In message , Anthony Fremont
writes
Ian Jackson wrote:


As I said, sorry for the ramble.
Ian.

OMG are you kidding, don't be sorry. Thank you way so much!!! :-))) You
should set your clock to way in the future and repost that message so it
sticks around for a while. ;-)

So to make a long story short I need to put an inductor of roughly 400uH
across the crystal to cancel the 5pF of C2. Wow that's a ton of
inductance, but about 27 turns on a FT50-43 ferrite torroid ought to do it.
I'll let you know how that works out. I found another crystal,
unfortunately it's identical and possibly from the same batch. I haven't
tried it yet, but I'm not expecting any miracles. I'm tired of burning my
fingers unsoldering parts, so I'm goint to tinker on the breadboard with
another 602 set up just for the oscillator testing with capacitor changes.
I will apply the new coil to the soldered up version though.

The receiver hears, as we just had a storm earlier and I could hear
lightning crashes in the distance. In my narrow tuning range, I can hear
what is likely the carrier of a broadcaster too, or maybe my TV. Later
tonight when the band opens up some more, I should hear something from W1AW
hopefully.

Thanks again



As you say, at around 3.5MHz, you will need a fairly large inductor to
resonate with 5pF. An alternative might be to make a bridge circuit,
where you actually use another (5pF) capacitor to balance out the
unwanted 5pF. I used to use an extremely simple balancing circuit to
make accurate measurements of the resonant frequencies and ESRs
(equivalent series resistance) of VHF crystals, and it should be
possible to use something similar in an oscillator. However, maybe
someone out there can advise on a tried-and-tested circuit which will
definitely work. [There's no point in re-inventing the wheel!]
Ian.
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