using new technology on old machines
ard at p850ug1.demon.co.uk
Tue Jun 16 23:29:36 CDT 2015
> that the original appears to use a 2k 10-turn pot, and a 7440 output buffer,
> neither of which are in my rather extensive junk box.
> In fact the 7440 are rather rare, and I see the cheapest I can get them for
> is around $4.00 each.
True. But you don't need either to build and test the oscillator circuit. The 7440 is
an output buffer, if you want to build a test version of the entire module you could
stick a '20 there or just about any other TTL inverting gate. But I was thinking of just
testing the oscillator part which is Q1 and Q2 (again the other 2 transistors are buffers)
The preset clearly sets the frequency. You could build the oscillator with say a 1k
resistor there at least to put a 'scope on various points to understand how it works.
> I am really used to RF circuits so am puzzled there is no inductor. It kind
> of looks like a Darlington Pair but it isn't.
RC oscillators have been around since the 1930s.....
> > I will leave the flames about Spice and simulation packages in general for
> > another day.
> You are touchy. Would it help if I used the original Spice2 written in
> Fortran IV. It still works. Sadly I don't have a real mainframe but have to
> use Hercules to run it....
OK, I sort-of forgot the smiley, but only sort-of.
The problem is that people try to understand a circuit by throwing it on the
simulator and really don't understand what is going on at all (this assumes
the simulator gets it right which is not always the case). I prefer to think about
it. work out how the capacitor (there is only one!) charges and discharges, since
that must be the key to the oscillations.
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