During the troubleshooting process of a circuit, especially when dealing with complex faults and without a circuit schematic for reference, it is necessary to draw a circuit schematic based on the actual components and printed circuits on the circuit board.

I. Basic Approach to Drawing a Circuit Schematic

1. Minimize the Drawing Scope
   • It is unnecessary to draw the entire circuit schematic of the device. Based on the fault phenomenon and potential inspection steps, narrow down the fault to the smallest possible scope and only draw the circuit schematic for this specific scope based on the physical components.
2. Determine the Type of Unit Circuit
   • Identify the circuit type based on the characteristics of the components on the circuit board, such as whether it is a rectifier circuit in a power supply circuit or an amplifier circuit, to determine the general direction of the circuit type.
3. Select a Reference Circuit
   • Utilize learned circuits as reference circuits based on the specific circuit type. For example, for a full-wave rectifier circuit, first draw a typical full-wave rectifier circuit and then compare it with the actual circuit on the circuit board, making individual adjustments as necessary.
4. Verification Method
   • After drawing the circuit schematic, conduct a reverse check based on the drawn circuit schematic and the actual situation on the circuit board, i.e., verify whether each component in the drawn circuit is correctly connected on the circuit board. If there are discrepancies, it indicates that the drawn circuit schematic is incorrect.

II. Simple Method to Observe Copper Foil Circuit Paths

When observing the connections between components and copper foil circuits on the circuit board, as well as the direction of the copper foil circuits, you can use a lighting method. Shine a light on the side with copper foil circuits, and you can clearly and conveniently see the connections between the copper foil circuits and each component on the component side. This eliminates the need to flip the circuit board, which can be cumbersome and prone to breaking the leads on the circuit board.

III. Method to Observe Copper Foil Circuits on Double-sided Circuit Boards

1. Double-sided Board Structure
   • The following diagram is a schematic of a double-sided circuit board. Both the component-mounted side (top layer) and the back side (bottom layer) have copper foil circuits. Surface-mount components can be mounted on either the top layer or the bottom layer.
2. Via Holes
   • To connect the copper foil circuits on the top and bottom layers, via holes are set on the circuit board, as shown in the diagram below. Whenever copper foil circuits on the top and bottom layers need to be connected, a via hole will be set, and it needs to be connected to the copper foil circuits on both layers when drawing the schematic.

IV. Method to Draw a Circuit Diagram Based on Components

When analyzing the working principle of a circuit, the key is to identify the power supply voltage terminal (+V or -V), ground terminal, signal input terminal, and signal output terminal of the unit circuit. When drawing a circuit schematic based on a circuit board (since the distribution of printed circuit diagrams on the circuit board is inconsistent with the rules of circuit schematics), it is necessary to first draw the interconnection circuits between components and then connect them to the respective terminals. Here, a transistor circuit is used as an example to illustrate the specific drawing method.

1. Step 1
   • First, draw the graphical symbol for the transistor circuit, as shown in the diagram below. It is found that two components, R1 and C1, are connected to its emitter. If there are more components connected to the emitter of transistor VT1, they should all be drawn.
2. Step 2
   • After drawing R1 and C1, find the components or circuits connected to the other ends of R1 and C1 on the circuit board, as shown in the diagram below. It is found that R1 is connected to the ground, so the ground symbol can be directly drawn. C1 is connected to another resistor R2, so draw the graphical symbol for R2, which is in series with C1.
3. Step 3
   • Continue to find the circuit connected to the other end of R2 on the circuit board and discover the grounding wire, as shown in the diagram below. Typically, when a component is connected to the grounding wire or power supply, the drawing of this branch circuit can be completed.
4. Step 4
   • According to conventional drawing methods, organize the sketched diagram, as shown in the diagram below, for easier analysis and understanding of the circuit.

V. Method to Draw a Transistor Amplifier Circuit

1. Step 1: Draw the Graphical Symbol for the Transistor Circuit
   • First, determine whether the actual transistor on the circuit board is an NPN type or a PNP type. Currently, NPN transistors are used most frequently.
2. Step 2: Draw the Collector Circuit
   • Find the collector of VT1 on the circuit board and then draw all components connected to the collector, noting which resistor is connected to the power supply circuit. The diagram below shows a resistor connected between the collector and the DC operating voltage +V terminal, and note that the coupling capacitor is connected to the next stage amplifier.
3. Step 3: Draw the Emitter Circuit
   • The components on the emitter of VT1 generally connect in the direction of the grounding wire. Typically, there are many components connected between the emitter and the grounding wire, which may be capacitors or resistors, as shown in the diagram below, where a resistor is connected.
4. Step 4: Draw the Base Circuit
   • The components on the base of VT1 have three directions: power supply direction, grounding direction, and previous-stage circuit direction. The diagram below shows R1 in the direction of power supply +V, resistors mostly in the grounding direction, and coupling capacitors mostly in the previous-stage direction.
5. Step 5: Draw the Entire Transistor Circuit
   • Combine the circuit diagrams from the previous four steps to form a complete circuit diagram, as shown in the diagram below. This circuit reveals that it is a DC circuit for a common-emitter amplifier.

VI. Method to Draw Other Transistor Circuits

When drawing other transistor circuits based on the physical components on the circuit board, the key is to understand the general function of the transistor, such as whether it constitutes an oscillator circuit or a controller circuit. After determining this general direction, identify the specific type of circuit based on the characteristics of the components.

1. Step 1
   • Draw the DC circuit of the transistor, as transistors have DC circuits in most of their application circuits. If it is found during the drawing process that a transistor does not have a complete DC circuit, it is likely that the transistor is not operating in amplification, oscillation, or other states, but instead constitutes a special circuit, such as a transistor-based ALC (Automatic Level Control) circuit. Such judgments require a solid foundation of circuit knowledge.
2. Step 2
   • Draw the circuits of components connected to the three electrodes of the transistor.
3. Step 3
   • Organize the sketched diagram for easier analysis of the circuit’s working principle. If obvious circuit errors are found during the analysis, it is likely that the circuit diagram is incorrect and needs to be verified against the circuit board.