2 Feb
2026
2 Feb
'26
11:26 a.m.
On Sun, Feb 1, 2026 at 6:59 PM Rob Jarratt via cctalk
<cctalk(a)classiccmp.org> wrote:
I'm coming in rather late...
Firstly, are you sure J3 is for the Yoke? It looks much more like it's
for the CRT base (electron gun electrodes). Green = g1 (control grid),
Yellow = cathode (with Q101 as the video amplifer, are you SURE this
is PNP???, an emitter follower there is ridiculous!), black = 1 side
of the heater, Red = first anode, Blue = focus anode.
FWIW I have the 2N5830 listed as NPN. That would make a lot more sense
in the Q101 position, emitter to the bias network and thence to video
ground (pin 10 of the edge connector), collector to the CRT cathode
via R105.
The horizontal output could actually work as drawn I think. Q414 is
the horizontal output transistor and should be PNP if the collector is
grounded (so emitter goes via the flyback transformer to a +ve
supply). Note that this not an emitter follower. The input comes from
the driver transformer which is connected between emitter and base. So
it's actually the common emitter amplifier you'd expect. If CR406 is
another PNP transistor, it could again be correct. The collector-base
junction here would appear as a diode with cathode to collector and
anode to base. That's the polarity I'd expect for the diode in
parallel with the horizontal output transistor, cathode grounded.
-tony
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? 