On 2026Jan 5,, at 12:54 PM, Rob Jarratt <[email protected]> wrote: >> -----Original Message----- >> From: Brent Hilpert via cctalk <[email protected]> >> Sent: 05 January 2026 19:54 >> To: General Discussion: On-Topic and Off-Topic Posts > <[email protected]> >> Cc: Brent Hilpert <[email protected]> >> 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.
If the H output circuitry were as in the schematic and the “CR406” is NPN and the collector on GND (as it appears to be and I think you mentioned) then it would be the wrong orientation to be functioning as the CR406 in the schematic. So I was looking for a way it might make sense for that C to be GND, thus the above. Just speculation, we’ll see what your RE schematic shows.
