Here is some info I have saved regarding replacing the CCFL in the 3400
Digi-Key Part Number 289-1056-ND
Manufacturer Part Number BF26250-24B
Description LAMP CCFL 1300V 2.6MM X 250MM
It is slightly larger diameter than the original which is 2.2 mm, not
2.6 mm. It is also slightly shorter at 250 mm where the original is
more like 255 mm, but it worked very well and does not have any dark
corners due to the shorter length.
Note: they are cheap enough that it may be prudent to buy two of them
just in case you break one during installation. I have never had that
misfortune but it is quite delicate work and could happen fairly easily
if you are not slow and careful.