Ok, so further observations of interest. Now that it's been powered up for over 2.5 hours, those leaky extra segments have gone, come back for a few minutes, and are now gone again. But when gone, multiple segments appear "hot" or over bright, particularly the slant segments both top and bottom,
but only while in the menu. At the DC voltage measurement mode, for example, brightness is consistent across all segments including the slants in the V in VDC.
Looks like some thermal variation causing segment lines initially shorted to break contact, and voltage or current to fluctuate. That or the brightness variations are just a characteristic of an aging VFD, but not sure why only when in the menu. Maybe because the menu display is static vs refreshing like in measurement mode? The brightness difference doesn't show as significantly on camera as it does in person, but this pic captures some of it. Notice how bright the slant segments on the R and V are:
hmmm... when one lower diagonal is on, so is the other. looks like it could be a short. do you see ANY characters displayed where one lower diagonal is ON, while the other lower diagonal is OFF?
Nope. When it happens, both lower slanted segments show whenever either one should be displayed. I agree it looks like a short on those segment lines.
i'd be checking the display PCB for the dreaded tin dendrils. these are fine whiskers of tin that can grow between electrical contacts, developing over a period of many years. microscopically thin, but can grow up to half an inch or so long.
That's an interesting notion I hadn't considered. If such a thing occurs over time, it would certainly make sense in a 20 year old instrument. And with such a delicate short, thermal expansion could easily break the short.
I have a sneaking suspicion it's a faulty component or driver IC, but I will look at everything suggested and will give it a good brushing just to cover the bases.
do the short display test
Hold shift and power up the meter and release shift, it will briefly light all the vfd sections ..
Yeah I've done the tests. All VFD segments light up, but that's to be expected -- I'm not having any issues with segments NOT displaying, just those displaying when they shouldn't.