mini h-bridge for driving VFD filament very warm to the touch

[Zero999]

I've had another idea. Since the frequency isn't important. It's possible to use an astable multivibrator to drive it. Two additional transistors can be added to the typical astable circuit to form a self oscillating h-bridge.
http://en.wikipedia.org/wiki/Multivibrator#Operation

Hi! Dear Sir, In the schematics of the cassette deck that I am currently restoring (TEAC V-770), I saw a very similar solution.
Unfortunately, I am not well in analog circuitry.
Can you please help me to understand, how can I slightly increase the voltage at the output of this generator.


https://ibb.co/x1rXDND

Thank you!

That circuit works on a different principle to mine. U16 is the oscillator and Q7 to Q9 from an H-bridge which drives the VFD filament. In my circuit the transistors also form the oscillator.

Why do you want to increase the voltage?

R80 and R85 are effectively in series with the VFD and drop some voltage. Reducing their resistance should increase the output voltage, with the VFD connected.

[Nifty]

That circuit works on a different principle to mine. U16 is the oscillator and Q7 to Q9 from an H-bridge which drives the VFD filament. In my circuit the transistors also form the oscillator.

Why do you want to increase the voltage?

R80 and R85 are effectively in series with the VFD and drop some voltage. Reducing their resistance should increase the output voltage, with the VFD connected.

Thank you! I want to try to increase the VFD brightness a little. It's dim, still usable, but the extra brightness will definitely add to the comfort.

[T3sl4co1l]

Possibly the 47's can be reduced a little, or a turn or two removed from the primary winding (that drives a transformer, right?), but that most likely overdrives the heater, leading to even shorter life.  VFD fade is usually due to phosphor wear, unavoidable.  (Come to think of it, I wonder what temperature it could be annealed at. Probably nothing you could do without an inert-atmosphere oven though, or melting the glass envelope for that matter.)  If it's due to fading cathode emission, raising cathode temperature will work momentarily.  Increased plate voltage may not be possible, due to other limits (driver voltage rating, grid cutoff voltage shift).

Tim

[floobydust]

Dim display I find it can be oxide contamination on the filament giving low emission, so a bake off say careful slow build up to max 2x V_FIL for ~20 seconds helps a lot.
Second, I recently found the control grid is really a screen grid and something builds up there. It makes segments/icons appear dim and washed out at the edges. I did a burn off there of something that went away after a few seconds and it never came back. I have notes on it somewhere.
I find phosphor wear is not common unless they drive the tube very hard, high voltage like 40-60V. The giveaway with phosphor is not all digits/segments are dim - some (rarely used) will light up bright like new, so you get a display with bright and dim.

edit: I would check the +20V and +30V rails, see if the electrolytic capacitors are OK or there's hum/ripple there, as well.

[schmitt trigger]

Before the current Russia-Ukraine war, I was able to procure some Soviet era VFDs.

I went through the same iterations as the OP has, but settled with a fully integrated H bridge, the LB1638MC from Onsemi.

[Zero999]

A bit if background:

Nifty, who resurrected this thread, originally sent me a private message, asking me about this and referred to a circuit I designed. Since I generally don't answer questions via private message and I had no idea which circuit he was taking about (it turns out it was nearly 10 years ago!), I politely asked him to resurrect the thread in question.

Now I know it's different to my circuit, but there was no way of knowing before.

That circuit works on a different principle to mine. U16 is the oscillator and Q7 to Q9 from an H-bridge which drives the VFD filament. In my circuit the transistors also form the oscillator.

Why do you want to increase the voltage?

R80 and R85 are effectively in series with the VFD and drop some voltage. Reducing their resistance should increase the output voltage, with the VFD connected.

Thank you! I want to try to increase the VFD brightness a little. It's dim, still usable, but the extra brightness will definitely add to the comfort.

How old is the VFD? Sorry, I suspect it's just old age and will eventually need to be replaced.

[Nifty]

Possibly the 47's can be reduced a little, or a turn or two removed from the primary winding (that drives a transformer, right?), but that most likely overdrives the heater, leading to even shorter life.  VFD fade is usually due to phosphor wear, unavoidable.  (Come to think of it, I wonder what temperature it could be annealed at. Probably nothing you could do without an inert-atmosphere oven though, or melting the glass envelope for that matter.)  If it's due to fading cathode emission, raising cathode temperature will work momentarily.  Increased plate voltage may not be possible, due to other limits (driver voltage rating, grid cutoff voltage shift).
Tim

No, there is no transformer, the outputs in the schematic diagram are directly connected to the VFD.
I measured the voltage, it's about 5V RMS, 400Hz.

[Nifty]

Dim display I find it can be oxide contamination on the filament giving low emission, so a bake off say careful slow build up to max 2x V_FIL for ~20 seconds helps a lot.
Second, I recently found the control grid is really a screen grid and something builds up there. It makes segments/icons appear dim and washed out at the edges. I did a burn off there of something that went away after a few seconds and it never came back. I have notes on it somewhere.
I find phosphor wear is not common unless they drive the tube very hard, high voltage like 40-60V. The giveaway with phosphor is not all digits/segments are dim - some (rarely used) will light up bright like new, so you get a display with bright and dim.
I would check the +20V and +30V rails, see if the electrolytic capacitors are OK or there's hum/ripple there, as well.

Thanks for the info. I'm still afraid to experiment with VFD, because it still works and I have no replacement. But today I found an inexpensive donor of spare parts, if the display there is in better condition, I’ll try to revive this one.
Voltages seem ok. All capactors is "Nippon Chemicon" branded, and according to the RLC meter, comply with the specs. The voltage on the VFD filament is approx. 5V RMS 400Hz.

[Nifty]

How old is the VFD? Sorry, I suspect it's just old age and will eventually need to be replaced.

I believe this particular device is from 1988.