November 6, 2017 Hi, its Monday! Soon December for all you know it. Let's learn AM radio before they invade North Korea. All Hell will break loose. See headline today. Ok, so far I explained the AM radio in general. Time to explain certain sections and componenets in details. Let us start with the front-end, the antenna. It's coils of wires. Well, there are two coils in there, the long one which is tuned by the variable cap. Then the short coil which connects to the B of the first tranny. Why 2? Well, first it has something to do with IMPEDANCE. This has been a difficult topic to explain really. It is also measured in ohms but your tester-ohmeter cannot test it. Well, lets talk of the speaker where many of us first encountered IMPEDANCE. Most speakers have 8 ohms. The world started with tubes and the electrons flowing through a tube is moving at a very high rate (that is the Voltage). Hi-Fi's has over 500V DC. It can operated at 90 V. Energy is in watts and to get it, it is voltage x amperes. You will note the tubes operated at high V and Low ampere in milli-amps in fact. It is very similar to the step down traffo you have for your power supply project. Primary is 220V and secondary is 12V. The wattage on either side is the same. Let us start with the seconday, 12V at 1 amp. that will be 12 watts. So, what will be the amps in the primary. It will be 12W/220V, very tiny. It is called inversely proportional. Back to the tube amp and the speaker. if one were not to use a final traffo, one has to use a very high impedance earphone or speaker. The final traffo is really just a step down traffo. Something like 550V to less than 20 V to the speaker. BUT THE CURRENT IS INVERSELY PROPO. SO, HIGH CURRENT DRIVES THE 8 OHMS SPEAKER. Now, the epeaker should have less turns but with heavier wire to carry more current. Impedance is close to heaving a 12 V bulb connected to a 12V source. Back to the antenna. The tuned signal from the long coil is transformed to a lower impedance in the short coil. The signal in the short coil actually had more current. It is fed to the B of the first tranny. One added advantage is that the tuner is isolated from the input load at the B of the tranny. OSCILLATORS How do they work? Look at the oscillator coil, the red traffo. The primary has e pins, the secondary has 3. It is the secondary that is tuned. See the capacitors across it. The primary has no cap and is picking up whatever go through the collector of the first tranny. A COIL AND A CAP IN PARALLEL IS CALLED A "TANK" CIRCUIT. If energy is introduced, the cap discharges to the coil and the coil will be magnetized but when the field collapses, it recharges the cap! It will oscillate at a frequency depending on the coil and the cap. The tuning cap is able to change the oscillation. The red ferrite slug affects the inductance of the coil and can also change the frequency. The local oscillator is designed to oscillate at about 1 khz to 2khz.