"Chris F." wrote in message
...
The unit has sound and seems to be functioning, but the display is
totally
dead. The filaments in the VFD look OK and are getting about 6V across
them
(not checked for continuity though). The power supply seems OK, but
without
a service manual there isn't much testing I can do. Are these displays
known
for going bad? If not, does anyone know where I can find a service manual
for this?
Thanks.



6v sounds quite a lot to be across the heater. It is usually in the region
of 1.5 to 3v AC. Most common reason for failure of VFD to light, is the main
negative supply being missing. This is typically -32v, but can be anywhere
from -20 to -40v. The heater supply is usually floated on this negative
supply, so you can measure if it's there or not, by clipping your meter +ve
probe to chassis, then measuring on the VFD heater pins with the meter -ve
probe. You can check on the heater pins at whichever end of the display is
easiest to get to - it doesn't matter, as both ends should be at about -32v,
with respect to chassis.

Assuming that the supply is missing, the cause will be an open circuit
electrolytic capacitor. The supply is usually derived from an AC coupled
voltage multiplier, fed straight from one of the windings on the power
transformer. Most commonly, it is the input capacitor which fails. Value is
usually around 47uF at 50v working. You will be looking for a small cluster
of caps ( 3 or 4 ) with a few small diodes scattered around them, in the
power supply area. The caps will often be very " tired " looking.

If you have a 'scope to hand, this is often the easiest way to locate the
caps. Find some likely looking candidates, then check on either end of them
with the 'scope. Sooner or later, you will come across one which has a dirty
great AC waveform on one side, and virtually nothing on the other.

Good Luck

Arfa