"James Sweet" wrote in message
news:tF8Te.14618$QN4.26@trnddc02...

"BoborAnn" wrote in message
...
I have a Sony DTC 700 DAT tape transport that the display has gone blank
on.
This is a blue display that can be turned off entirely or partially or
the
intensity changed
I don't know the technology but it appears to be a gas based display
given
the display unit shows the remains of the glass connections used to
evacuate
it and has an intact getter
I've probed around and it seems odd that the unit is provided a 4-5 VAC
source.
Hoping somebody can provide some guidance on what these displays are
called
and how they work so I can keep this unit out of a landfill
Aside from the display the unit runs fine
Thanks in advance
Bob



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