> Well, all modern test equipment is normalized for 50 or (more rarely)
> 70 ohms. That is the so-called 'system impendance' and there's not
> much one can do about it. However, I really just want to see where the
> resonant point is for each of these devices. I only had the time last
> night to do one (a green one) which turned out to be 10.7Mhz, so I can
> now sell them off as such. I'll do the other colors when I have time.
> It'll be interesting to see the difference in construction between a
> 10.7Mhz and a 455khz one. Interesting for *me* at any rate, although I
> accept that others might fail to see the point of this. Each to his > own!
A long time ago, when the Earth was much younger, FM didn't exist where I live but there was an AM station transmitting at 540kHz.
I used to take a 455kHz IF transformer, replaced the built-in 200pF cap with 150pF and used it as the collector load for an RF transistor (an AF117 IIRC). I tuned the IFT directly to the station and the secondary fed a Ge diode (usually OA79) detector. The DC output from the detector served as an AGC bias while the audio signal went to an amplifier. I had a very narrow range of parts to choose from and the amp was usually an AC126 driving an AC187/AC188 complementary pair or a pair of AC128's in parallel push-pull with an output transformer..
The RF input side was a coil of solid copper wire (I didn't have Litz) on a ferrite rod, paralleled by another 150pF cap and tuned by sliding the coil along the ferrite rod.
I must have built at least half a dozen such "radios" with some variations and gave them away to friends. It was a lot of fun.