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This is from an Elenco AM/FM superheterodyne radio receiver kit.

The shown area is the AM antenna and mixer stage. The wire leading from the top right corner is Vcc.

Q7 simultaneously oscillates at the IF and mixes this with the received signal.

It appears that the right side of L4 is for the IF oscillator tuning and the left side picks up the signal from the air.

I do not understand what capacitors C28 and C30 are for. The instructions are slightly vague at this point.

Could someone please explain what they may be for?

Why is Vcc leading into the RF tank circuit?

The schematic section being referenced

Full schematic

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  • \$\begingroup\$ Check the schematic .Maybe the Lefthand side of C28 is ground .So C28 is just RF bypass . \$\endgroup\$ – Autistic Dec 11 '18 at 1:14
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    \$\begingroup\$ It is drawn by an artist, not electrical designer so it does not flow like it should. Q7 is tuneable LO and Q8,9 D4 is AGC while C28,C29 couple the LO cap to the DC biased collector L5 \$\endgroup\$ – Sunnyskyguy EE75 Dec 11 '18 at 1:16
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    \$\begingroup\$ He forgot to put a dot on C29, R31 \$\endgroup\$ – Sunnyskyguy EE75 Dec 11 '18 at 1:28
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    \$\begingroup\$ ... he ran out of solder \$\endgroup\$ – Chu Dec 11 '18 at 1:38
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    \$\begingroup\$ L4 is tuned to the RF signal (not IF). The large coil wound on a ferrite rod is an antenna too. Its smaller coil matches impedance going into the mixer (Q7). Q7 does oscillate via L5, at a frequency 455 kHz above the RF signal frequency. The difference frequency (455kHz) is selected by T6 - your schematic fails to show its internal resonating capacitor. This is the beginning of the intermediate-frequency (IF) amplifier string. \$\endgroup\$ – glen_geek Dec 11 '18 at 15:03
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If this is a really old-style receiver, it most likely has vane-type tuning capacitor for selecting the receivefrequency. These are notorious for having intermittent shorts between the movable and stationary vanes.

Look at the value of the capacitors in question. If they are large value, the capacitors are most likely to be simple coupling caps intended to stop damage from occurring should the tuning capacitor develop a shot.

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Here is half an answer. C30 at 150 pF could well be a padder capacitor. Its function, together with the trimmer capacitors on the variable capacitor gangs, is to assist with the tracking of the local oscillator and RF tuning. The local oscillator operates at a higher frequency than the RF but at a fixed offset from it. Its relative tuning range is therefore smaller. The padder helps tracking at the lower end and the trimmer at the upper end. C28 looks like decoupling but I can't work out why it's where it is.

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  • \$\begingroup\$ Your supposition about C30 could well be true - without a careful examination of the variable tuning capacitor, one cannot tell. Sometimes the plates of the main dual-capacitors were stamped into such a shape that coils of a very specific inductance would auto-track throughout the band. The capacitance value of C30 as a "tracking capacitor" can vary, but hundreds of picofarads is a ballpark figure for AM broadcast band receivers. \$\endgroup\$ – glen_geek Dec 11 '18 at 15:26

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