Hi all,
has anyone ever seen a VFD (vacuum fluorescent display) failure in which half
the display is very dim and the other half looks good?
I have this 16 character (16 segments) 1 row display and the first 8-10
characters have a normal brightness, the rightmost ones are very dim, barely
readable.
I first suspected a problem with the filament supply, but swapping the
filament pins has no effect, so I'm a bit puzzled.
Waveforms look identical on all the 16 grids too.
Unfortunately I have no schematic of this instrument (digelec 824 eeprom
programmer), nobody else in the world seems to have this programmer also,
so I could not ask anyone for some quick waveform comparison.
I'm starting to suspect a bad VFD but I can't imagine how would it fail
like this.
Part of the filament supply is made with a NE555 that's getting quite hot
and also a 120 ohm 1/4W resistor near it gets too hot to touch, but all
components I could test looks ok. I swapped the 555 and it doesn't make
any change. If I leave the 555 out, the display has no filament supply.
Any hint is really welcome.
Regards

Frank IZ8DWF

On Wed, 7 Sep 2016 07:38:16 +0000 (UTC) frank wrote in
Message id: :

Hi all,
has anyone ever seen a VFD (vacuum fluorescent display) failure in which half
the display is very dim and the other half looks good?
I have this 16 character (16 segments) 1 row display and the first 8-10
characters have a normal brightness, the rightmost ones are very dim, barely
readable.
I first suspected a problem with the filament supply, but swapping the
filament pins has no effect, so I'm a bit puzzled.
Waveforms look identical on all the 16 grids too.
Unfortunately I have no schematic of this instrument (digelec 824 eeprom
programmer), nobody else in the world seems to have this programmer also,
so I could not ask anyone for some quick waveform comparison.
I'm starting to suspect a bad VFD but I can't imagine how would it fail
like this.
Part of the filament supply is made with a NE555 that's getting quite hot
and also a 120 ohm 1/4W resistor near it gets too hot to touch, but all
components I could test looks ok. I swapped the 555 and it doesn't make
any change. If I leave the 555 out, the display has no filament supply.
Any hint is really welcome.
Regards

Just a WAG, but perhaps in normal operation the dim digits were constantly
lit, while the bright ones weren't?

JW wrote:

Just a WAG, but perhaps in normal operation the dim digits were constantly
lit, while the bright ones weren't?

could be, but I doubt it. The brightness starts going down from digit 10 and
it is really progressive, each digit is dimmer than the one on its left.
Things without a schematic are difficult sometimes.
What really puzzled me is the fact that swapping the filament pins didn't
make any difference. I would suspect the driver (Rockwell 10937), but
grid waveforms look all the same.
Frank

On Wed, 7 Sep 2016 12:09:50 +0000 (UTC) frank wrote in
Message id: :

JW wrote:

Just a WAG, but perhaps in normal operation the dim digits were constantly
lit, while the bright ones weren't?

could be, but I doubt it. The brightness starts going down from digit 10 and
it is really progressive, each digit is dimmer than the one on its left.
Things without a schematic are difficult sometimes.
What really puzzled me is the fact that swapping the filament pins didn't
make any difference. I would suspect the driver (Rockwell 10937), but
grid waveforms look all the same.

Interesting. Under a bright light can you see if the phosphor for each
digit incrementally looks darker for the dimmer digits?

I've never seen that fault before, but I have noticed that when VFDs get
dim the phosphor looks darker than a new one.

JW wrote:
On Wed, 7 Sep 2016 12:09:50 +0000 (UTC) frank wrote in
Message id: :


Interesting. Under a bright light can you see if the phosphor for each
digit incrementally looks darker for the dimmer digits?

The phosphor looks identical on all digits, as far as I can tell.

I'm going to try to swap the last and first grid connection, that should
give me some more hints (I hope).

Frank

frank wrote:

I'm going to try to swap the last and first grid connection, that should
give me some more hints (I hope).

swapping the grids leave the bad digits at the same physical place, of
course the displayed characters swap too.
This is another point in favour of bad VFD, which is unobtanium anyway,
it's a futaba 16-SY-03Z.

