2 Feb
2026
2 Feb
'26
1 a.m.
On 2026Feb 1,, at 10:58 AM, Rob Jarratt <robert.jarratt(a)ntlworld.com
<mailto:robert.jarratt@ntlworld.com>> wrote:
Your schematic shows both Q414 & CR406 as PNP transistors. Electrically this makes
sense for both of them, in regards to conduction polarities.
Both BU407 & BU411 types are NPN, so are not going to work for the presented
schematic.
2SB411 which the installed transistors are indicative of, is a PNP, high-V, high-current
power transistor, albeit Ge as you mentioned earlier in the thread.
So given your schematic, it does make some sense that those Ts may actually be 2SB411,
though rather unusual for Ge being used for such that late. Or someone repaired it with
old stock.
PNP HOT transistors were/are far less common than NPN, or were so by the late-70s period
of the VT100.
Without having gone looking, I don’t offhand have a suggestion for a (more modern, Si) PNP
HOT.
Given that the base drive for the HOT is a floating transformer secondary, it could
conceivably be rewired to use a common NPN HOT such as the 407 you mention with same or
something for CR406, or an NPN HOT with integrated reverse clamp D.
—
The absence of the 555 (H osc) means that the monitor will not self-excite (produce
high-V), or self-scan, it requires the H & V sync pulses from the terminal to do so.
—
A graphic in the VT180 service manual:
http://bitsavers.org/pdf/dec/terminal/vt180/EK-VT18X-TM-001_VT180_Technical…
<http://bitsavers.org/pdf/dec/terminal/vt180/EK-VT18X-TM-001_VT180_Technical_Man_Feb83.pdf>
PDF page 7-32
shows a monitor board that - for as much as is drawn in - matches yours.
It is referred to as the “Digital / Elston monitor”.
Unfortunately the schematic is not in that document.
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.…
<https://bitsavers.org/pdf/dec/terminal/vt100/MP00633_VT100_Schematic_Feb82.pdf> 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
<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…
<https://rjarratt.wordpress.com/wp-content/uploads/2026/02/5414131-partial-schematic.pdf>
This is a picture of the board
https://rjarratt.wordpress.com/wp-content/uploads/2023/12/img_20231221_1122…
<https://rjarratt.wordpress.com/wp-content/uploads/2023/12/img_20231221_112233.jpg>
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?
