VFD with CDS maybe?

[hermitengineer]

My microwave has a really dim VFD display.  I decided to take a close look at it to see if there is anything easily fixable.  I know VFDs leak, but this is only a few years old.

Anyway, one thing I noticed on visual inspection is that there is a black spot on the front of the display in the lower right corner.  It is also clouded around the right side.  I noticed that there is a metal ring (or at least a half-ring) which is mounted at a diagonal to the front pane.  It is connected to a wire strip on both sides which goes down to the rest of the internal contacts and there is a third strip that comes up between the other two.  I can't see through the cloudy black part well enough to get more details than that.  But I wonder, is this a light sensor intended to adjust the brightness?  If so, being covered in a black cloud would definitely explain why it keeps the display so dim.

Either way, is that black spot likely to indicate that the display itself is failing?

[BrokenYugo]

That's the "getter", a sort of vacuum pump to catch any errant molecules. You can tell the tube has lost vacuum if the spot turns white.

https://en.m.wikipedia.org/wiki/Getter

[hermitengineer]

Ok, so the black spot probably means it hasn't leaked yet then.

Now, on schematics for VFD-based displays, I see that there are two sets of pins, labeled F1 and F2, that seem to be the filament power?  On this one, F1 and F2 go directly to one of the secondary windings on the step-down transformer.  The voltage between those two connections is 3.5VAC.  Does that sound reasonable?

The other secondary is 24V with a center tap.  There is a 7915 regulator on the board for a -15V supply to something, probably display-related.  One chosen reference, a plug with a black wire, shows voltages between -20 and -15V on various VFD pins.  Not sure if this is a true ground yet though.