Tek 2440 PSU repair.

[grtyvr]

A couple of weeks ago got a dead Tek 2440.  I am trying to figure out what is wrong with the PSU.  When I try injecting voltage at the diode in front of the primary supply the output follows the voltage.  There is no threshold of 18 volts to trigger the 12v output.  That case does not seem to be covered in the troubleshooting flowchart.  Anyone able to point me in the right direction?