If you own an Icom IC-705, you might already know that the PTT (SEND) keying circuit is one of the radio’s few "Achilles' heels." Many users have reported the internal MOSFETs blowing out, especially when triggering external amplifiers or high-inductance relays.
The Problem: Fragile Factory MOSFETs
The IC-705 uses a pair of LN2306LT MOSFETs (Q31 and Q32) in parallel to handle the PTT switching to ground. While fine on paper for very light loads, they are quite vulnerable in real-world ham radio environments:
Voltage Limit: The LN2306LT is rated for only 20V (Vds). In a 13.8V system, any "flyback" voltage spike from an external relay can easily exceed this 20V ceiling, causing an instant short circuit.
Current Handling: It’s rated for roughly 2.3A, which sounds like enough, but it has very little "thermal headroom" when things get messy.
The Solution: The AO3400 Upgrade
When my PTT circuit inevitably failed, I decided not to just replace the parts with the same fragile factory components. Instead, I upgraded both MOSFETs to the AO3400.
Why the AO3400 is a superior choice:
Voltage Margin: It bumps the Vds limit from 20V to 30V. That 50% increase in voltage tolerance provides a much safer buffer against inductive spikes.
Current Capacity: The AO3400 is a beast for its size (SOT-23), rated for up to 5.8A. It handles the switching load with zero sweat.
Efficiency: It has a significantly lower On-Resistance , meaning it stays cooler even under heavy use.
Adding the "Armor": Protection Diode & External Buffer
To make this repair permanent, I didn't stop at the transistors. I added two more layers of protection:
The Protection Diode: I installed a high-speed diode (like a 1N4148 or a Schottky) across the PTT line to ground. I soldered it on the back of the PCB, right at the solder pads of the PTT connector. By placing the cathode to the signal and anode to ground, any back-EMF or voltage spikes from external gear are clamped and shunted to ground before they even reach the MOSFETs.
External PTT Buffer (Optional) : As a rule of thumb, I now run an external PTT buffer between the IC-705 and my amplifier. This ensures the radio's internal circuit only sees a tiny, controlled load, while the buffer handles the "heavy lifting" of the external equipment.
A Word on the USB-C Mod Boards
You may have seen replacement boards online that swap the Micro-USB port for a USB-C port and include "upgraded" transistors. While those boards are convenient, they are often expensive and do not increase charging or data transfer speeds; they are essentially a physical port swap with a transistor repair.
If you are comfortable with a soldering iron, performing the AO3400 upgrade yourself is a much more cost-effective and technically satisfying way to fix the issue. You aren't just swapping a board; you are actually improving the engineering of the radio.
Conclusion
The IC-705 is an incredible piece of kit, but the PTT circuit needs a bit of love to be truly "field-ready." By upgrading to AO3400 MOSFETs and adding a simple protection diode, you can key your external gear with total peace of mind.
