As I’ve mentioned, I am building a superhet receiver around the beautiful National Radio gearbox/dial that Armand WA1UQO gave me. First step was to build the VFO. Before I started, I went back to Doug DeMaw’s books and read his words of wisdom on how to build stable VFOs. I followed his advice:
— Air core coils.
— Tuning capacitors with bearings at both ends.
— NPO fixed capacitors.
— All frequency determining parts in a separate box
— Run the oscillator stage at lower voltage (6 volts)
— Stable solid physical construction.
— One-sided PC boards.
I went a bit further. I wound the main coil on a cardboard tube from a coat hanger. I coated it with several layers of clear nail polish. I glued it down with a generous dose of gorilla glue.
There are some fixed caps in the circuit. I didn’t want them physically hanging off other parts, so I used bits of balsa wood to support them.
I put the actual oscillator stage in its own Altoids tin and attached this tin to the bot that held the main coil and capacitor. I put the 6 volt Zener diode and its dropping resistor on the outside of the box to minimize heating. The buffer and amplifier went into another Altoids tin. I used a wooden grilling plank from Whole Foods as my base.
At first, Armand’s gear box and reduction drive didn’t seem wo work very well. There seemed to be a lot of “play” in the mechanism. Some words of wisdom from Pete N6QW and the blog Dave AA7EE provided the solution. There is a spring in the gearbox that hold the teeth of the gears together and prevents the kind of play that I encountered. With guidance from Dave, I was able to put some adiditonal tension on the spring and the gears. This resolved the play problem.