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Old 12th January 2009, 09:36 PM   #1
Salar is offline Salar  Germany
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Default Replacing a VFD Display with LEDīs - possible?

Can I replace this VFD Display- circuit

Click the image to open in full size.
with LEDīs.?
The image of the circuit schematics is pretty large, so you have to zoom out a lot.

It is from 20 years old Nak CD-Player and the flourescent segments of the VFD start fading.
So I guess, replacing the whole VFD with an array of longer lasting LEDs would not be a bad idea.
The LED array would be hidden behind printed foil, digits and letters shining through - I even took the fonts from an actual Nakamichi pdf-brochure, they did not change their designin 20 years and they fit perfectly to the original silk printed writings on the front panel.

But I do not know how do deal with the circuit:

The IC driving the VFD is externally fed with -25V (the function table of the IC says -22V), so I guess I could change that to +3.6V for driving blue LEDs, consisting of 7 segment digit displays and and single diodes for the commands like "Disc" "Time" "Remaining" etc...

As far as I understood the principle of a VFD, two layers of anodes (the lower layer with posphoric paint, the upper a metal grid) and a cathode (heating wire) are needed to emit light;
If both Anodes get the same negative current, no light is emitted. If the lower anode gets a current only, light is emitted. This combination of switching ON/ON, ON/OFF in a fast time cycle displays the digits. Correct?
If this is correct how do I "translate" the interaction of both anodes to switching LEDīs? Could anybody help me with this?
All the best,
Salar

(BTW, there is a remark in the schematic in german: "Wechselstrom für Heizdraht" means AC-power for the heating wires - the cathode)
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Old 17th January 2009, 04:22 PM   #2
Colt45 is offline Colt45  Serbia
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The screen isn't an anode as such, it's like a control grid in a triode.
if both the grid and the phosphor segment (anode) are high, it glows, otherwise it doesn't... It's the same Idea as the multiplexed 4digit 7segment LED displays.. except grid (common cathode/anode) is inverted from the segments there.


So.. we need to find the voltage being delievered to the anode segments (a-h) and the grids (G1-7). I'm assuming something like -22v when off, and +5v when on, but you should really measure... If it is indeed 5V when on, then it should be cake.. clamp the signal to 0-5V, buffer it, send it to LED segments... invert the grid signal, send it to the common element of the 7seg.

then the remaining disc time etc are probably on 'h'. guess you'll have to find out though.
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Old 17th January 2009, 05:35 PM   #3
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Yeah, on the face of it this shouldn't be too big a problem, it's just a logic-type display, just identify a set of signals and levels that correspond to illuminated segments. You can just do this physically with a meter or scope.

Unfortunately you can't just connect 3.6V to the IC where -25V is currently connected.

Although this is fairly simple question of level-translation, it's in the implementation that there may be problems.

Messing about with this 20-year-old player is a risk, it's not straightforward to explain without the complication of trying to cover every contingency with the benefit of only a fragment of the schematic. Perhaps it might be better to tolerate its fading display or retire it.

w
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Old 17th January 2009, 06:08 PM   #4
Salar is offline Salar  Germany
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Hello Colt, hello wakibaki.
Thanks a lot for your proposals.
There is no risk in messing around, Iīhabe two of the players,
one serves for providing repair parts. I also still have unused lasers, belts and motors on the shelf and the player still looks like it was bought yesterday, so rejuvenating the display would be nice.
My players are identical, except the display, because one player offers more functions (program play, Remote Control), while the player for repais parts has only basic functions and a different display. Even though the rest is absolutely identical (you just have to add some connections and IR-sensor to get full functionalety. I guess, they build two versions of the display, because nobody should notice the extra function digits in bright sunlight. They charged the customers around 500$ for extra functions which were already there.
Thatīs why I only have one Display...

But still, if something goes wrong, repair will not be a big problem.
The -22V are taken from an external Voltage source and dispatched bei the IC, which controls the Display, so there is not a big problen in changing this Voltage.
All the best
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Old 18th January 2009, 11:03 AM   #5
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Post the rest of the schematic and some good sharp pictures (preferably not flash) of the assembly showing how it comes apart.

Have you identified replacement LED displays of suitable size?

Do you understand the basics of how a multiplexed display works?

w
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Old 12th February 2009, 07:51 AM   #6
Salar is offline Salar  Germany
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Hello wakibaki,
yes, I understand the principle of multiplexed displays, but I am not a technician.
So coming up with a schematic and proposals, which parts to choose would help me a lot.
What I already got, mechanically, is a black sheet or mask, which has to be silk-printed on acrylic glass and will be clear, where the LEDs will shine through.

This setup will replace the original VFD glass. There is still enough room for additional drivers and transistors. There is also already a strong +5V power supply in the player, because I did put some LEDīs in the tray some years ago.

From the look the LED display will not differ from the original VFD Display
I want a blue display, but for a reasonable price, only green 8mm 7-digits LED are available. I did already start to cut up the green digits and glue blue smd
LEDs behind the segments. This is cheaper and I am free to choose a common anode or commos cathode later. The letterings will be also blue smd LED, glued behind small sheets silk acrylic glass to diffuse the light.

Again, I am not a technician (but, as one example managed to provide this player with a digital output with much help from this forum), but I guess, it would be safer
to use the IC switching voltages for an independ circuit working on itīs own power supply? Because, I guess, the LEDīs will draw much more power than the original grids and anodes?

I did not have the time to evaluate yet, which letter on the schematic stands for which segments and for which grid. But I guess, this is not so much needed in this moment, because the principle of translating the VFD voltages to LED will stay the same?
All the best,
Salar
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