Some operators remove the internal resistors or add capacitors to modify the tone frequency. Be careful, as hacking the active circuit can inject unwanted RFI (Radio Frequency Interference) straight into your transmitter.
The "UP" and "DOWN" buttons interface with the transceiver's logic via a dedicated pin (Pin 3 or 4 depending on configuration). 3. Yaesu MD-100 Pinout Diagram (8-Pin Round)
Because the output level of an electret element is relatively low compared to what some older Yaesu rigs expect, the MD-100 includes a built-in preamplifier circuit board located under the main chassis.
This dual op-amp processes the signal, providing gain and active filtering capabilities. Filters:
The schematic will show a momentary switch (Push-To-Talk) that grounds a specific pin (usually pin 4 on the 8-pin round connector).