27-04-2021, 08:02 AM
Amie Wrote:As an aside, I had always wondered how it was that sets with a frame coil could be accurately aligned
It seems to me, a lot of them just couldn't be accurately aligned. And, it's not restricted to sets with a frame aerial.
Ever Ready Model K has a non-adjustable frame aerial. So does Vidor CN420a and Bush DAC90a. Cossor 500 has a non-adjustable RF coil. As does Pye 67a. That's 5 come to mind.
Alignment technique, after getting the oscillator 'right' is to tune to somewhere in the middle of the band (usually specified), and adjust a trimmer capacitor. It'll be spot-on at this point. But it's like trying to define a straight line by giving one point only - if the gradient of the line is wrong, there will be an error each side, of opposite sign. Component tolerances and Sod's law says that, in production, the gradient WILL be a bit wrong, and tracking WON'T be optimum.
A lot of radios leave something to be desired, and it hurts sensitivity.
Being able to vary both the coil's inductance and the tuning gang's parallel trimmer, does allow tracking to be adjusted spot-on at two points, and the error elsewhere is then very much less. Down side is, the extra time spent in test puts up the price, as does a coil slug if used. But it does allow the circuit to give more nearly its best, which is more rewarding from a designer's point of view.
I've not seen the technique described, but I have read about it. Though blobs of wax would've been far nicer than a staple...
