Thanks for the image - tbh I'm not comfortable in using that to try to diagnose further; I've just reviewed my images of a strip down of a 2DM860H repaired (details on this forum) and I've a DM860T stripped down in front of me. Both have SMDs on front and rear of board - the 2DM having lovely silk-screen printed ADC monitor points, the 860T is rather closer to the EM806 board in being rather non-descript.

If you look at the board it's a fairly conventional, common design. The HV stuff is on the left, largely isolated from the LV stuff on the right (the break point being around the 8-pin devices U9/U10/U11/(U12?) - centre of board. What you're looking for is a signal line that provides a HV feed from the left to the right and then into a resistor cluster. The problem is the resistor cluster may be on the rear of the board (it looks to be on the 2DM...)

Something like

Click image for larger version.  Name: Screenshot 2020-07-04 at 09.25.57.png  Views: 1271  Size: 1.84 MB  ID: 28514

but again - pure speculation. The image isn't sufficiently detailed to sensibly diagnose from a distance, and I do think there's stuff on the rear of the board that can come into play here. I was hoping to find a couple of resistors that looked shifty enough to probe to find your 73V, your 0V, and somewhere as a ratio (based on resistor values) between this as a feed into the ADC.

I'd say try to drop your terminal voltage, but as Neale said - the device should work to 80V (I've read more of the manual and I concur with his statement - though the manual also injects a useless 68V to confuse even more). But, beyond that and a truly high res image of both sides of the board I think trying to diagnose remotely isn't going to be very forthcoming. Previous experience is you'll need to be flipping the board frequently to trace potential signals through board vias... and I'm guessing the forum is processing the images - there's a lot of compression noise when you blow them up.