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?
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.
Has the terminal ever been seen working in its present state?
Is the body of Q414 connected to HORIZ GND or is it one of the pins of Q414 is
connected to HORIZ GND? Is there any evidence of modification by a previous
owner here?
Is there any evidence that pin 4 (or pin 3 even) of T403 could have been
originally connected to HORIZ GND and was modified by the same previous owner?
Could it be that Q414 was originally an NPN silicon transistor but a previous
owner modified the terminal in order to replace Q414 with a quite different
transistor they had available to replace a possibly failed NPN Si transistor
they didn't have and couldn't get, more in hope than conviction?
If an NPN Si transistor like the BU407D was fitted in the Q414 position, can
you see a plausable way that its emitter could have originally been connected
to HORIZ GND and its collector (tab / body) could have been connected to the
horizontal output transformer? I mean for example with the outer two leads
going through the holes in the heatsink and PCB and soldered to the underside
of the PCB and a bolt through the tab, the heatsink and one of the remaining
holes in the PCB that presently have the mounting bolts for the B411 there?
As drawn, the schematic suggests that there would be a relatively high voltage
peaks between the primary and secondary of T403. I think it is less likely the
manufacturer would have laid out the ciruit this way. If they were designing
a circuit from scratch to use a PNP transistor, I think they would have
connected the emitter of the horizontal output transistor to the positive
supply and connected the horizontal output transformer primary between the
collector of the transistor and HORIZ GND. This would allow a bit less voltage
stress on T403 and allow a less highly specified part to be used in this
position. Or possibly they could have laid out the circuit completely
differently using a negative supply rail instead of a positive one.
I find it hard to imagine the manufacturers using a transistor identical to
the horizontal output transistor as a clamp diode in the CR406 position. I
am similarly suspicious of the apparantly redundant mica washer under CR406.
This seems more like the work of an enthusiastic amateur who had several of
these B411 parts on hand (which we are still not sure of the identity of).
I think the manufacturer would have been more likely to use a diode designed
specifically for the application in the CR406 position. This is likely to be
cheaper than employing only part of a second relatively hi-spec and likely
expensive horizontal output transistor.
If the B-E junction of Q414 as drawn is forward biased, I think C443 would
end up with a small negative voltage across it and probably ought to be the
other way around if T403 is connected as shown and Q414 is a PNP transistor
and the supply rail feeding the horizontal output transistor is a positive
one.
Unfortunately, I don't have any suggestions for a replacement part to use.
If my speculation above is correct, a BU407D might be suitable with its emitter
connected to HORIZ GND and its collector connected to the horizontal output
transformer and C438 and not to T403. Pin 4 of T403 would need to be moved
to HORIZ GND. This ought to leave C443 with the correct orientation.
However, if the manufacturer didn't originally use a BU407D, the specification
of the horizontal output transformer may be different to that needed for a
BU407D unfortunately.
If the terminal has never worked and you have a BU407D on hand, I guess
there is not much to lose in trying it, with the above changes also made.
If the original transistor really was a germanium PNP power switching transistor,
I imagine replacements are rare and expensive if obtainable at all, even if
we can be sure of the correct identity of it.
As far as I can see, the BU407D has an integral clamp diode and shouldn't need
an external one so you should be able to leave the CR406 position empty.
(I am not sure where BU411 came from, I think maybe you meant to say B411?)
Regards,
Peter Coghlan
