[TenTec] 422 Centurion blowing fuses...

[Bry Carling]

I have a Tentec 422 Centurion that is blowing the main 20 Amp fuses...

Looking for insights to this amp failure.  It started spontaneously.
I can turn on the main power OK and the filaments light up on the
3-500Zs.  The 12 volt power supply is OK because the pilot lights come on.
However when I flip the standby - operate switch to the Operate positionm
the fuse or fuses will blow.  I replaced them with good 250V 20A cuses.

I checked the plate transformer windings and everything seems normal for
all three windings. Nothing is shorted. The safety ground for the cover-open condition is not
across the HV plate supply. I checked that nothing else is shorting out the plate supply
either. 

Does anyone have any idea what might be going on here?

Many thanks - Brian Carling, AF4K