The MD-100A7 does not use a simple dynamic element. It uses an electret capsule requiring 2–5V DC. The Yaesu "GX" radios provide a low-current +5V DC on the mic line (Pin 1 of the RJ-12). Inside the MD-100, a 3-terminal regulator (often a 78L05 or a surface-mount equivalent) stabilizes this to clean 5V for the FET inside the electret capsule.
Let's apply the schematic to real-world problems.
Before you close the mic base:
The MD-100 is a tank. With a $5 capacitor kit and this schematic in your head, you can keep it running for another 20 years.
Have you repaired an MD-100? Found a different schematic variation? Let me know in the comments below.
— 73, and happy soldering.
The complete schematic for the Yaesu MD-100/100A8X Go to product viewer dialog for this item. Yaesu Md 100 Schematic
covers the microphone head, the active filtering circuitry in the base, and the output wiring for both 8-pin round and modular (RJ-45) connectors. Key Circuit Sections
Microphone Element: A high-quality dynamic element with a 500-600 ohm impedance.
Active Filters: Powered by +5V from the radio, the circuit uses μPC4572C op-amps for "High Emphasis" and "Low Cut" filtering. Controls:
Main Thru/Filt Switch: Bypasses or engages the active audio processing.
Low Cut Switch: Offers two stages of low-frequency attenuation.
High Emphasis Switch: Amplifies highs while cutting lows for DX clarity. The MD-100A7 does not use a simple dynamic element
Scanning Control: A rotary control for Up/Down and Fast/Slow frequency tuning. Wiring Pinouts The MD-100 uses a standard Yaesu 8-pin configuration. Wire Color 1 2 3 4 Fast (on some models) 5 6 7 Mic Ground (Shield) Shield/No Color 8 Documentation Resources
For a full high-resolution "complete piece," you can view the official technical sheets here:
Full Schematic Diagram (QSL.net) — Includes all component values and PCB traces.
MD-100A8X Technical Sheet (RadioManual.info) — Detailed circuit breakdown for the A8X variant.
Official Operating Manual (Yaesu) — Covers switch settings and installation.
💡 Pro Tip: If you're using this with an older radio like the FT-897, keep the base switch set to "THRU" to avoid distorted audio reports. The MD-100 is a tank
Are you looking to modify the audio levels or repair a specific part like the PTT switch or the scanning dial? YAESU- MD-100/100AX schematic.pdf - QSL.net
I’m unable to directly provide or host the full schematic for the Yaesu MD-100 desktop microphone, as it is a copyrighted technical document. However, I can point you toward legitimate sources where you may find it:
Mods.dk – This site hosts many Yaesu schematics (requires free registration).
Repeater-Builder – Often archives Yaesu microphone and radio service info.
If you are troubleshooting or modifying the MD-100 (e.g., for pinout or electret/element bias), I can summarize the critical electrical connections or draw a simplified block diagram from memory/common knowledge. Just let me know what specific detail you need (e.g., pinout to FT-847, FT-817, or general 8-pin round connector wiring).
The schematic for an MD-100A8X (designed for the FT-100/FT-817/FT-857 series with an 8-pin round plug) is different from the MD-100C (designed for Icoms with a modular plug). Ensure you are searching for the exact suffix.
| Pin | Function | Wire Color (Internal) | Voltage | | :--- | :--- | :--- | :--- | | 1 | GND (shield) | Black | 0V | | 2 | MIC + (Audio out) | White | 0.5-2V DC bias | | 3 | PTT (Push to Talk) | Yellow | 0V when pressed | | 4 | +9V DC (from radio) | Red | 9-13V (via radio’s mic jack) | | 5 | NC (not used) | - | - | | 6 | DOWN (scan down) | Green | 0V pulse | | 7 | UP (scan up) | Blue | 0V pulse | | 8 | GND (second ground) | Black | 0V |
⚠️ The MD-100 is active – it needs +9V on pin 4 from the radio (e.g., FT-857, FT-991, FTDX series).