Micro DC motor speed stabilization circuit

Micro DC motors are commonly used in tape recorders, and the matching electronic speed stabilization circuit is shown in the attached figure.

R2, RP, R3, R5, and R6 form a voltage-dividing sampling circuit. RP is a speed control potentiometer, VT1 is an error voltage amplifier, VT2 is an adjustment tube, R5 and R6 (in parallel) are current-limiting resistors, L is a high-frequency choke inductor, C1 is a power supply filter capacitor, and VD1, VD2, and C2 are used to eliminate high-frequency noise to ensure stable operation of the speed control circuit. VD3 and VD4 play a compensation role. The error voltage is taken from both ends of the motor M. When the voltage across M increases, causing its current to increase and the speed to increase, the voltage drop of R5 and R6 increases accordingly. The base potential of VT1 rises. The current of the collector of VT1 (i.e., the base of VT2) decreases, and the dynamic resistance of VT2 increases, which eventually reduces the current passing through the motor M and reduces the speed to a level close to the normal level, thereby achieving loop speed control; vice versa.

The main fault of the speed stabilization circuit is the deterioration of the speed stabilization performance, which makes the motor M rotate too fast or too slow. According to maintenance experience, the main cause of the fault is the deterioration of the performance of VT1 and VT2. Other components generally do not have problems. When the penetration current of VT1 and VT2 becomes larger, the motor speed will increase; when their β decreases, the motor speed will slow down, etc. VT1 can be replaced with 3AX22, 3AX31B, 3AX52C/D, etc., and VT2 can be replaced with 3DG12B, 9013, 8050, etc.