This is the XTUGA 4-channel wireless microphone system, UHF set (includes 4 handheld microphones) S400. Regarding the B microphone, it seems the power ON/OFF switch has poor contact, and it doesn't connect immediately after being turned ON. It connects to the receiver if the power switch is slightly adjusted up and down. Also, the negative spring in the battery case of the C microphone has come off, so a general-purpose replacement has been inserted as shown in the photo. About this product: [Professional UHF Wireless Microphone System] Features automatic channel synchronization with UHF fixed frequency band design. Intelligent tuning automatically detects and avoids frequency interference, achieving clear tone reproduction faithful to the original sound. High-durability design with environmental noise resistance. [Long-Distance Transmission Performance] A wireless system capable of long-distance use up to 80 meters in a clear environment with a wide open space. The improved dual-antenna design with two antennas built into the back of the receiver significantly improves radio wave reception sensitivity. This enables high-precision frequency stability and reliable signal transmission. [Professional Sound Quality Design] The rugged all-metal construction for the quad-channel UHF wireless microphone set receiver and high-sensitivity unidirectional handheld microphones is particularly resistant to impact and corrosion. The dynamic microphones pick up clear, pure full sound, faithfully reproducing from rich bass to transparent treble, perfect for use at parties, weddings, meetings, etc. [Wide Compatibility] Four XLR outputs and one 1/4 inch (6.35mm) mixed output allow this versatile system to connect to most amplifiers/mixers/powered speakers on the market. [Multi-Scenario Support] Designed for simultaneous use by 4 people, they can be used perfectly for karaoke with friends and family, parties, churches, DJs, weddings, business meetings, classroom activities, outdoor or indoor stage performances, etc. Thank you for viewing.