Jay Honeck wrote:
It's some sort of a solid-state thing, with what looks like maybe a
luminescent strip behind the words/sybols?

You can get night-lights like this; they are very thin and flat and emit
kind of a greenish glow. If you've seen one of these signs illuminated
and it was kind of greenish, than that may be what you have, and it
probably takes AC. Or, it could be surface-mount LEDs on a thin circuit
board; that could take either AC or DC. Are there any polarity markings
(+, -, GND, etc) on it? What color are the lead wires, if any?

There would be no room for any kind of a bulb.

Since the 9 VDC didn't impress it, I'd probably vote for the 28 VAC
400 Hz or 115 VAC 400 Hz options that have been suggested. If it wanted
12 V DC, the 9 V battery should probably have made it light up (if it
was connected the right way around). If it wants 28 V DC, the 9 V
battery might have made _something_ happen, but maybe not... if you've
got some more 9 V batteries, you might try two or three of them in
series. If three in series doesn't impress it, then it's probably AC.

You can test the 28 VAC case first with a transformer with a 120 V
primary and a 24 V secondary. You can buy one at Rat Shock, or look
around at the wall-warts on the gadgets you have until you find one
that has an AC secondary. Most of them are DC, but answering machines
often have an 18 VAC transformer (which is probably close enough for
this test) for some reason. If something between 18 and 24 VAC doesn't
impress it, then it's time to (CAREFULLY) try 120 V AC. If you're brave
you can just stick the wires in the wall socket and see what happens.
Or, wire something like a 4 W incandescent night-light bulb in _series_
with the sign before plugging it in. This will act as a current
limiter; if the incandescent bulb lights up brightly, then unplug the
sign immediately. More likely the bulb will light up dimly or not at
all, but the sign will light up, which is what you want. If the sign
lights up OK with the bulb in series, you can probably dispense with
the bulb and run the sign directly from the wall socket.

If it turns out the sign does take AC, I'm not sure what will happen
long term if you feed it 60 Hz instead of 400 Hz. If it's LEDs inside
there, it probably won't care at all, as the AC is probably getting
rectified to DC right away. If it's an electroluminescent panel,
though, it might care - as I understand it an EL panel is basically a
light-emitting capacitor, and it might be less happy with other than the
design frequency. You could feed it with an "official" DC-to-400 Hz AC
inverter - a quick Google gives http://www.kgselectronics.com/SC3.CAT.html
or http://www.kgselectronics.com/SD3.htm , but just the mating connector
for those boxes is liable to cost you three or four times what you paid
for the sign.

Standard disclaimers apply. Be careful when working with mains voltage.
I don't get money or other considerations from any companies mentioned.

Matt Roberds