[Electronic M]

That thing half cut off at the top center of the pic is probably the boost diode, but unless it is weak/dead it probably will be too high a voltage for your meter....That is why I said to measure it through the screen pot.

If you can't clip directly to the screen pot wiper terminal, then pull the CRT socket, stick a wire about the thickness of the CRT pins into a pin that connects to a white wire with a color stripe and use that as the wiper pin test point for a screen/G2 pot of the color of the wire stripe. Measure voltage to ground with the pot at min, and turn up the pot if the voltage does not rise with the pot setting and get high enough, then the boost system is weak or dead.

Another option would be to measure the B+ boost with reference to the high power transformer derived B+.....Connect the meter negative to the positive of the first lytic fed by the diode bridge you replaced, and connect the positive to the B+ boost line. The boost voltage will be the voltage of the bridge WRT ground + the voltage you measure... The meter may still get more voltage than it can measure this way so the G2 wiper method is preferred.

Do you have a schematic? IIRC I have a Bietman's most often needed Color book with the schematic for the D12 chassis that I could probably scan or take photos of for you.