How to repair imported electric screwdriver

First, the repair tools

For this repair task, you'll need a few essential tools. Two multimeters—one for analog and one for digital (the analog one is crucial), diagonal pliers, needle-nosed pliers, a soldering iron with a stand, flux, solder wire, desoldering braid, fine sandpaper, heat shrink tubing, a wire stripper, small and medium-sized slotted screwdrivers, small and medium Phillips screwdrivers (including one with a rubber handle), and an art knife. These tools will help you perform precise and safe repairs.

Second, the working principle of the circuit

The circuit operates in the following way: Input (from a protector and micro switch) → Rectification → Filtering → Positive/Negative Switch → Motor → Clutch. Understanding how each component functions is key to identifying and solving problems. Here are the normal parameters for each part:

(1) Power line: Measure the DC resistance. A normal value should be close to zero. Common issues include open circuits or poor contact.

(2) Micro switch: Check resistance between normally open (which should be close to infinity) and normally closed (close to zero). Faults may involve burnt contacts, failure to reset, or physical damage.

(3) Protector: The resistance should be zero under normal conditions. It can reset automatically after a minor overload but may become permanently disabled if the overload is too severe. Some protectors also disconnect due to high temperature and reset when cooled down.

(4) Rectifier bridge: Test the DC resistance between the positive and negative outputs, which should be around 15K ohms.

(5) Positive/Negative switch: The pin connected to the toggle button is common, and the two positions of the switch are connected accordingly.

Third, circuit detection and analysis

When dealing with an electric batch that has an unknown fault, start with a preliminary test. Set the multimeter to ×1K range, press the micro switch, and measure the DC resistance at the input. If it reads around 15K (or about 1.0V on a digital multimeter in diode mode), the circuit is likely functioning properly. Another method involves shorting the motor after turning it on. Normally, the input resistance should be slightly lower than the initial reading, excluding the motor’s own DC resistance.

Fourth, the components of the motor

A typical electric motor consists of several key parts: the armature (rotor), carbon brush holder, motor front cover, bearings, magnet (stator), carbon brushes, and a motor gasket. Each plays a vital role in the motor's performance:

(1) Armature: This is the heart of the motor. Its condition determines whether the motor works properly. To check its quality, measure the DC resistance between adjacent poles on the commutator. Under normal conditions, this should be around 15 ohms (varies by manufacturer; values over 20 ohms may indicate issues). Signs of damage include worn commutators, a misaligned central axis, or open circuits between poles. Open circuits can prevent the motor from starting or cause it to stop when the carbon brush hits the break.

(2) Carbon brush holder: Inspect visually for damage or internal discoloration (blackening).

(3) Motor front cover: Check for visible damage or deformation.

(4) Stator: Look for internal disk damage or displacement, which can cause magnetic imbalance. Use a flathead screwdriver to test the magnet’s strength; if it’s weak, replace the magnet.

(5) Bearings: Listen for unusual noises or resistance during rotation. Excessive friction may lead to loud motor sounds.

(6) Carbon brushes: Replace them when they wear down to about 2–3 mm. Regular replacement is recommended.

(7) Gasket: Ensures the armature remains stable and prevents excessive movement. Proper shimming is important for smooth operation.

Fifth, clutch

The clutch is another critical component. Common faults include:

(1) Inability to grip the screwdriver head: This could be due to a broken drive shaft, which needs to be replaced. If the slot becomes too large, the drive shaft may fall off automatically.

(2) Missing parts: Replenish or replace as needed.

If the drive shaft is damaged, check the appearance of the screwdriver head for heavy wear. If it’s severely worn, replace it. For internal breaks, insert the fan into the clutch and snap it. Then, use a screwdriver to twist the drive shaft and feel for any resistance or irregularity. This helps determine if the drive shaft is still functional.

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