You're nearly there with the two IF's, 455kHz is the AM IF and 10.7MHz is the FM IF, you will already see that in the IF amplifier, the primary tuned circuit of the AM transformer and the primary tuned circuit of the FM transformer are connected in series between the HT feed and the anode of the IF amplifier, in comparative terms the AM tuned circuit will be high Q and it's inductance value will be high, the FM tuned circuit will be a lower Q, and it's inductance value will be low, the higher the Q the narrower the bandwidth, the lower the Q the wider the bandwidth, this suits the type of transmissions being discussed just fine because the bandwidth of the AM transmission is only a few kHz while that of the FM transmission is much higher and is several 10's of kHz.
You should also be able to see how those two tuned primaries can work in series at the different IF frequencies, you've already done the understanding of the reactance bit for L and C so let's take the AM 455kHz tuned circuit first, at that frequency it will be resonant and it's impedance will be high, the tuning capacitor of the FM transformer will have a high reactance at that frequency because the capacitor has a relatively low value, but the value of the inductor is low and therefore it's reactance at 455kHz will be low so in effect the anode end of the 455kHz transformer is now connected to the IF amplifiers anode via a low reactance path, eg: the anode load at that frequency in effect just consists of the high impedance primary tuned circuit of the 455kHz transformer, this is what we want because the maximum voltage will be developed across it at resonance.
Now the FM 10.MHz tuned circuit, one end is connected directly to the anode of the IF amplifier and the other end to one end of the AM 455kHz transformer, all still in series, this time were dealing with an IF of 10.7MHz, the FM transformer is resonant and therefore has a high impedance at 10.7MHz but the HT end of the transformer needs a low impedance path to chassis in order to decouple it, the decoupling capacitors are C40 and C41 which are connected in series, the path to them has to be via the AM IF transformers tuned circuit, that tuned circuit has a high value of inductance and a high value of capacitance relative to the values used in the FM transformers tuned circuit, this means that the AM IF transformers tuned circuit inductor will have a high reactance value at the FM IF frequency but the capacitors reactance will be low, around 50 ohms in fact, so now in effect we have a high impedance anode load at 10.7MHz which will develop the maximum voltage across it at that frequency, this again is what we want.
Hope I've got all that right and that is of some help.
Lawrence.
You should also be able to see how those two tuned primaries can work in series at the different IF frequencies, you've already done the understanding of the reactance bit for L and C so let's take the AM 455kHz tuned circuit first, at that frequency it will be resonant and it's impedance will be high, the tuning capacitor of the FM transformer will have a high reactance at that frequency because the capacitor has a relatively low value, but the value of the inductor is low and therefore it's reactance at 455kHz will be low so in effect the anode end of the 455kHz transformer is now connected to the IF amplifiers anode via a low reactance path, eg: the anode load at that frequency in effect just consists of the high impedance primary tuned circuit of the 455kHz transformer, this is what we want because the maximum voltage will be developed across it at resonance.
Now the FM 10.MHz tuned circuit, one end is connected directly to the anode of the IF amplifier and the other end to one end of the AM 455kHz transformer, all still in series, this time were dealing with an IF of 10.7MHz, the FM transformer is resonant and therefore has a high impedance at 10.7MHz but the HT end of the transformer needs a low impedance path to chassis in order to decouple it, the decoupling capacitors are C40 and C41 which are connected in series, the path to them has to be via the AM IF transformers tuned circuit, that tuned circuit has a high value of inductance and a high value of capacitance relative to the values used in the FM transformers tuned circuit, this means that the AM IF transformers tuned circuit inductor will have a high reactance value at the FM IF frequency but the capacitors reactance will be low, around 50 ohms in fact, so now in effect we have a high impedance anode load at 10.7MHz which will develop the maximum voltage across it at that frequency, this again is what we want.
Hope I've got all that right and that is of some help.
Lawrence.