Frank

On Wed, 7 Sep 2016 13:16:38 +0000 (UTC) frank wrote in
Message id: :

frank wrote:

I'm going to try to swap the last and first grid connection, that should
give me some more hints (I hope).

swapping the grids leave the bad digits at the same physical place, of
course the displayed characters swap too.
This is another point in favour of bad VFD, which is unobtanium anyway,
it's a futaba 16-SY-03Z.

I think you're right. That's a real weird failure mode, though.
Not unobtanium.
http://www.plccenter.com/en-US/Buy/FUTABA/16SY03Z
but ridiculousium.

Any reason you can't move to a newer programmer? You're not programming
2708s or anything?

JW wrote:
http://www.plccenter.com/en-US/Buy/FUTABA/16SY03Z
but ridiculousium.

yeah, the programmer costs much less


Any reason you can't move to a newer programmer? You're not programming
2708s or anything?

Actually I'm programming a lot of old things, including MCM68764, 2716,
most of the MCUs that this old programmer supports...
You would be surprised to discover how few programmers support (correctly)
the 2532s and MCM6876x (or ricoh pin compatible parts) for example.
Anyway... I can live with a magnifying glass for the moment.
Frank

Just a WAG, but perhaps in normal operation the dim digits were constantly
lit, while the bright ones weren't?

could be, but I doubt it. The brightness starts going down from digit 10 and
it is really progressive, each digit is dimmer than the one on its
left.

That would suggest to me that the filament is being driven by DC,
rather than by AC or a square wave.

The filament is the cathode in the tube. The voltage difference
between the cathode, and the fluorescent anode sections, greatly
affects the brightness. If you have a voltage drop across the width
of the filament (with one side grounded) you'll have different
cathode-to-anode voltages across the width of the display, and
different digit brightnesses.

One way to avoid this (the usual way) is to heat the filament with an
AC or square-wave-DC signal, typically symmetrical around ground.
This results in the same average voltage between the anodes, and
different points on the filament, and evens out the brightness. If
you drive the filament with AC from a separate transformer winding,
it's necessary to create some sort of ground return reference for it
(for the cathode-to-anode electron flow). This can be done by
grounding a center tap on the transformer secondary, or grounding a
center point on the filament itself.

The fact that swapping the filament pins didn't affect the problem,
makes me suspect that there's a fault inside the tube. Possibly
there's an internal short between one end of the filament, and ground.

So, take a look at the voltages and waveforms on the filament drive
pins. Ideally, they should be exactly mirror-opposites of one
another, with respect to ground. I suspect you'll find that they are
not.

Dave Platt wrote:

[...]


The fact that swapping the filament pins didn't affect the problem,
makes me suspect that there's a fault inside the tube. Possibly
there's an internal short between one end of the filament, and ground.

I haven't been able to measure any short between filament and the rest of
the internals (grid, anodes). I've also spent some time checking for shorts
between grid-grid, grid-anodes and even anodes-anodes. Since characters
are generally made correctly, I didn't expect any short, but I was out
of ideas.


So, take a look at the voltages and waveforms on the filament drive
pins. Ideally, they should be exactly mirror-opposites of one
another, with respect to ground. I suspect you'll find that they are
not.

That's the first thing I checked actually. The voltages are not symmetrical
respect to the ground, so I swapped the filament pins expecting to have
an opposite behaviour, but, the filament supply didn't change (it did not
follow the pin swapping) and the brightness situation of all the characters
didn't change.
The filament waveform is a square wave with rather low duty cycle (and
this also probably explains why they got away with this non symmetrical supply
in the first place), the DC component is about -11V and it is present on both
filament pins, and the square pulses go to about -21V and are applied on one
side of the filament. It is obtained with a NE555 based circuit, the duty
cycle is probably ok as I measured an RMS value of 6V on the filament and
this particular VFD wants 5.8V nominally.
I can't still exclude a rather obscure (at least to me) problem in the
filament supply, but components are very few and they are all ok.
This instrument is rather large with more than 100 ICs scattered around, so
I'm not going to reverse engineer a schematic out of it in my lifetime.
All the tests in my opinion points to lazy phosphors or low emission from
the filament in the last digits.
If it was the asymmetrical filament supply, the dim digits would follow
the pin swap of the supply.
Maybe the hypothesys that the last digits were the ones most used isn't that
bad after all.
VFD are really nice looking displays, but I must remember to avoid old
instruments with those
Frank

