While I've not worked on that model of Sony, some later Sony VCRs had a 1uF cap on the VFD display driver board that would open, causing no display. The 1uF cap was a low-ESR type, available only from Sony. The "fix" was to use a common 3.3uF cap of the same voltage (IIRC, 160V or so...) to get the same low-ESR effect.
The cap was the filter for the switching "boost" converter supply for the VFD. The output of the supply was AC in the 30-80KHz range.
Some VFD's use a variable duty cycle to vary the display brightness - you may have one similar - you'd have to scope the filaments to see if you have a small duty cycle (narroww pulses) - the microprocessor typically controlled that. In operation (tape playing/REW/FF/loading), the display is brighter, and when idle, the display is dimmer - all done by varying the duty cycle of the filaments.
EDit: The third possibility is that, yes, your VFD is tired/worn out. I've seen this in car clocks, and digital scales used by trucking firms.
Most of those Karaoke VCRs went for 600 bucks or more in the 90s.
__________________
Brian
USN RET (Avionics / Cal)
CET- Consumer Repair and Avionics ('88)
"Capacitor Cosmetologist since '79"
When fuses go to work, they quit!
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