[sdiy] VCO drift - Update2

[rob]

Hi,
The BC847 dual transistor worked, but has terrible frequency tracking,
completely unusable (1% tracking error). Okay for one octave only. I could
redesign the HF trim circuit, but that seems a waste of time. An unmatched
pair of 2N3904/06 tracks very well over 6+ octaves, 0.1% in the upper
octaves. I am surprised how well ARP designed the HF as it needs no
trimming/setup.

The VCO circuit is temperature sensitive on a number of resistors other than
the tempco and NPN/PNP transistor pair.
So for the next version of the VCO I am going to recreate a potted sub
module - just like the original ARP design.

Does anyone have the 4027-1 PCB layout (I have the schematic) ???

Regards
Rob
www.emulatorarchive.com
PS I'll post some more measurements on my web site, with E-mu Systems
Modular VCO and Oakley VCO. They have different HF trim designs.