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NEC LD8121 VFD Tube pinout
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sergey:
Hello everyone,

It happened so i've got NEC LD8121 tube salvaged from an old piece of gear and would like to try lighting it up.

There're some specs in ebay about this puppies which provides voltages, current and impulse characteristics , but i'm failing to find actual pinout of this tube.

One old russian book about indicators says IV-27 is an analogue for LD8121, but pinout wouldn't match fr sure (IV does have 11/15 pins, LD does have 13pins on each side).

Maybe it happened so someone does have pinout? Or maybe there's a technique to figure it out?

Thanks in advance!

P.S. Attached a photo of this guy just in case someone is interested how it looks like :)
c4757p:
Either probe around, or just look, to figure out which pins go the filament. (The two which have a low resistance - around 50 to 1k ohms - between them.) Apply about 2V to these, then group all the other pins together and apply about 12V to them. You can play with the voltages to get a reasonable brightness (you'll probably end up with 5V on the filament and about 20V on the anodes - be careful with the filament, don't run it glowing hot). Then just start removing pins one at a time from the anode voltage and seeing what shuts off.

Make sure you check that there are only two pins for the filament - you wouldn't want to get one of the tubes with four filament pins and stick 12V up it.
PA0PBZ:

--- Quote from: c4757p on October 09, 2013, 04:51:39 pm ---then group all the other pins together and apply about 12V to them.

--- End quote ---

Aren't the segments multiplexed in some way? I can't see how you control close to 100 segments with only 24 (26-2) connections otherwise. And if they are multiplexed will you get anything by just connecting them all to 12 volt?
Not criticizing your advice, just wondering...
sergey:

--- Quote from: c4757p on October 09, 2013, 04:51:39 pm ---Either probe around, or just look, to figure out which pins go the filament. (The two which have a low resistance - around 50 to 1k ohms - between them.) Apply about 5V to these, then group all the other pins together and apply about 12V to them. You can play with the voltages to get a reasonable brightness (you'll probably end up with 5V on the filament and about 20V on the anodes - be careful with the filament, don't run it glowing hot). Then just start removing pins one at a time from the anode voltage and seeing what shuts off.

Make sure you check that there are only two pins for the filament - you wouldn't want to get one of the tubes with four filament pins and stick 12V up it.

--- End quote ---

Thanks for the quick reply :)

Just to clarify, do i need to use 1/16 duty cycle with 40us PW or for such a quick test 5/12V DC is not gonna to make any damage?
sergey:

--- Quote from: PA0PBZ on October 09, 2013, 05:05:20 pm ---Aren't the segments multiplexed in some way? I can't see how you control close to 100 segments with only 24 (26-2) connections otherwise. And if they are multiplexed will you get anything by just connecting them all to 12 volt?
Not criticizing your advice, just wondering...

--- End quote ---

They are actually multiplexed. Not sure about this articular device, but for IV-27 you've basically got 7 pins for segments and 12 pins for digit selection.

But i guess it's just more fun for figuring out segments/digits and powering all up is just gonna to make all segments to highlight?
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