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Below is a functionally equivalent schematic of the MD-100’s internal preamp board.
The original factory layout for the maps as follows: Pin Number Factory Wire Color Pin 1 UP Controls frequency upward scanning Pin 2 +5 Volts DC Powers the internal active op-amp Pin 3 DOWN Controls frequency downward scanning Pin 4 FAST Speeds up VFO scanning speeds Pin 5 Ground (GND) Logic, PTT, and chassis ground Pin 6 PTT Push-To-Talk control line Pin 7 MIC Shield Bare Shield Wire Isolated ground explicitly for audio Pin 8 MIC Audio Positive (+) microphone audio signal
– This site hosts many Yaesu schematics (requires free registration). Yaesu Md 100 Schematic
The MD-100A8X typically uses a round 8-pin microphone connector, though it can be adapted to RJ-45. According to standard Yaesu MD-100 documentation , the 8-pin pinout is as follows: 2 3 4 5 6 7 MIC Shield 8
Before touching the PCB, discharge static – the 2SC945 transistor is sensitive. For high-power HF operation, keep the mic cable as short as possible (<2m) to avoid RF ingress. Below is a functionally equivalent schematic of the
: A three-position switch (Off, 1, 2) that provides rolloff at approximately 150 Hz and 300 Hz. High Emphasis Filter : Provides a +6 dB boost at 3,000 Hz to help voice signals cut through interference. Dual Connector Interface
To build a custom cable or troubleshoot continuity issues, you must match the schematic nodes to the physical pins on the microphone jack. Yaesu has largely standardized its 8-pin round assignment for decades. Pin Number Signal Name Description UP / DOWN / FAST Control line for frequency scanning resistors Pin 2 +5V to +8V DC Phantom power from the rig (used for active filtering/LEDs) Pin 3 Push-To-Talk control line (Ground to Transmit) Pin 4 Dedicated ground return for the audio signal Pin 5 Unbalanced microphone audio output Pin 6 EARTH / GND Common chassis ground for PTT and shielding Pin 7 NC / MODULE No connection (or used for specific modular configurations) Pin 8 SPEAKER / NC According to standard Yaesu MD-100 documentation , the
The schematic will show a momentary switch (Push-To-Talk) that grounds a specific pin (usually pin 4 on the 8-pin round connector).
Let's apply the schematic to real-world problems.
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