I dabbled a little into tubes but let us go back to the transistor radio. The first lesson I would offer the nubie is about the transistor. THE TRANSISTOR (TRANNY FOR SHORT) Ok, the tranny is a semiconductor device. It is natural material, silicon. It has 3 layers, the outer layers the Emitter and Collector and the middle layer is the Base. Tickle the based and electrons flow between the E and C. Let us talk of NPN trannies. Electrons flow from the E to the C if B is made a bit positive. Let us use an analogy, the faucet. The E is where water comes in, the C is where it comes out. The B is the rotary valve. We can shut off the faucet, open it slightly or open it full. We can make the outlflow of water dance by playing with the knob. Back to the tranny. We ground the E to the minus rail. If we connect the C to the plus and put some plus to the B.ZAP! we can blow up the tranny. Too much current flowing and we shorted it! We should limit the current even if the tranny is fully ON. How, we put a resistor in series as a ballast. When we turn in on fully, the resistor heats up and the tranny might survived. Or even better, we series an LED from the plus to the C. Get a 1k R and tap the plus and the Base. VOILA! the LED lights up and the LED itself limits the current flowing between the E and C. Now get a 5k volume control, ground one end and put the other end to Plus. The mid pin goes to the base. We have a voltage divider. Turn the POT and control the brightness of the LED. Okay, Okay.... 1. When a plus is put on the Base, it is called a FORWARD BIAS. 2. When it is minus, it is called REVERSED BIAS. Take a look at the first tranny in the AM radio. It is about 820k connected to the Base and the plus line. So, the tranny is forward biased a bit. The faucet is just slightly open. That is the region where the tranny is very sensitve to the signal to its Base from the antenna coil and the signal from the oscillator coil to its E. The signal from the OSC coil is NOT shorted cuz the E is not grounded. There is a resistor there to ground. So, in respect to ground, the E is a plus! What are the parameters of the tranny? Well, first it has to be able to handle a certain frequency. The 9011 is an RF tranny and can handle probably up to 100 Mhz. The next parameter is the current handling capacity. For the RF tranny, it does not have to be able to handle high currents. For the audio ampli trannies, the 9012 (PNP) and 9013 (NPN), these are high current trannies but does not have to be high frequency trannies. The third parameter is the BETA, the amplification ratio or factor. EXERCISE: Google the trannies and note their parameters. THE INTERMEDIATE FREQUENCY TRANSFORMERS These are no different from the traffo in your power supply project. They have a primary and a secondary windings. They also have ratios. In your p.s. traffo, the primary and secondary ratio is 220:12 if your secondary is to give 12 volts. The IFTs is designed to be able to operate MAX at a certain frequency, 455 khz. To fine tune, the ferrite slug is moved up or down. Now, take note. From the yellow IFT to the next tranny, there is no wired connection! The signal is magnetically transferred to the secondary and on to the next tranny. This is called "isolation". BE CAREFUL YOU DO NOT SHORT THE PRIMARY OR SECONDARY OF THE IFTs, YOU CAN BURN THEM. AND THAT IS WHY BEFORE YOU INSTALL THE IFTs, CONDUCT CONTINUITY TEST TO THE BOTH PRIMARY AND SECONDARY. SET TESTER TO RX1 AND TEST. THE OSCILLATOR COIL The OSC coil is also a traffo cuz it also has a P and S. DO THE SAME TEST AS THE IFT. The OST(lets call it that) gets fed a signal from the C of the first tranny. The signal is magnetically transferred to the other side of the OST which has 3 pins. The outer pins are paralled the second gang tuning capacitor. It tunes the OST. TAKE NOTE OF THE CAPACITOR TO THE MIDDLE PIN. IT GOES TO THE E OF THE FIRST TRANNY. SO, WHEN A SIGNAL IS FED THROUGH ITS PRIMARY, IT IS FED BACK TO THE TRANNY VIA ITS SECONDARY. AND SO AN OSCILLATION TAKES PLACE. THE DIODE This section is called the detector cuz it detects the audio. But what it does, it simply converts the AC to DC just like your power supply project. Note the caps before and after the resistor after the diode. That is the filter! THE AGC This is something you will never learn if you jut assemble an AM radio from a kit. AGC stands for Automatic Gain Control. Some stations are very strong and some are weak. We need something like a governor. In steam engines, they have a mechanical governor. Two balls is rotated by the engine. The balls fly out further the faster the speed. Two levers are set on the center rod which compresses the higher the speed. It pushes a valve off to slow down the steam. SAME PRINCIPLE IN THE AM RADIO. Take note, the DC from the diode is brought back to an earlier tranny via a resistor in series. It connects to the Base. Take a look at the polarity of the diode. It is a minus out A strong signal will make the minus greater and it is fed back to the base of the tranny to slow it down. When a signal is weak, that tranny is more sensitive. Got it?
To be continued.........