The problem is the output of the PBGA seem to be producing a 1HZ pulse
>which is expected when the 28V is on but not expected when the 28V is
>turned off. The 1HZ pulse has a amplitude of 2V when the 28V is off
>but when the 28V is on, the amplitude is 5V.
>I have tested the power supply stand-alone and it works perfectly.
>When the 28V is off, the PBGA power should be zero but in this case,
>the voltage is 1.4V when connected to the PCB.
What's the voltage on your supply rails/planes on the side that you expect to be off?
My guess is that you have some signals going from the expect-to-be-on section to the expect-to-be-off section, and the protection diodes are kicking in and connecting the energy from the signals to the power rail and there is enough power getting through for the logic to start to work.
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