On 2023-May-01, at 2:25 AM, Peter Coghlan via cctalk wrote: > It seems a bit odd that a power supply from someone like DEC of that era would > be designed to depend so critically on the absolute value of a rail used > for startup purposes.
Further to Peter's point above, the 1988 NatSemi databook - which is to say, from the era of this power supply - specs the 7812 output min-max to be 11.4 to 12.6V (+/-5%). Your measured Vstart=12.4V is well within this. Looking at the schematic, nothing stands out where the distinction from 12 or 12.1 would matter. You still haven't reported the IC power pin connections. If the neg-supply pins are supplied by -12 rather than GND, it could explain the odd voltage seen on the E3d +input. There are 3 explicit components in the design which provide -12V at startup. They didn't throw those components in there just to fill up board space and look pretty. Why would you expect the control circuit to be testable for valid startup state when you haven't provided the startup environment?
