> On 2026Feb 2,, at 4:07 AM, Peter Coghlan via cctalk <[email protected]> > 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.
