No probs, glad it was a help. I've often wondered the same thing as to why
go to the trouble of putting a winding on a custom tranny, and only making
it half the voltage you need ... Such is the esoteric world of commercial
design ...

Arfa

"BoborAnn" wrote in message
...
Got it fixed thanks to your advice
There was no -24V which was generated from a voltage doubler.Seemed odd
that they would put a winding in just for this voltage and then double it
All the caps and diodes were fine but the 4.7ohm 1/4 watt limiting
resistor between the transformer and the diodes was open
I put in a 5 ohm 1/2 watt unit and its running fine 24V source is at 23.5
Thanks again
Bob

"Arfa Daily" wrote in message
...

"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