I = Current K = Cathode
AKB failure is related directly to the current being sensed off the
picture tube during the three scan lines checked during the vertical
interval.

Usually in a Sony it is due to the picture tube having cathodes that
are so unabalanced in current that the AKB circuit cannot compensate
and provide a balanced gray scale, hence they blank the video.
Basically the AKB circuit and video drive circuits including the
voltages all need checked per the Sony training manual.

Lots of postings here regarding AKB and Sony video blanking. Do a
Google groups search.