5 Jan
2026
5 Jan
'26
12:54 p.m.
-----Original Message-----
From: Brent Hilpert via cctalk <cctalk(a)classiccmp.org>
Sent: 05 January 2026 19:54
To: General Discussion: On-Topic and Off-Topic Posts
<cctalk(a)classiccmp.org>
Cc: Brent Hilpert <bhilpert(a)shaw.ca>
Subject: [cctalk] Re: Identifying This Component from VT100 Monitor Board
- Were those measurements made in-circuit or out?
- The photo-board has the same component IDs as the Mar82-board
schematic.
However the photo-board layout differs from the Mar82-board layout.
So, for as much as can be discerned from the photo, it appears or suggests
they did make two board layouts for the same circuit.
Yes a lot of the part labels and values are the same, but there are
differences. I am gradually reverse engineering a schematic in KiCad.
What could make some sense for the photo-board is if
they were using one
of
the B411s as a fast non-saturating clamp diode -
actually wired and
intended
to function as CR406.
That is indeed how it is being used. The emitter is not connected to
anything.
In which case you would expect:
CR406.C == GND (A)
CR406.B == GND (A)
CR406.E == Q414.C (K)
This would explain why (one of) the collectors is connected to GND.
This would want a high BE reverse breakdown V though.
Might there be a BC connection underneath the transistor on the component
side?
Not sure what you mean by this.