A tale of five revisions

As can be seen in my previous post, i am currently working on an analog synthesizer, as a project to maintain and develop my analog/macro-circuit skills.

I started out doing some research, and decided to base my build on Thomas Henry's VCO-1 design.
I stripped some of the extra features off, (Sine converter/ pwm input) and proceeded to breadboard the resulting design, which immediately worked as designed.

What follows is an interesting study in what can go wrong with a design.

REV 1

I then started laying out a small two-sided design in eagle, aiming to fit everything within a single 5*5 cm square (this being minimum unit for  both the student hackspace i frequent, as well as the http://www.seeedstudio.com pcb service). This being a one-off side project in a busy period, i may have rushed the design a bit more than i should have, something repeated head -> desk moments later would confirm.

Revision 1 was etched, soldered and powered, but refused to work.
As an evening of inspection and probing gave little results i reinspected my circuit diagram, revealing that my oscillator had somehow between the original and my schematic become non-inverting.

REV 2

Revision two was mostly identical to revision one except for some inverted signs.
It took some time to get running, as i had broken one of my own rules for home etched (non-plated) designs to optimize space and minimize vias. After some debugging i discovered that one of my top-side component pads were unsoldered.
When i had corrected this, the oscillator immediately started running, and watching my scope i slowly turned up the voltage to full drive voltage. And, not watching the PSU, i then proceeded to run it well past the maximum.

I would later find out that the circuit working must have required some other mistake in the soldering.

REV 3
Believing that the second revision worked, i proceeded to revise the circuit to ease correct soldering (at the cost of some extra vias), re-etched, and soldered up the third revision of my circuit.
It never worked, and through several sessions of debugging i made no progress, and decided to scrap the current prototype and start anew. 


REV 4
Still convinced that the basic circuit was correct, and that the problem with the third revision was some artifact of my production process, i then adapted the circuit for production-grade pcb (less phobia of vias and top-level connections), and proceeded to order a set from seeed.
When the boards arrived, i then carefully tested all the transistors before soldering them, working off the theory that some of the components might be defective or have the wrong pinout.

Turns out, the footprint for my monolithic pair was just .. wrong. checking both the datasheet and the component, i then proceeded to solder the component "cross-legged", and the oscillator worked.
I am fairly certain that i checked all the footprints against the datasheets, and i have no idea how this managed to slip by me through four revisions.


Revision five will hopefully be a stable, correct implementation (although a greenwired rev.4 still works), and i hope someone out there will see this and at least be spared one of these mistakes.