8 Jan
2017
8 Jan
'17
9:14 a.m.
On 08/01/2017 16:18, "Mouse" <mouse at Rodents-Montreal.ORG> wrote:
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.
>>
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. 
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?