Cgs synth

Hi,

please, note that I could be wrong about any of this, but if I had this
problem, this is what I would be doing. I know troubleshooting is
frustrating. Reccently, I spent a week searching a filter circuit for a
very simple problem.

I think we should look at this area a bit more. The reason, is that you
get sort of the opposite results that I expect. If you are measuring
left and right transistor as shown in the schematic, then the right side
should be at a higher voltage than the left side as the pot is
increased, for atleast most of the pot range. At minimum pot position,
the left side might be a little higher than the right, depending on
transistor matching. The diodes go from the right side to the left, to
forward bias them, the voltage on the right has to be greater. This
allows an adjustable current flow through the diodes, which causes them
to act like variable resistors.

I ran a simulation of the biasing section only. The results matched
pretty well with my analysis on paper. here is some screen shots as the
voltage 'V3' (basically pot setting) is changed from 0V to +15V:

simulation circuit:
http://www.sdiy.org/destrukto/temp/steiner-biasing.gif

diode string voltage:
http://www.sdiy.org/destrukto/temp/steiner-biasing_plot.gif

left diff pair transistor base voltage:
http://www.sdiy.org/destrukto/temp/steiner-base_voltage_plot.gif

diode current (CV pot from 0 to +7.5V)
http://www.sdiy.org/destrukto/temp/steiner-diode_current.gif

that last plot shows the current through the diodes. only half the pot
range is used, because beyond that, the biasing is no longer linear,
which causes the diode current to no longer increase exponentially
(exponential is what we want). On the diode string voltage plot; You can
see the two voltage curves intersect, the point of intersection seems
fairly dependent on the transistors and thier matching. There is a part
where the diodes are not forward biased. My diode current plot shows
that it still behaves mostly exponentially in that range, but I don't
really understand how it does.

now, on to your results. I didn't think before, but you measuring could
effect the circuit behavior. not sure what the impedance of the volt
meter is. anyway, I think there still might be a problem here.

ken johnston wrote:

> Near as I can tell those all check out fine. These are the readings I
> got on the meter w/common connected to ground:
>
> Left transistor collector: 7.04 to 7.32 (freq pot low to high)
> Left transistor emitter: -.64
> Left transistor base: -22.6 to -.33

the left transistor base doesn't seem right. the pot should go from 0V
to +15V. My simulation shows that at 0V, the base voltage will be
slightly negative, but for most of the pot range, it won't be. the
collector voltage should be decreasing as the pot in increased.

>
> Right transistor collector: 5.60 to 5.80
> Right transistor emitter: -.64

this collector current increases as it should, but I expect a much
larger change in the voltage. the emitter voltage looks good, so your
transistors are turned on.

> Center diode string: 4.86 to 5.06

a little lower than I expect, but possibly there is a reason for that.
maybe the volt meter effects it?

ok, so make sure your CV pot is connected correctly (0V and +15v), and
make sure the voltage at the pot wiper goes from 0 to 15v. sure that
sound obvious, but I would double check it.

I'll try to think of other possible problems, nothing comes to mind
right now, but I'll think about it.

-ryan