EEVblog #717 - How To Hack Vacuum Fluorescent Displays

[vlad777]

Why does the tungsten cathode have burn marks on one end? (and not the other end)
I am guessing it is metal atoms deposited on glass, but why?

[bundy]

Why does the tungsten cathode have burn marks on one end? (and not the other end)
I am guessing it is metal atoms deposited on glass, but why?

It is caused by the getter: http://en.wikipedia.org/wiki/Getter

[vlad777]

Why does the tungsten cathode have burn marks on one end? (and not the other end)
I am guessing it is metal atoms deposited on glass, but why?

It is caused by the getter: http://en.wikipedia.org/wiki/Getter

Thanks.

Damn, once I broke off a CRT electron gun and until now I wondered what this is:

[max_torque]

Found the datasheet:
http://www.eevblog.com/files/Babcock-VF0240.pdf

^^^ More of an "Infosheet" than a Datasheet really  ;-)

[vlad777]

Can someone help me out with this beauty?
I had it laying around for quite some time. But now I can use pic2550 with USB to control this.
Chip symbol is Rockwell International.
I don't know about that T_scale symbol, anybody?

Is LE latch enable?
What is VCT?

Is VFIL AC few volts RMS?

Many thanks.

(Made in USA, USA, USA, USA, ... )

PS:

P/N is product number. What is A/N?

[bktemp]

Is LE latch enable?
What is VCT?

Is VFIL AC few volts RMS?

VCT is probably center tap (center tap of the tranformer winding for the filament, if you do not have a suitable transformer, a simple voltage divider build from two 100 ohm resistors should do it for the first test). The filament voltage must not be connected to the -27V supply, because it is connected internally probably using a z-diode to float a few volts above the -27V.
Guessing from the size around 4Vrms. As I said on the first page, either measure the filament resistance and then adjust the voltage until the hot resistance is around 4x the cold resistance, or until the filament glows dim.
It could be either a complete controller like the VFD in Dave's video, or just a simple shift register.
LE (latch enable) sounds more like a simple shift register. RST maybe resets the grid counter to the first digit and each LE pulse stores the serial data and switches to the next digit.

[mikeselectricstuff]

Probably data,clock and latch, like a 74HC595. clock in all the data bits then toggle the latch pin.

[KD0CAC John]

I've got a unit that I took out of a commercial convection oven and trying to fix [ if not too much for friend ] or if a lot labor / parts , I found a replacement on Digikey site for $145.49 .
No engineering at this end just fix stuff if I can .
This does not light up at all , It had power to board close 5v , the SMD fuse was OK , the 2 sets of 3 pins at either end of board had just under 3 volts .
I tried to download PDF for some of the chips , but can not seem to get , some times just keep on getting linked to buying , even after clicking on PDF logo - just jumps to sales site ?
Any help ?
Thanks
John

[BoomBrush]

Hi Dave, I liked the video but it reminded me of the more conventional arduino displays, you know the 16 x 2 ones.

Anybody know how you would light up custom sections of a digit? Another way to word it is you dont want a character that exists in ascii, but maybe where you define the custom sections of it. Kinda like making your own font. Would that be possible with a display shown in the video? I would be interested to know.

eg if you wanted a digit where just only the outer sections of the digit were on.

Also Dave: good job explaining how these vacuum tubes work but why is a vacuum necessary? Would it be because it would be picking up the electrons from the air instead of the ones being emitted? So all the segments would always be on... is that correct? Thanks guys!

[bktemp]

Anybody know how you would light up custom sections of a digit? Another way to word it is you dont want a character that exists in ascii, but maybe where you define the custom sections of it. Kinda like making your own font. Would that be possible with a display shown in the video? I would be interested to know.

eg if you wanted a digit where just only the outer sections of the digit were on.

It depends on the instruction set the display supports.
If it only has a fixed character set, it is impossible.
Many VFDs use the non printable ASCII codes 0-31 as commands for changing brightness, setting cursor or changing font. Since most of those displays use a standard microcontroller it is easy to customize the instruction set and supported features by changing the firmware. Most of the VFDs I have seen do not support custom characters.

[JackOfVA]

Hi Dave, I liked the video but it reminded me of the more conventional arduino displays, you know the 16 x 2 ones.

Anybody know how you would light up custom sections of a digit? Another way to word it is you dont want a character that exists in ascii, but maybe where you define the custom sections of it. Kinda like making your own font. Would that be possible with a display shown in the video? I would be interested to know.

eg if you wanted a digit where just only the outer sections of the digit were on.

Also Dave: good job explaining how these vacuum tubes work but why is a vacuum necessary? Would it be because it would be picking up the electrons from the air instead of the ones being emitted? So all the segments would always be on... is that correct? Thanks guys!

If not evacuated, electrons emitted by the cathode would not be able to freely flow from cathode to plate (grid bias permitting) but rather would collide with gas atoms. Some would be ionized creating space charge, also not a good thing. 

Additionally, the hot cathode - in "modern" receiving tubes would likely be poisoned reducing emission. The heating element itself would oxidize and burn.

That being said, a few specialized tubes were designed to operate with specific gases inside the envelope. Voltage regulator tubes, such as OA2, might use a combination of neon and other gasses that would break down at a defined voltage. (The vacuum tube version of a Zener diode.) Some rectifiers were also made with a similar design, OZ4, for example.  And, high power / high voltage rectifiers uses some more exotic designs, such as mercury vapor (tube must heat up before HV applied so that the mercury vaporizes) or Xenon. 

