On 08/01/2017 16:18, "Mouse" <[email protected]> wrote:
>>>> http://www.binarydinosaurs.co.uk/STCExecutelSyncCircuit.jpg >>> I don't believe it's correct as drawn. For one thing you have V333, >>> an NPN transistor, with the collector grounded (and no -ve supplies >>> on the circuit. > > Bipolar transistors are to some extent symmetric under interchange of > collector and emitter. Simplified pictures showing a bar of > semiconductor with the ends being emitter and collector and a thin base > layer in the middle are, well, simplified, but there is _some_ truth > lurking in them. > > However, the only reason I could see that being done deliberately is if > the circuit is analog and the transistor's behaviour is relevantly > different from the normal way around. This circuit doesn't look like > that to me. Redrawing it a bit less confusingly with V333 E/C swapped > makes it look as though V333 and V326 are being used in their switching > regions, not their linear regions. Yes, I got the emitter and collector swapped. The software I'm using (Fritzing) is excellent given it costs nothing, but occasionally you can make a change then alter something else in the properties of the item and the change is lost. > The lack of negative supplies is hardly conclusivein itself; negative > voltages could be developed in any of many ways. However, V325, R322, > and V326 would make it difficult for the R335/V325/V333 junction point > to get too far below 5V (admittedly this is much less true if R322's > value is actually significantly higher). > > I also question the way R311/R310/R309 are all different. I would > expect the red, green, and blue circuits to be electrically more or > less identical, and different pulldown values does not fit with that. There's been a couple of resistor failures with this board so I'm at the stage of starting with a DMM, swapping probes to hopefully get a consistent reading then reading the bands and using the calculator here for ones I don't instantly recognise: http://www.allaboutcircuits.com/tools/resistor-color-code-calculator/ If the two don't match I take it out of circuit. -- Adrian/Witchy Binary Dinosaurs creator/curator Www.binarydinosaurs.co.uk - the UK's biggest private home computer collection?
