DX100 VFO questions

[John K5PRO]

Has anyone replaced it or done some serious improvement/mods to make it more temp stable? I tore mine apart months ago, and tried to use a JFET and buffer, and a 2N5109 with a step up transformer to get approx 15 VP-P RF to the grid of the next stage. It was a disaster, on some bands pulled when the RF came up, others it had FM from the RF being on, causing a hum on RX, and it was regenerative on one band. So I pulled the silicon and put the 6AU6 back. I did notice that the plate RF choke for the 6AU6 was chosen with intent to make a signal "rich in harmonics", as it was not optimal for clean sinewave. I believe that the separate VFOs like the VF1 and HG10 used bandswitching on the choke or an RF inductor for low band versus 7 MHz.

[w3jn]

Take a lesson from National and Squires-Sanders.  They temp compensated VFOs by using a butterfly cap, each leg in series with idential fixed caps but of different temp compensation characteristics.  One was a NP0 (or maybe a P050) and the other was a N750 or N1000.  By adjusting the butterfly cap you are essentially adjusting the temp compensation ratio, as the total capacitance in the circuit remains constant.

Look at the skizmatic for the VFO in a NCX-5 for an example of this.

You can plug temp compensation caps in there on a more or less trial and error basis - but this takes millions of years and costs thousands of lives 

73 John

[John K5PRO]

W3JN, Tnx for pointing this out. I will check out that idea.

[WA1GFZ]

Also consider a class a buffer stage if going solid state.