More circuit help required please

Adrian Graham witchy at
Sun Jan 8 11:14:48 CST 2017

On 08/01/2017 16:18, "Mouse" <mouse at Rodents-Montreal.ORG> wrote:

>>> 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:

If the two don't match I take it out of circuit.

Binary Dinosaurs creator/curator - the UK's biggest private home computer

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