craigm wrote:
wrote:
Most of the mass-produced radios lack the RF coil to properly match the
ferrite antenna. As you adjust the oscillator coil you will find a spot
where the gain of the circuit is optimum. You are not adjusting the
ferrite antenna: you are matching the oscillator to wherever the
ferrite is tuned to. This will produce stronger signals. Try it.
That would do winders for dial accuracy.
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Actually, this type of radio doesn't have good dial accuracy out of the
box, so the tuned version is not much different than the original as
long as the tuning device is adjusted.
This adjustment is made to have the oscillator 455 khz higher than
whatever the ferrite is tuned to. Since there is no easy way to adjust
the ferrite, adjust the oscillator coil until it is 455 khz higher than
the antenna (ferrite) coil. You will find a spot where the signals are
the loudest. This is where the correct difference frequency is 455 khz
and the gain is maximum through the IF. For example, if the station you
want to receive is at 600 khz, and the antenna is tuned to 610 khz due
to manufacturing tolerances, the oscillator must be tuned to 1065 khz
to have the correct difference frequency. Since the oscillator is the
only thing that can be adjusted, you adjust it. The IF is happy: all it
wants is a 455 khz signal. The increase in gain is especially
noticeable from 550 to 700. All the radios I have tuned could easily
add the new 1600 to 1700 frequencies. The 455 khz IF coils are quite
stable: I haven"t run into any that needed adjusting.