VFD (Vacuum Fluorescent Display) doesn't display anything

[synth-dude]

Hi,

I have an amp which works fine sound-wise, but has no display.  The VFD is blank.  Doesn't matter what buttons are pushed, I also reset the processor, still nothing.  Some suggested it might be in “audio-direct” mode, but its not.

Looking at the Vacuum Fluro Display itself it seems to have everything it needs, it has 6VAC to the cathode (the thin wires running horizontally) and they glow slightly when its powered on, and the processor is sending signals to the other pins for the characters (which is consistent with proper operation).  So I have verified the VFD is getting power and the display processor is functioning.  Yet still the VFD doesn’t display anything.  I thought maybe its not got vacuum, but it has as the black dot in the corner hasn’t turned white (the black dot turns white when the vacuum is broken).

I tested all the components in circuit from the transformer to the VFD, they test fine.  Is a simple circuit (maybe 3 resistors and 2 caps).

The only thing odd is the circuit diagram says the VFD cathodes should be -24V, but I’m measuring it at 6VAC.  But nothing in the circuit appears to be faulty.  The transformer puts out 6 VAC (to power the circuit).

Only thing I can think of is the transformer is putting out 6V, when it should be 24v.  But this seems very unlikely.  It would either short circuit (and then go open) or just go open circuit.

I’ll post below the circuit diagram (it's quite simple) for the cathode power.  Has me perplexed why the display won’t start.

Any help or ideas would greatly appreciated - I'm completely stuck where to go next.

[synth-dude]

One further thing, I did the "zapping" of the VFD with a higher voltage, which is a way to successfully bring back to life dim or none-working VFD's, it made the cathodes glow slightly brighter than before, but still the display wont show anything.

[Gyro]

Is sounds as if you're missing the -24V DC cathode (filament) bias. This is separate from the 6V AC heating supply. Looking at the bit of the circuit you highlighted, it looks like some kind of voltage doubler. I would suspect either the 1uF 50V capacitors or the diodes. It's hard to tell without being able to trace the whole circuit.

I recommend that you don't do any more 'zapping' until you have the -ve bias correct, you'll probably do more harm than good.

EDIT: You need to find out where that -26.3V DC signal comes from... and why it isn't!

[stj]

replace the 1uf electrolytics - they form a booster probably.

[synth-dude]

OK - success!

I replaced the 1uF caps but that didn't fix the problem.  So I traced the -26v rail to the power supply, I found no -26v rail at all.  I replaced all the caps there, but still no joy.  Everything tested ok, as in tested very well.

So I put it on the CRO to see the actual waveform of what was coming from the transformer, turns out one side of the centre tap output was dead.  So looking at the transformer it turned out to be an open circuit resistor on the circuit board soldered to the transformer.  Replaced and everything works now.

Amazing, a 5c component. 

Thanks for everyone's help.