[ARC5] BC-453B dial calibration saga.

[Bruce Long]

I think Neil is giving very good advice here.  BC-453 and other ARC-5 type receivers are known for stable, and predictable dial calibration.  The fact that there is one receiver that acts differently with no obvious damage/prior ham modification of the tuning components and which acts just as band with a new coil drawer suggests there is something wrong in a place in the schematic where the tuning circuit can be affected but not part of the tuning circuit as traditionally considered.


      From: AKLDGUY . <neilb0627 at gmail.com>
 To: ARC-5 List <arc5 at mailman.qth.net> 
 Sent: Friday, July 28, 2017 5:50 PM
 Subject: Re: [ARC5] BC-453B dial calibration saga.
   
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 theoscillator will not be adequately decoupled. The only thing left is wiring capacitance. Thismeans 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 presentthere.
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 rangebecause the wiring capacitance becomes progressively less adequate as a decoupler asthe frequency goes lower. So, instead of fiddling with coils and slugs which never shift, lookfor bad decoupling around the 12K8.
73 de Neil ZL1ANM
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