The vacuum fluorescent display on this machine has gone extremely dim. The machine is about 10 years old, but hasn't had a huge amount of use
- perhaps 300 hours or so, so I'm hoping the problem isn't the display itself. (The fine 'heater' wires are glowing dull-red like they always did, and the machine works perfectly.) Does anyone have any experience of a 'dim display' problem like this on this or any similar equipment, or any general experience of working with these displays? I'm especially interested to know what sort of voltage the supply and drive waveforms should be on this kind of display. Also, any ideas where I could find a schematic for this machine (for free or low cost)? Thanks.
Typical supply voltage is around 1.5 to 3v AC for the filament supply (that's the pins at either end of the display which are normally separated from the rest of the pins by one space, and may be just a single pin at either end, or two pins at either end) and -29v (that's *negative* 29v) for the segment drives. This voltage is very typically derived from a capacitively coupled voltage multiplier, using electrolytic caps of around
50 to 100uF. As electrolytic caps are not really designed to have low frequency significantly-sized alternating voltages across them, it is very common for them to dry out or go high ESR in these multiplier circuits, the result being that the -29v supply drops low, and the display goes very dim.
Start with a decent digital multimeter and check that the negative supply is around the value that it should be. If it is, check the filament supply just to make sure that is ok (there used to be a Sony cassette deck which AC coupled the filament supply with electrolytics which went open, but it is much more common for this suply to be derived directly from the power transformer) and if it is, then unfortunately, it will likely be the VFD panel itself at fault. If the supply is low - say -18v or less - then look for the multiplier stage, and either check the caps' ESRs if you have an ESR meter, or check for AC getting through them with a 'scope.
All of this assumes that it uses a linear supply. If it uses a switcher, same voltages apply, but the negative supply will be directly derived from the switching transformer secondary, via its own diode and smoothing cap. In this case, suspect this cap for high ESR. Also with a switcher, the AC supply to the filament may be at switching frequency, in which case your digital multimeter will not read a correct value.
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