For a high level view of vacuum tube design look at the first section of the RCA tube manual. www.tubebooks.org/tubedata/RC30.pdf  and the transmitting tube manual. w5jgv.com/downloads/RCA-TT-5.PDF

[Macbeth]

Maybe I've lost my puerile sense of humour...

But the cringe is strong with this one 

[vlad777]

@mikeselectricstuff

Hi Mike, I got it displaying some random stuff.
Didn't connect to PC yet, I am using debounced switches.

So here is what I got:
Reset is active low.
LE does  nothing.
Last bit on Data must be one , out of eight Clocks (low to high).

To get it started I have to click in eight ones.
00111111 is F
00110011 is 3
00001111 is clear
After inputting, next char is accessed.
Sometimes on random input random segments lit up within one char.

Does this remind anyone of some protocol ?
Many thanks.

[Len]

00111111 is F
00110011 is 3

Guessing from these two examples: If the first 4 bits are 0011, the next 4 bits are displayed as a hexadecimal digit.

[vlad777]

00111111 is F
00110011 is 3

Guessing from these two examples: If the first 4 bits are 0011, the next 4 bits are displayed as a hexadecimal digit.

Seem so but if last bit isn't 1 , nothing happens.

Edit:

Actually you are correct , must have been my clicking finger  .
0011 seems to be an opcode

[vlad777]

@bktemp

Filament cold resistance is 15 Ohm.
I am driving it with DC. Is that OK or will it damage something, in the long run?
At 5V DC resistance is about 50 Ohm.

[twice11]

@bktemp

Filament cold resistance is 15 Ohm.
I am driving it with DC. Is that OK or will it damage something, in the long run?
At 5V DC resistance is about 50 Ohm.

Typically, the filament is AC driven, so the average voltage between filament/cathode and the grid is the same over the whole length of the filament. With DC drive, you will get a brightness gradient. If the voltage at the negative end of the DC driven filament does not exceed the expected average voltage, you should be definitely save.

[nixfu]

Dave, two words:

Bus Pirate

[nitro2k01]

Dave, two words:

Bus Pirate

Bus Pirate is great for playing with serial protocols but not parallel ones. Adweeeno was a good choice for this quick hack.

[vlad777]

What kind of display would have five-two-seven groups of digits?
Any idea what was this displaying?

[blue-v]

Thank's for the video, Dave!

Speaking of reverse engineering these displays:
I'm struggeling for quite a while with an VFD in an HP Designjet 650C plotter.
First time the heater windings in the transformer were broken. I fixed that by adding a new winding to the transformer.
That worked a while.
Now the display went completely dark. One of the OKI C1162 had a short. Replaced that one but the display is still off.
Unfortunately you need the display for operating the plotter. And vice versa: The plotter needs the display too. Removing
it completely, the plotter hangs on startup.
I found out the display has a serial bus with two handshake pins. 16 bit are shifted in consisting of 8 data bits and 8 control bits.
Also status information is read out of the display. This explains, why the plotter hangs without the display.
Unfortunately I could not find any detailed information.

I really would like to replace that thing with a controller and HD44780 LCD. But I have not enough information about the display to build a replacement.
Does someone have the specs of that thing?
Or maybe an idea what might be wrong?
High voltage, heating and handshake to the interface are fine. But no data is shifted out by the controller to the tube.

Thank's!
Lothar

Here's some photos:

[bktemp]

Maybe the same happened as here:

The driver failed and shorted the high voltage and 5V together and this damaged the microcontroller.

[richfiles]

I picked up a pair of IEE Model S03601-40-040 VFDs from surplus. They are 2x20 character VFDs with HUGE 11mm characters! They were made by IEE, and feature an 8-bit parallel interface very similar to the VFD Dave has.

I contacted IEE, and unlike the data sheet Dave has, mine is VERY detailed... It's 17 pages long, with a complete listing of all the characters and sets, all the control codes, timing, modes, and everything.

[aargee]

I've got one of these that I'm thinking of putting to use some how. All the VFDs are run by OKI shift registers, as are all the push buttons.

A bit of work involved...

[blue-v]

bktemp,

yes that might be the case.
The OKI C1162 I replaced, shorted the high voltage to nearly zero.
But with the chip still in place I measured the other voltages and they were just fine.
The inverter got a little warm (not hot) and also the defective OKI had some temperature.

After replacing the OKI all the voltages are within expected ranges.
And the 8052 controller seems to work, because the plotter recognizes the display and starts up fine.
It's just, the controller doesen't output anything to the display side.
Of course I also would replace the 8052, but at the moment I have noreplacement handy.
I think I'll desolder the thing and try to read out the ROM contents. Hope that is still possible.
Atmel produced a AT89C52, which is compatible, but comes with a flash on chip. This will replace the 8052 just fine.

The thing is, I'm fed up with this non-reliable VFD board and would really like to replace it with a common LCD.
It's the second time now the thing breaks!
Also it was already very pale before the OKI chip died.

Is there a way to refresh faded VFDs, btw?

Lothar

D'oh!
I just read the 8052 has a security bit that prevents readout of the ROM if set, which it most likely is.