2 Feb
2026
2 Feb
'26
12:46 p.m.
On 2026Feb 2,, at 4:07 AM, Peter Coghlan via cctalk
<cctalk(a)classiccmp.org> wrote:
…
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.
...
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.
To drive the Q414 PNP per the schematic, T403 would be producing a pulse with pin 4 + and
pin 3 –.
C443 would develop a slight charge across R471 with the polarity consistent with the C443
orientation shown, so it’s orientation looks correct to me. The RC appears to be there to
provide a little bit of reverse bias to help turn & keep Q414 off (when the drive
pulse from T403 goes away, a little bit of + bias from the C is presented on Q414.B
relative to the E).
To adapt the circuit for an NPN, pin 3 (not 4) would have to be moved to GND, and pin 4 to
the RC / base.
Adapting the circuit for an NPN like this leaves it almost identical to the way it is
drawn in the manufacturer VT100 schematic Rob ref’d at the thread beginning, though in
that schematic the base series RC are absent and instead a C from GND to B is present. It
kind of looks like they went through this same reasoning to redesigning the board to use
an NPN.
—
Rob, if you were to put up decent photos (better lighting) of both the component side and
solder side of the board, we out here may be able to double-check some of the issues being
raised rather than having to speculate.
If the B411s are still removed (IIRC, you mentioned earlier you had removed them for
testing), all the better.