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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, 2018 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\$ Dec 11, 2018 at 1:16
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    \$\begingroup\$ He forgot to put a dot on C29, R31 \$\endgroup\$ Dec 11, 2018 at 1:28
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    \$\begingroup\$ ... he ran out of solder \$\endgroup\$
    – Chu
    Dec 11, 2018 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, 2018 at 15:03

3 Answers 3

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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, 2018 at 15:26
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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.

No, L4's purpose is strictly as an antenna, tuned to the received radio frequency signal. It has a link winding to impedance-match a transistor's low input impedance at its base.


C29 couples the antennas' low-impedance RF winding to the transistor base. C29 also serves another purpose: it causes Q7 to operate as a grounded-base oscillator as the higher-frequency local oscillator.
The dual-purpose autodyne mixer (RF amplifier plus local oscillator) complicates capacitor selection. The AM broadcast band is wide:

  • Oscillator frequency must track linearly above RF frequency by 455 kHz. over the whole span of the AM band.
  • Oscillator must operate as grounded base (grounded by C29), even at its lowest frequency where C29's impedance is higher.

The choice of C28, C30 are also subject to these same constraints. Tracking is sometimes complicated by the mechanical construction of the variable tuning capacitor. Practically all multi-section tuning capacitors turn plates of the RF-tuning section along with plates of local-oscillator tuning section on the same shaft.
In some variables, plate shape of the RF-tuning section differs from plate shape of local-oscillator-tuning section, to aid tracking. In other variables, plate shape for both sections is identical: tracking is accomplished by choice of fixed coupling capacitors - an approximation to proper tracking. The choice of C28 and C30 would involve tracking.

Any designer ensures that Vcc does not vary at any frequency of interest, be it audio, RF, or local oscillator: you can assume it is at ground potential as far as AC signals are concerned. So one end of C28 is essentially grounded.
Why did they choose Vcc instead of GND? Perhaps a board layout issue: Vcc was more convenient. Or perhaps impedances of the Vcc trace (or GND trace) favoured stable operation...sometimes these circuits also oscillate at far higher frequency.

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