1 Feb
2026
1 Feb
'26
7:58 p.m.
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.
I have finished partially reverse engineering the schematic for the monitor board in my
VT100. I have tried to make it follow the layout used in the printset. I hope I
haven't made any mistakes, but it is quite possible that I have.
As a reminder the Feb 82 printset
(https://bitsavers.org/pdf/dec/terminal/vt100/MP00633_VT100_Schematic_Feb82.… on p58 of
the PDF) has a schematic for this board. My board seems to follow mostly the same
schematic (including part labels, values etc), but my board differs in some substantial
ways. Q414 in particular seems to be a BU411 and nothing like the BU407D in the schematic,
but I also have CR406 installed on my board, which the printset shows as optional,
presumably because a BU407D includes the diode. The other big difference is the absence of
the 555 (E1 in the schematic) that drives Q413. I have not reverse engineered the lower
half of the schematic because the problem is to determine whether Q414 as installed on my
board is correct or not. I have obtained a BU407D which I could now install at Q414
(although quite how I make it fit I am not really sure)
The schematic is drawn in KiCad 9 and is here:
https://rjarratt.wordpress.com/wp-content/uploads/2026/02/vt100.zip
A PDF of the schematic I drew is here:
https://rjarratt.wordpress.com/wp-content/uploads/2026/02/5414131-partial-s…
This is a picture of the board
https://rjarratt.wordpress.com/wp-content/uploads/2023/12/img_20231221_1122… Q414 is
the TO3 on the heatsink at the left edge of the board with the marking "B411".
CR406 is the other TO3 just above it without the heatsink.
Would appreciate advice on whether to replace Q414 and CR406 with a BU407D or something
else?
Thanks
Rob
