Help with Display problem

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It's a VFD, vacuum fluorescent and works on a similar principal as a CRT. The AC voltage is for the heated cathode wires, there should be several stretching horizontally which may glow a dull red if you turn the lights out. IIRC you need about 30v on the anodes (display segments) to light them and there's usually also a control grid in between sections of segments and the cathode wires.

All agreed except one thing I would say is that the display segments are invariably driven by a negative voltage, so you will be looking for drives at -30v. It is usually this supply that's missing. Check for it by measuring from ground to one or other of the filament connections which you have already discovered at either end of the display. Although the filament overall has around 2 - 5 v AC across it, it is floated -30v below ground so that there is not a huge potential diference between the filament wires and the display drives.

If this supply is missing, go looking for a voltage multiplier in the power supply, as this is the way that this supply is almost always derived. It will comprise 3 or 4 small diodes in close proximity to 3 or 4 small electrolytic caps. These are often around 47uF at 50v working. The input capacitor to the multiplier is particularly stressed by having large amounts of AC across it all the time, and has a tendency to fail open circuit. If you have a 'scope, it's easy to find the offending cap. It will have a dirty great AC waveform at line frequency at one side, and nothing, or very little at the other.

Hope this helps

Arfa

Got it fixed thanks to your advice There was no -24V which was generated from a voltage doubler.Seemed odd that they would put a winding in just for this voltage and then double it All the caps and diodes were fine but the 4.7ohm 1/4 watt limiting resistor between the transformer and the diodes was open I put in a 5 ohm 1/2 watt unit and its running fine 24V source is at 23.5 Thanks again Bob

No probs, glad it was a help. I've often wondered the same thing as to why go to the trouble of putting a winding on a custom tranny, and only making it half the voltage you need ... Such is the esoteric world of commercial design ...

Arfa