Re: More VFO troubles (W1FB NB p. 111 again)

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From: Michael Silva (mjsilva@ted.win.net)
Date: Sat Jun 04 1994 - 03:47:31 EDT


 
>
>I'm still working on the rig, and I've made progress. Many thanks for the
>help!
>
>I'm still having troubles with the VFO. For some reason, the output link
>and capacitor are behaving as the frequency-determining components. If I
>detach the link from the output capacitor, the VFO works exactly as one
>would hope, on frequency and everything. If I leave the link connected by
>play with the proper frequency-determining components, the frequency changes
>little if at all. The dead giveaway was when I replaced one of the tank
>capacitors with a trimmer and changes of 10x in the tank capacitance produced
>no change in the VFO output frequency!

I don't remember if it was in a private or group e-mail that you
mentioned putting a 0.01 in place of the 0.001 as the link output
cap. That translates to about 2 ohms instead of 23 ohms, in series with
the 0.01 / diode combination. which is one heck of a heavy load on a VFO.
Ignoring the diodes, the difference is a load of 4.5 ohms vs. 25 ohms,
which reflects back to the tank as 120 ohms vs. 720 ohms. The tank
impedances are about 200 ohms, so you get a tank Q of either 0.6 or 3.5
(disregarding other loading by VFO circuit and diodes). No wonder your
VFO is gasping! I would first try putting the 0.001 back in the
circuit (and maybe take out that other bothersome 0.01 in the LP
filter), and if that doesn't work, try a simple class-A buffer to give
your poor VFO tank a break.

><sigh> Next time I build a kit. :-)

Don't you dare! We're having too much fun "helping" (?!). Now I'll
post this and let everyone check my math....

Mike, KK6GM


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