Yaesu MD-100 A8X Go to product viewer dialog for this item. is a popular dynamic desk microphone featuring built-in audio filtering and scanning controls. Its schematic highlights a sophisticated pre-amplification and filtering circuit centered around op-amps like the PC4572C. 🛠️ Wiring & Pinout Guide Go to product viewer dialog for this item.
typically uses a reversible cable that supports both 8-pin round and RJ-45 modular jacks. Color (Standard) 1 UP / Fast Scan Pink / Yellow 2 3 4 FAST / MIC Yellow / White 5 6 PTT (Push-To-Talk) 7 MIC Ground / Shield 8 White / Red Note: Wire colors may vary by manufacturing batch. 📉 Audio Filtering Controls
includes a "Main" bypass switch and two specialized filters located on the bottom of the base: Low Cut Filter: Position 1: -3dB roll-off from 1000Hz down to 150Hz.
Position 2: -3dB roll-off from 1000Hz down to 300Hz (for noisy conditions).
High Emphasis: Boosts higher frequencies by +6dB at 3000Hz to improve clarity in pileups.
Main Switch: Must be set to "FILT" for the Low Cut or High Emphasis settings to take effect. 🔍 Technical Resources YAESU- MD-100/100AX schematic.pdf - QSL.net
This is the heart of the schematic. A low-noise NPN transistor (often a 2SC945 or similar) amplifies the weak signal from the capsule. The gain is set by feedback resistors around the transistor. Power for this stage comes from the transceiver’s microphone jack via the +5V or +9V line.
If you own a Yaesu FT-857, FT-897, FT-950, or any of the "GX" series mobiles, chances are you’ve either coveted an MD-100 desktop microphone or you have one sitting on your bench with a dead up/down button or distorted audio.
The MD-100 is beloved for its heavy base, adjustable stand, and surprisingly good electret condenser element. But like any piece of gear from the late 90s and early 2000s, the electrolytic capacitors dry out, the muting switch gets scratchy, and the proprietary voltage regulator circuit can fail.
Finding an official Yaesu MD-100 schematic is surprisingly difficult. Yaesu treats these like trade secrets. After reverse-engineering several units and consulting service bulletins, I’ve reconstructed the essential schematics you need to troubleshoot this mic.
While we cannot reproduce the entire copyrighted schematic here due to legal restrictions, we can describe the most critical sections that every technician should recognize. If you obtain a legitimate copy from Yaesu or a service manual archive, look for these reference designators:
Because the DC supply from a radio can be noisy (especially during transmit), the MD-100 includes an RC (resistor-capacitor) filter network and sometimes a Zener diode regulator (e.g., RD5.6EB) to provide a clean, stable voltage to the electret capsule.