"frank" wrote in message
...
Hi all,
has anyone ever seen a VFD (vacuum fluorescent display) failure in which
half
the display is very dim and the other half looks good?
I have this 16 character (16 segments) 1 row display and the first 8-10
characters have a normal brightness, the rightmost ones are very dim,
barely
readable.
I first suspected a problem with the filament supply, but swapping the
filament pins has no effect, so I'm a bit puzzled.
Waveforms look identical on all the 16 grids too.
Unfortunately I have no schematic of this instrument (digelec 824 eeprom
programmer), nobody else in the world seems to have this programmer also,
so I could not ask anyone for some quick waveform comparison.
I'm starting to suspect a bad VFD but I can't imagine how would it fail
like this.
Part of the filament supply is made with a NE555 that's getting quite hot
and also a 120 ohm 1/4W resistor near it gets too hot to touch, but all
components I could test looks ok. I swapped the 555 and it doesn't make
any change. If I leave the 555 out, the display has no filament supply.
Any hint is really welcome.
Regards

Frank IZ8DWF

You might take a look at this:

http://www.eevblog.com/forum/chat/va...-rejuvenation/

In article , frank
wrote:

Hi all,
has anyone ever seen a VFD (vacuum fluorescent display) failure in which half
the display is very dim and the other half looks good?
I have this 16 character (16 segments) 1 row display and the first 8-10
characters have a normal brightness, the rightmost ones are very dim, barely
readable.
I first suspected a problem with the filament supply, but swapping the
filament pins has no effect, so I'm a bit puzzled.
Waveforms look identical on all the 16 grids too.
Unfortunately I have no schematic of this instrument (digelec 824 eeprom
programmer), nobody else in the world seems to have this programmer also,
so I could not ask anyone for some quick waveform comparison.
I'm starting to suspect a bad VFD but I can't imagine how would it fail
like this.
Part of the filament supply is made with a NE555 that's getting quite hot
and also a 120 ohm 1/4W resistor near it gets too hot to touch, but all
components I could test looks ok. I swapped the 555 and it doesn't make
any change. If I leave the 555 out, the display has no filament supply.
Any hint is really welcome.

If the filament were made up of two parts in parallel, and one of the
parts failed open, could that account for both the dimness and the
overheating 555?

Take the thing into a dark room; often you can see a dim glow from the
filament. Do you see it over all parts of the display, or only over the
"normal" part?

Isaac

isw wrote:
In article , frank
wrote:

If the filament were made up of two parts in parallel, and one of the
parts failed open, could that account for both the dimness and the
overheating 555?

the filament has two wires that run for all the display length. However
I can't tell if these two wires are in parallel or in series electrically.
Supposing that the two wires are in parallel, then since they run
for all the display length, it shouldn't make any difference on only
the rightmost digits. One open wire would also mean higher resistance
so probably the 555 would run cooler too.


Take the thing into a dark room; often you can see a dim glow from the
filament. Do you see it over all parts of the display, or only over the
"normal" part?

I'll try to verify this, just for the sake of it.

Frank

"frank" wrote in message
...
isw wrote:
In article , frank
wrote:

If the filament were made up of two parts in parallel, and one of the
parts failed open, could that account for both the dimness and the
overheating 555?

the filament has two wires that run for all the display length. However
I can't tell if these two wires are in parallel or in series electrically.
Supposing that the two wires are in parallel, then since they run
for all the display length, it shouldn't make any difference on only
the rightmost digits. One open wire would also mean higher resistance
so probably the 555 would run cooler too.


Take the thing into a dark room; often you can see a dim glow from the
filament. Do you see it over all parts of the display, or only over the
"normal" part?

I'll try to verify this, just for the sake of it.

Frank


I dunno - I've seen many VFD's that simply were aged and had similar
problems.

Replacement was the fix. Increasing the filament voltage might help, but I
suspect the dimmer segments still won't be as good.

I once fed about 60 volts to a nominal 5 volt filament before it failed, so
I don't think they are terribly fragile.


Mark Z.

