Cgs synth

Steve has kindly answered my question over at the SergeModular group:

http://launch.groups.yahoo.com/group/SergeModular/message/6497

- basically the emitter-base junction is used in reverse as a 7V zener diode!!

Tim

--- In cgs_synth@yahoogroups.com, "Tim" <timothy@...> wrote:
>
> Hi list,
> I recently posted this at the SergeModular Yahoo group, but it was suggested that here might be a better place, so here goes!:
>
> I'm trying to simulate the Negative Slew from this schematic:
>
> http://www.cgs.synth.net/synth/serge/pic/serge_negative_slew_schem.gif
>
> I'm trying to investigate how well the CV controls the negative slope, but I
> can't get the simulation to run properly, mainly because I can't see how on
> earth the main integrator (amp pins 8/9/13) resets: for this to happen base 6
> will need to climb to about 2 diode drops above ground, but I simply see no
> route for any current to enter the 220k there - all the diode junctions in the
> two transistors would appear to prevent this. Anybody know if this schematic is
> accurate, or can explain how it does work (if indeed it does)? (I've checked
> that part of the circuit against the PCB layouts given in other pics there, and
> they look like they agree.)
>
> I also think the (kind of) 'diode connected' transistor 9/10/11 would cause
> grief to the CA3086 if the 'diode' could in fact conduct: this would effectively
> put the transistor into saturation, which might be bad news for the chip, as I
> think it would turn on the parasitic PNP transistor (formed from
> base-collector-substrate), causing current to flow out of the substrate.
>
> Has anyone any thoughts or ideas?
>
> Thanks, Tim
> __________________________________________________________
> Tim Stinchcombe
>
> Cheltenham, Glos, UK
> www.timstinchcombe.co.uk
>