Haible Trapezoid - Works too well!

[cnicht]

I’ve built a stripboard version of the Haible Trapezoid generator and it constantly cycles irrespective of the setting of the ‘OFF’ pot.

The controls all work and the manual trigger functions if the cycle is in an off phase to trigger the output waveform.

I’ve tried changing some of the bias resistors round the window comparator and the differential pair transistors are ‘reasonably’ well matched.

Any thoughts? What might cause this behaviour?

I’ve built a couple of the discrete versions of this and it was equal fun getting them to cycle!

A ‘two pipe problem’?

Edit: Reduced R19 to 680k now it’s off, triggerable and cyclable

[vladosh]

i had in mind to build this for long time now ,and i wonder a little thing ,on the LM394 the first collector connects to +V or ground ,not sure cause elsewhere on the scheme +V is labeled +ve ? Thanks

edit: jus checked the original synthi schematic so it connects to ground i seems

[cnicht]

It does connect to GND and I used a pair of transistors which I matched

[vladosh]

made a layout for this using BCM847

[cnicht]

That’s excellent! I built mine on stripboard as I just needed a one-off.

There is a PCB available for it at Modular Addict