These parts are made to resonate at three basic ranges of frequency in a 40-channel or 10-meter radio.
The channel frequency at 27 MHz.
The high IF frequency at 7.8 or 10.695 MHz.
The local-oscillator (LO) frequency at 35 MHz (Uniden) or 16 MHz (RCI).
IF transformers meant for the 455 kHz AM IF frequency are built differently and won't have the hollow threaded coil form with the skinny threaded slug. Those are the ones with the larger adjustment hole and a screw slot visible on top. Haven't found myself needing to try this trick on those.
The internal cap for a 35 MHz coil is probably under 10 pf. For a 27 MHz circuit, under 20 pf, more or less.
The 10.695 MHz IF transformers are probably closer to 30pf, and 7.8 MHz parts a bit more.
It's like asking how many turns of wire are inside that metal can. It's just not documented anywhere that you or I can get access.
The schematic diagrams would sometimes show that internal capacitor value in the bad old days of tube-type radios, but usually not. The schemo for a solid-state radio will usually show the capacitor inside the dotted lines around the RF/IF coil, and where it's connected inside but without any description of the part's capacitance value.
It's a cut-and try sort of thing. If your "patch" trimmer cap adjusts all the way to maximum and won't resonate, it's too small.
I'm accustomed to setting the slug to the depth inside the coil core where I'm used to seeing it, then setting the trimmer cap for a peak. Sometimes, moving the tuning slug's peak up or down by tweaking the trimmer cap will improve the signal level.
Your mileage may vary.
And if you're working on a 23-channel radio that combines multiple crystal frequencies to select the channel, it can get pretty squirrelly.
73
The channel frequency at 27 MHz.
The high IF frequency at 7.8 or 10.695 MHz.
The local-oscillator (LO) frequency at 35 MHz (Uniden) or 16 MHz (RCI).
IF transformers meant for the 455 kHz AM IF frequency are built differently and won't have the hollow threaded coil form with the skinny threaded slug. Those are the ones with the larger adjustment hole and a screw slot visible on top. Haven't found myself needing to try this trick on those.
The internal cap for a 35 MHz coil is probably under 10 pf. For a 27 MHz circuit, under 20 pf, more or less.
The 10.695 MHz IF transformers are probably closer to 30pf, and 7.8 MHz parts a bit more.
It's like asking how many turns of wire are inside that metal can. It's just not documented anywhere that you or I can get access.
The schematic diagrams would sometimes show that internal capacitor value in the bad old days of tube-type radios, but usually not. The schemo for a solid-state radio will usually show the capacitor inside the dotted lines around the RF/IF coil, and where it's connected inside but without any description of the part's capacitance value.
It's a cut-and try sort of thing. If your "patch" trimmer cap adjusts all the way to maximum and won't resonate, it's too small.
I'm accustomed to setting the slug to the depth inside the coil core where I'm used to seeing it, then setting the trimmer cap for a peak. Sometimes, moving the tuning slug's peak up or down by tweaking the trimmer cap will improve the signal level.
Your mileage may vary.
And if you're working on a 23-channel radio that combines multiple crystal frequencies to select the channel, it can get pretty squirrelly.
73
