Troubleshooting a CGS75 - Voltage-Controlled Slope..

[MC1495]

Currently attempting to build the CGS75 from scratch (no PCB) - and running into a little bit of difficulty. Obviously, this is a minefield - but maybe I can explain the problem sufficiently to merit suggestions.

There seems to be a lot that is hinged on the PNP/NPN differential pairs - specifically in the "Exponential 1v/oct" that is attached to the bases of the bottom NPN and PNP.

With the voltage divider attached to the bases, I get short blips - and if I adjust the Rise/Fall CVs, eventually I get to a point where the Envelope (and LED) is sticks at varying voltages after being triggered. Without the voltage divider, I seem to be able to effectively control the "Rise", but have no control of the "Fall."

I'm assuming this potentially has some relationship to the LM3900 - which is ideally configured to control the positive- or negative-going slope, depending on a threshold?

Any advice would be greatly appreciated!

[Mongo1]

Hi MC,

I haven't built the VCS, but I did build the DUSG which is similar. With the complexity of these boards I would really suggest buying the PCB from Ken. There's an awful lot that can go wrong, and isolating is going to be very difficult.

Barring that, I'd suggest posting a photograph of the front and back of your project. That way people can at least see where things stand.

Good Luck -
Gary

[MC1495]

Don't have a way to photograph this circuit at the moment.

I will say that I just realized I had the input resistors for the "In" stage arranged incorrectly and that does seem to fix, or at least "change" the problem.

[Mongo1]

That's good news!