Replace/repair an old vac fluorescent display?

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pneumatic
 
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Replace/repair an old vac fluorescent display?

Post by pneumatic »

Greetings Hardware hackers,

I've come across a beautiful old (c. 1980) TDD machine.
The whole machine, with manual and case
The whole machine, with manual and case
TDD_whole.jpg (46.93 KiB) Viewed 3730 times
It doesn't work; the power light will come on, but no other LEDs or the display will light up, and the modem doesn't make sounds even in origination mode. This is most likely due to the fact that the huge NiCd pack in the back had leaked, and I suspect the vacuum leaked out of the display ;-).
The guts, showing a 32 character display and keyboard
The guts, showing a 32 character display and keyboard
TDD_display.jpg (65.93 KiB) Viewed 3730 times
I've got an idea for a cool project, but I'd need to get a working display. Does anybody know where I can find a replacement vacuum fluorescent character display of approximately the right size? I'm also thinking of a 40x2 LCD, but that won't look "stock".

Thanks
-- Mitch

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zener
 
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Re: Replace/repair an old vac fluorescent display?

Post by zener »

Pneumatic wrote: I suspect the vacuum leaked out of the display
What makes you suspect that? I never heard of that happening.

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pneumatic
 
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Re: Replace/repair an old vac fluorescent display?

Post by pneumatic »

What makes you suspect that? I never heard of that happening.
I had an alarm clock from 1986 that used a VFD. By 2 years ago it was so dim that even on "bright" I could only make out the display in very dim light. I was told it was because the VFD wasn't holding a vacuum properly any more, and therefore it wasn't getting good discharge from the gas inside any longer. I assumed that the same was true for all old VF displays.

I'll see if I can drive the display without the (dead) main logic board and post the results.

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mike31416
 
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Re: Replace/repair an old vac fluorescent display?

Post by mike31416 »

One possibility is: Light output drops over time due to falling emission and reduction of phosphor efficiency

http://en.wikipedia.org/wiki/Vacuum_fluorescent_display

Mike

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pneumatic
 
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Re: Replace/repair an old vac fluorescent display?

Post by pneumatic »

Thanks Mike,

That sounds like a plausible explanation. I wonder if it's worth trying to up the drive voltage. I have found a current feedback amplifier chip. I might be able to figure out the voltage regulation and tweak it.

I'm thinking that it's easier to just get a new display. Does anybody have a recommendation for a good source for vacuum fluorescent display board, or a good character LED display of approximately the same size (8" x 0.75")?

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stinkbutt
 
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Re: Replace/repair an old vac fluorescent display?

Post by stinkbutt »

Before you get a new display confirm the power's ok. There are two cases of "low-hanging-fruit", in my opinion:

1. The VFD's tapped because the phosphor's been depleted. However, this is a consequence of use - The effect should be minimal if the VFD's been sitting inert in a box somewhere for the past 25-30 years.

2. The VFD's not functioning because it's not being driven with sufficient voltage. You've got a 30 year old power supply in there. It could easily have a bad cap in there. Or you could not be driving it at all, since as you say, the NiCd leaked all over the place. I think this is more likely.

Assuming you can power the thing up, you should check the voltages going into the VFD to determine if it's really getting 50-90V, which is what it needs to operate. The amplifier chip's a good place to start, I'd guess. You might also want to look for a terminal on the VFD board labelled VCC or VDD.

Oh, and do so carefully. If you're going to pull the VFD, do it while the thing's been unplugged. For a few minutes. 90V can give you quite a nasty shock. It's unlikely the VFD driver's capable of sourcing any significant amount of current, but who knows? They might've been stupid enough to connect the VFD directly to rectified mains power, for all we know.

pstemari
 
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Re: Replace/repair an old vac fluorescent display?

Post by pstemari »

I've taken apart clock-radios in the past where the "power supply" was just a big power resistor. No transformer, no switcher, just the big resistor.

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pneumatic
 
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Re: Replace/repair an old vac fluorescent display?

Post by pneumatic »

It turns out that what I thought was the current feedback amplifier, was a DC/DC converter, a TI TL497ACN. That is only rated fr up to 30V though, so I'm thinking that there must be something else for the full 50-90 required by the display, (unless they were doing something tricky like feeding the switching output into a transformer, and there is a little metal box on the board I can't identify.)

I'm not sure it's worth saving this display though. The project I have in mind is going to be driving by an arduino or other ATMega chipset, and it'd be nice to work with something that had a library already written for it. If I make this work, I still have to write a library to drive it, and that will be days of reverse engineering and testing. There are some VFD modules that are designed to be plug/pinout compatible with supported LCD displays--they're just a little expensive.

-- Mitch

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pneumatic
 
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Re: Replace/repair an old vac fluorescent display?

Post by pneumatic »

Oh, and the ac adapter for this thing is 12AC, but it charges an internal 12v battery pack (now missing) that actually runs the system. The logic all seems to run off 5V, though it's a mix of TTL and CMOS. I don't know if the 12V is used for anything else, since I haven't traced the circuit yet, and may not bother. The only things I may end up saving are the keyboard and the case (and I wish I could think of a good use for the acoustic modem couplers.)

-- Mitch

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stinkbutt
 
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Re: Replace/repair an old vac fluorescent display?

Post by stinkbutt »

If the input is 12V (AC or DC) then there almost has to be a voltage boost somewhere in the circuitry to drive the VFD. If it's AC then not only is a boost converter a possibility, but a transformer is, as well.

Can you post a hi-res pic of your board? Perhaps we can dope out what's on the board and where.

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