Elmer 101 Preliminary

AC and DC

You should already have a pretty good idea what DC is. AC could be RF signals, audio signals or noise. In many electronic circuits, AC signals are combined with DC so that transistors operate correctly. And this is where a lot of folks run afoul. DC biasing of transistors is necessary so that they can amplify AC signals linearly.

The AC signals move about the DC bias point, swinging more positive or more negative. If AC amplitude is so large as to change polarity on the peak of a swing, any transistor attached can't handle the polarity switch, and ceases to operate linearly, during that portion of the cycle.
You should become comfortable separating DC from AC, even though they may co-exist. For example, connecting a voltmeter to a circuit switched to a DC scale, will ignore any AC signals and give you the DC bias point only. When switched to AC voltage scale, the DC bias point is ignored, and the AC amplitude (usually RMS) is displayed. Oscilloscopes also have "AC" mode and "DC" mode. In AC mode, any DC bias voltages are blocked, and all AC voltages are shown swinging above and below zero volts. However, in DC mode, all voltages (both AC and DC) are shown.

DC biasing Transistors

In the SWxx+ radio, Q2, Q4, and Q5 are all biased with DC voltages so that they operate linearly. Actually, Q2 is an oscillator which starts off linear, and some short time later, the AC amplitude builds to swing more than the initial DC bias voltages and currents. It is designed to operate this way, and in normal operation, runs non-linearly. We'll look at it as a linear stage for now.

Let's strip all the AC components from these three transistors, leaving only the transistors, and the resistors which supply bias voltage and current. Any capacitors appear as open-circuits at DC, and any inductors appear as short-circuits for DC. Resistors treat AC and DC exactly alike, so we must include them in our DC model. With practice, you can scan through a schematic with your mind in "DC-mode" and see where DC currents flow, and where they do not. For example, Q4 is connected to Q5 through C34, a capacitor. Since no DC current can flow through it, the Q4 stage is DC-isolated from the Q5 stage.
Similarly, the transformer T4 between collector of Q5 and DC supply voltage appears as a short-circuit for DC. For AC signals, these capacitors and inductors appear very different, but we're in DC mode for now.

The DC voltage at the top is the supply voltage that ideally remains very constant. For Q2, supply voltage comes from a regulator U2, that supplies a very stable 8.0v regardless of your external (12v) DC supply stability. For an oscillator, DC supply stability aids frequency stability. At Q4 and Q5, stability is less important. These stages only see the supply voltage while transmitting.  Q3 acts as a switch, pulling supply voltage up near 12v when you hit the key. It drops to zero volts while receiving (key up). Here, it is shown @ 12v, and we're assuming that the transmitter is active.
At the bottom is zero volts, which is ground. This is where the external supply has its negative terminal connected. In Dave's schematics, these zero-voltage ground points are each shown as an isolated ground symbol, but they're really all connected together.  Both the supply voltage and ground voltage should contain no AC signals! It is almost always assumed that supply and ground are solidly DC only. Satisfy yourself that the schematic shown here, and the schematic in Dave's manual are the same for the condition of DC-only.

All the resistors shown in the schematic above are DC bias resistors. They all establish the DC operating point so that the transistor they're attached to operates linearly. Many considerations go into choosing the operating point. One of the most important for Q4 and Q5 is the one outlined above - where AC signals must have "swinging room" so that voltage and current polarity always remains the same, at each terminal of the transistor.
For the collector of Q4, that's easy - it always remains at +12v. For the base and emitter of Q4, we need to know through what range the AC signals will swing. Dave has conveniently marked his troubleshooting schematic with AC amplitudes. Both base and emitter will see an AC signal that swings through a maximum range of 1.5v. He's marked these as peak-to-peak voltage swings. This means that both base and emitter must be biased above zero volts by at least half of 1.5v, so that the negative- going swing won't go below zero volts. Since the base for any linearly-biased transistor is always about 0.6v higher than the emitter (for these NPN transistors), the minimum bias voltage for the base is 0.75v + 0.6v = 1.35v DC.
Dave has allowed generous head room, by biasing the base at 3.475v DC. This bias point is established by the resistor voltage divider, R22, R23. The emitter resistor R24 of 500 ohms sees about 0.6v less, at 2.875v DC. Thus, DC emitter current of 5.75 mA flows. With DC current gain of about 140, base current will be 140 times less, at about 0.04 mA.
There are many values of resistors that could establish this DC operating point - all could be scaled up or scaled down in value. Choosing the actual values is a compromise between too-high and too-low. You want them high, because these resistors load down and dissipate precious AC power. But you want them low, so that the DC operating point is firmly established.
You might think that that small 500 ohm emitter resistor is far lower in value than the multi-kilohm base resistors, and severely loads down the AC signal passing through. But at the base, that 500 ohm resistor appears far larger because of Q4's current gain. Although the AC signal at Q4 base is the same amplitude as at Q4's emitter, Q4 still has significant power gain because of this impedance scaling. Conversely, you can imagine that Q4 accepts a high-impedance signal at the base, and provides low-impedance drive at its emitter. Although the 500 ohm resistor does dissipate more AC power than the base resistors do, Q4 has excess power available at its emitter to drive the 500 ohm resistor, and then some.

DC bias of Amplifier Q5