This week I stuffed the PCB of my Elektor Formant based NGF VCO Core one. Before soldering the precious uA726 in, I checked the footprint on the PCB, because I developed it in Eagle by myself. Lucky me! I screwed it up! I did the connection between pins and pads the other way around. I have to correct this before the next run of boards. To make use of the board anyway for testing I soldered the uA726 from the back and used some wires to connect the pins with the right pads.

I left some resistors out because I wanted to determine their right values while testing. None of them necessary for the basic function. I put the control voltage IC’s REF02 and REF102 in and tested the voltage. Fine. I put the other IC ‘s in, fired it up and the VCO worked as intended. A nice clean saw at the output.

Octave switch worked, fine adjust worked, sync in worked. FM lin, FM log worked with input resistors yet to be determined. Temperature control resistor is set to120k at the moment but should be checked again later. The Elektor Formant uses 200k here, the example in the datasheet of the uA726 says 75k with 15V. The Mini Moog used 49k9 with 10V on the second version VCO. So everything fine? Nope. I forgot that the output peak voltage should be at 5Vpp. I simply forgot the output amp and the output C to eliminate the d.c. offset as well.

The sync output pulse was derived directly from pin 8 of the 74LS13. But because you can sync the VCO with any input wave form I just eliminated this part completely. This gives me the chance retrofitting the 5Vpp output with the now freed OpAmp.

This week left me with a quite good working VCO. The PCB board must be redone but that’s not a big thing.