[TenTec] Centurion 422 Fuses blowing

[Bry Carling]

I havea nice Tentec Centurion 422 and the main 20A fuses blowing.

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-ofrf 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