Yaesu Md 100 Schematic -

Inside the Yaesu MD-100: A Schematic Deep Dive & Repair Guide

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 Yaesu MD-100A8X is a dynamic desk microphone that includes an internal preamplifier and active filtering. The schematic reveals a circuit centered around the $\mu$PC4572C operational amplifier. This IC handles the audio amplification and the "High Emphasis" and "Low Cut" filtering stages. Circuit Overview The schematic is divided into several functional blocks: Microphone Element: A 600-ohm dynamic element. Yaesu Md 100 Schematic

  • D1 (1N4148): Prevents voltage from the LOCK switch backfeeding into the MOMENTARY switch circuit.
  • SW1 (Lever Switch): The side lever. In the "LOCK" position, it physically latches a second set of contacts that permanently ground the PTT line (through Pin 4) until released.
  • SW2 (Momentary): The footswitch jack or front button – a simple normally-open pushbutton.

Active Filters: Powered by +5V from the radio, the circuit uses μPC4572C op-amps for "High Emphasis" and "Low Cut" filtering. Controls: Inside the Yaesu MD-100: A Schematic Deep Dive

: A master bypass switch. In "Thru" mode, the signal bypasses the active EQ, which is essential for radios with their own internal EQ. Low Cut Switch : Offers a standard response and two stages of attenuation. Position 1 provides a -3 dB roll-off from 1000 to 150 Hz, while Position 2 D1 (1N4148) : Prevents voltage from the LOCK

4. Common MD-100 Problems & Fixes

| Symptom | Likely Cause | Solution | |---------|--------------|----------| | No audio, but PTT works | Dead electret capsule or Q1 transistor | Replace capsule (Panasonic WM-61A works great). Check Q1 voltage: collector should be ~4.5V. | | Weak / muffled audio | Dried-out C2 (1µF) output cap | Replace with 2.2µF film cap for improved bass. | | RF feedback on transmit | Missing C3 (100pF) or broken ground | Solder 100pF from Pin 2 to Pin 1 (GND) near connector. | | UP/DOWN buttons erratic | Dirty contacts or broken 100Ω resistors | Clean switches with DeoxIT; replace resistors. |

Important Notes

  1. No Unauthorized Schematics: Yaesu (and most manufacturers) do not publicly share full schematics for commercial models. For genuine models, contact Yaesu or consult a licensed repair shop.
  2. Safety First: Always discharge capacitors before working on power supplies.
  3. Alternative Resources: Use simulation tools like LTspice to model RF circuits.

Final Check

Before you close the mic base:

Active Filtering: Note that the High Emphasis and Low Cut switches are disabled if your older radio doesn't provide voltage at the mic jack. 3. Common Troubleshooting & Mods