramp up over many seconds, to at most around 2X the normal voltage or so at the tube, and hold for 30 seconds with a cozy orange glow.
Don't make it light up white like a light bulb! Don't bump/bang it around when glowing orange hot!
Don't nail the tube with filament over-voltage when it's cold, the in-rush current will blow the filaments. You can't just connect a DC power supply to it at 2x voltage, ramp it up and use the resistor.
3. Disconnect power and Let it cool down for a minute afterwards.

I do another rejuvenation stage involving the grids, but it's best if the tube is out of the board/circuit for that.