[ARC5] BC-453B dial calibration saga.

[AKLDGUY .]

On Sat, Jul 29, 2017 at 2:00 AM, AKLDGUY . <neilb0627 at gmail.com> wrote:

> I would look for a bad or missing decoupling cap near the oscillator.
>
> An open circuit or missing cap won't pull down the circuit to RF ground,
> so all that's decoupling the circuit is wiring capacitance.
>


I have a habit of not explaining myself, figuring that what I've suggested
is blatantly obvious.
Perhaps I better explain.

If the decoupling cap near the oscillator is missing or is destroyed
internally, the B+ to the
oscillator will not be adequately decoupled. The only thing left is wiring
capacitance. This
means that the bottom end of the oscillator coil is not pulled down to RF
ground by the
(I think 0.05 uF) decoupling capacitor and only 30-50 pF of wiring
capacitance is present
there.

The effect of this is that the oscillator coil is now in a circuit like
that of the pi-coupler (a coil
with a capacitor to ground at each end). The tuning is affected at the low
end of the range
because the wiring capacitance becomes progressively less adequate as a
decoupler as
the frequency goes lower. So, instead of fiddling with coils and slugs
which never shift, look
for bad decoupling around the 12K8.

73 de Neil ZL1ANM
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