Hi,
Most machines sign with florescent lamps will share a common ballast and starter. So if only on sign lamp is blowing it could be the ballast, wiring or possible the lamp socket. For $25 in parts I would replace everything the ballast being the biggest part of that cost. I'm all for new technology when it makes a difference, but with the average life a florescent lamps and their cost it would take a very long time to go through $75 worth of lamps and if they are brighter or dimmer then the florescent lamps it may change the look of the sign. .....Just my 2 cents worth....
Also, if you don't have a manual for you machine you can get one for $14.95 @
http://www.colamachines.com/xcart/home.php?cat=399It's a PDf download I think they are worth it I ordered one for my machine.
Heres a good site if you want to learn more about how florescent light work
http://members.misty.com/don/f-lamp.html#int0Ray