No power to line 6 fbv express. Tried 4 different rj-45 cables. Using brand new spider IV as source.Tried recalibration procedure as recommended by line 6. Suspect main circuit board kaput

If you read down almost to the bottom of this link, you'll find a working link to it.

Thread LIne 6 Spider IV 30 Schematic request

For the best solution, contact line 6 Support. The number is on this page-
http://line6.com/support/tickets

line 6 only. sorry

Google is your friend... searching for the manual we find Line 6's support:

http://line6.com/support/manuals/?families=11&submit=Submit

Choose your unit and download the manual.

You can't download a manual? Why?

Here is site for downloading... find your unit manual and download in whatever language:

http://line6.com/support/manuals/?families=11&submit=Submit

Press toe-switch D

Typical guitar input on a schematic for line 6 shows an inductor between the input jack sleeve connection and the "earth" or safety ground.
I do NOT understand how you could fry one of the voltage regulators via the guitar cable unless you have more than a guitar (other devices) connected to the jack.
ALWAYS power any interconnected audio equipment from the same receptacle or power source to avoid ground bounce/fault damage to equipment.

Read this cut and paste and verify compatibility as a first step:

<<<
*FBV is not compatible with Spider II 15 or Spider III 15.

**FBV is only compatible with POD 2.0 units with pedal jacks labelled "FBV Pedal." It is not compatible with older units which have pedal jacks labelled simply "Pedal."

(NOTE: FBV is NOT compatible with Ax212™, Flextone™ I, II or HD, POD® Pro, Bass POD®, Bass POD® Pro and Spider™.)
>>>

Note there are apparently several versions. Check compatibility at this site:

http://line6.com/footcontrollers/fbv.html

User manuals here:

http://line6.com/support/manuals/?families=45&submit=Submit&1st=1st

You may need to do a reset and a calibration procedure. The info you need is on this page:http://line6.com/community/docs/DOC-4499 If you have the MkII there is also a software update for it.