[Elecraft] NB problem...one for the experts?

[Ron ZL1TW]

Hi all,
          I have a wee problem with the noise blanker in my K2.  It had 
been working fine, and it hasn't been taken out of it's mounting for nearly 
2 years now, but here is the problem.
If I press NB I get the message "not inst"  or alternately, "instnot". :-(
If I disconnect the power lead to the PA and plug the supply into the old 
supply jack I used pre-KPA100 days, the Noise blanker works fine again . :-)
Swap back to the supply via the KPA100 and it says "not inst" again. :-(
The rest of the set appears to be working just fine whether the noise 
blanker is accepted or not.
At the moment my tired brain cell can't wrap itself around that 
one......any ideas?
Cheers......Ron ZL1TW