The VT420 board shown with my finger was not powering on, so we stole D605 off of the circuit to fix the VT520 on display that went up in smoke- that VT520 was heavily soiled internally thanks to a prior possum party atop of it.
The VT420 board that was faulty is being kept for parts or future restoration, as the CRT was shattered in transit at some point.
The Good board from the shattered VT420 was moved onto our other VT420.
Thus, our VT420 is now working as we have taken a good power/control circuit out of a CRT damaged unit.
The faulty board from that VT420 has supplied D605 component to restore our VT520
The VT520 had a full wash with “Safe Wash” and was dried out, and is now in much healthier condition overall.
Here is a damaged DEC VT420 CRT part number along with shattered yoke/neck and implosion of the phosphor coating from inside of the CRT during loss of vacuum.
Here is the damage VT520 board part number along with the damaged D605 component and its replacement taken from D605 on the VT420
You can see just how filthy the board was prior to cleaning
D605 was a BY584 diode.
We identified it via the VT420 manual. As the VT520 flyback and circuit for HV looked similar we did take a gamble in the hope the component was the same, as there is no. Known VT520 schematic- as the components looked the same and were labeled the same, and were effectively the same family but a newer iteration there was little risk involved.
It paid off.