1 May
2023
1 May
'23
10:23 p.m.
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?