On 9/7/2016 3:38 AM, frank wrote:
Hi all,
has anyone ever seen a VFD (vacuum fluorescent display) failure in which half
the display is very dim and the other half looks good?
I have this 16 character (16 segments) 1 row display and the first 8-10
characters have a normal brightness, the rightmost ones are very dim, barely
readable.
I first suspected a problem with the filament supply, but swapping the
filament pins has no effect, so I'm a bit puzzled.
Waveforms look identical on all the 16 grids too.
Unfortunately I have no schematic of this instrument (digelec 824 eeprom
programmer), nobody else in the world seems to have this programmer also,
so I could not ask anyone for some quick waveform comparison.
I'm starting to suspect a bad VFD but I can't imagine how would it fail
like this.
Part of the filament supply is made with a NE555 that's getting quite hot
and also a 120 ohm 1/4W resistor near it gets too hot to touch, but all
components I could test looks ok. I swapped the 555 and it doesn't make
any change. If I leave the 555 out, the display has no filament supply.
Any hint is really welcome.
Regards

Frank IZ8DWF

Hello all,

I've seen several VFD's that have this issue, usually on the segments
that are rarely turned on, like a 12 digit calculator. One temporary
fix is to type in a ll "8" on the display, to light the greatest number
of segments and let it burn in that way for 24 hours. That usually
fixes the problem for a few months.

Regards,
Tim
Bristol Electronics

P.S. One amplifier manufacturer I deal with and had problems with their
display issues a new software revision that had a "light everything on
the display" function which they instructed you to leave on for 24
hours, and it did work. I think contamination plates rarely used segments.

Tim Schwartz wrote:

Hello all,

I've seen several VFD's that have this issue, usually on the segments
that are rarely turned on, like a 12 digit calculator. One temporary
fix is to type in a ll "8" on the display, to light the greatest number
of segments and let it burn in that way for 24 hours. That usually
fixes the problem for a few months.

Thanks for the informations, sounds interesting. I have to make some
experiments on this display, either using the driver or making a temporary
circuit to light all segments. It's 16 digits (characters really) with
16 segments each.

Frank

"Tim Schwartz" wrote in message
...
On 9/7/2016 3:38 AM, frank wrote:
Hi all,
has anyone ever seen a VFD (vacuum fluorescent display) failure in which
half
the display is very dim and the other half looks good?
I have this 16 character (16 segments) 1 row display and the first 8-10
characters have a normal brightness, the rightmost ones are very dim,
barely
readable.
I first suspected a problem with the filament supply, but swapping the
filament pins has no effect, so I'm a bit puzzled.
Waveforms look identical on all the 16 grids too.
Unfortunately I have no schematic of this instrument (digelec 824 eeprom
programmer), nobody else in the world seems to have this programmer also,
so I could not ask anyone for some quick waveform comparison.
I'm starting to suspect a bad VFD but I can't imagine how would it fail
like this.
Part of the filament supply is made with a NE555 that's getting quite hot
and also a 120 ohm 1/4W resistor near it gets too hot to touch, but all
components I could test looks ok. I swapped the 555 and it doesn't make
any change. If I leave the 555 out, the display has no filament supply.
Any hint is really welcome.
Regards

Frank IZ8DWF

Hello all,

I've seen several VFD's that have this issue, usually on the segments that
are rarely turned on, like a 12 digit calculator. One temporary fix is to
type in a ll "8" on the display, to light the greatest number of segments
and let it burn in that way for 24 hours. That usually fixes the problem
for a few months.

Regards,
Tim
Bristol Electronics

P.S. One amplifier manufacturer I deal with and had problems with their
display issues a new software revision that had a "light everything on the
display" function which they instructed you to leave on for 24 hours, and
it did work. I think contamination plates rarely used segments.




Interesting. I don't think this would help with some dot-matrix type VFD's
like the newer Yamaha's, where a few I've seen will light every other row
unevenly, or like a heavily used Fluke 8840 with each segment lit more in
it's middle and dimmer at each edge, but seeing as how Yamaha has a "light
all" option in their diagnostic menu I plan to keep your post here for
posterity. Might come in handy (or not - I'm retiring soon; don't know how
much I'm going to be doing audio in the future)

:-(

Mark Z.

Is the DC bias on the filament a nice flat line on the scope ?

wrote:
Is the DC bias on the filament a nice flat line on the scope ?

yes.
Looking better, some segments all over the places are dimmer than the
others. Looks like this display is at the end of its useful life.
I can't tell if the dim segments were the most used or most unused ones.
Probably a lot depends on how the previous owner(s) used this thing.
Frank