Microlab FC730 background in speakers, improvements
Minor repair and modifications to a Chinese Microlab FC730 multichannel amplifier. A humming noise is present in the speakers. According to the owner, it developed over time. The cause is the resistors overheating in the electrolytic capacitors. The electrolytic capacitors dry out, disrupting the operation of the voltage regulator. An additional regulator was installed, changing from 24 volts to 15 volts, and the single-transistor regulator was replaced with a 7809 integrated circuit. I also replaced all 0.1 μF electrolytic capacitors with film capacitors, as it's advisable to use electrolytic capacitors with a capacitance reserve when using them as decoupling capacitors. Here, they also function as a high-pass filter, cutting off the low frequencies to the satellites (in a system with a subwoofer).