IC-705 TX Inhibit and ALC
Aug 28, 2026
I use an Icom IC-705 and a homebrew Arduino-based sequencer for my 10 GHz EME station and have completed many QSOs over the past 18 months. I put the sequencer together a couple of years ago and it follows the same pattern as many other designs, protecting the waveguide antenna switch and LNA. Part of the control circuitry is a transmit inhibit stage which applies -4 V to the ALC/SEND connector on the IC-705. This prevents RF from being generated when the PTT is keyed until the ALC voltage drops to 0 V. I used the same technique many years ago with an Icom 251E on 144 MHz, and decided to implement it here after reviewing the sequencer designed by Bertrand, VE2ZAZ / VA2IW [1].
Slow RF power ramp up
During some recent bench tests using the Tune mode in WSJT-X I noticed that the IC-705 ALC meter was peaking at the start of the transmission and RF output slowly ramping up. I always assumed this was part WSJT-X and part IC-705, as my 144 MHz station using an Elecraft K4D also ramps up power, but over a shorter period. I decided to take a closer look with a scope and ran a series of tests with and without ALC control.
All tests were run with the IC-705 @ 144 MHz, RF power drive level 2% (~20 dBm / 100 mW), connected to the scope via a 10 dB attenuator.
No ALC TX Inhibit
To obtain a baseline I started by measuring how PTT over CAT behaves without a negative ALC voltage. When Tune is pressed, WSJT-X issues the PTT over CAT command and the IC-705 SEND output triggers the sequencer PTT input. Without the ALC clamp active, RF generation is not synchronised with the sequencer. If the WSJT-X TX Delay is set too low, RF would emerge before the waveguide switch has completed changing over, risking a hot-switching failure.
- Trigger 0 ms: PTT Input on the sequencer drops to 0 V (Yellow).
- Delay 0 - 350 ms: WSJT-X TX Delay 200 ms + sound card latency 150 ms.
- 144 MHz RF 350 ms envelope at full amplitude (Blue).
When Tune is stopped, WSJT-X halts the audio stream, the RF envelope drops, and after ~500 ms the PTT line goes high.
ALC TX Inhibit
For the next test I re-connected the ALC line to the IC-705. With the ALC clamp now active as the final sequencer stage for the receive to transmit condition, RF generation can only occur when all sequencer stages are complete and the waveguide switch is in the correct position.
- Trigger 0 ms: PTT Input on the sequencer drops to 0 V (Yellow).
- ALC 0 - 200 ms: sequencer completes its intermediate stages, IC-705 ALC input drops to 0 V (Purple). 200 ms WSJT-X TX Delay completes within the same period.
- 144 MHz RF envelope begins to emerge ~ 1.5 s after ALC reaches 0 V and requires a further ~ 2 s to ramp up to full power.
Carsten, OZ9AAR noted a similar issue with the IC-9700 in 2024, but the delay was much shorter at 80 ms [2].
LTspice model
After locating the ALC circuitry in the IC-705 Service Manual, I put together a LTspice model to simulate the behaviour I was seeing on the bench.
The IC-705 uses a large 10.4 s RC time constant network that is shared by the ALC loop and other protection circuits. The ALC circuit only utilises a small portion of the total available RC curve and has two distinct phases.
- ~1.2 s Op-amp Threshold Delay: The period between ALC clamp voltage (Red) being removed and the op-amp node (Green) crossing 0 V. During this time the ALC output (Blue) stays at its maximium, meaning zero RF power is generated.
- ~3.0 s RC Decay Window: The time required for the ALC output (Blue) to slowly decay from maximum suppression down to 0 V.
These simulation traces mirror the slow 144 MHz RF envelope ramp up observed on the scope.
Using SEND
Although I haven’t experienced any problems completing WSJT-X Q65 QSOs on 10 GHz EME with the 3.5 s RF ramp up, I decided to reconfigure the sequencer. Now when Tune is pressed, WSJT-X issues the PTT command over a virtual COM port (RTS) triggering the sequencer PTT input. When the sequencer completes all the intermediate stages it pulls the IC-705 SEND low, putting it into transmit.
The timings are now the same as the no ALC test and the modules remain protected.
- Trigger 0 ms: PTT Input on the sequencer drops to 0 V (Yellow).
- SEND 0 - 200 ms: sequencer completes its intermediate stages, IC-705 SEND input drops to 0 V (Purple). 200 ms WSJT-X TX Delay completes in the same period.
- 144 MHz RF 200 - 350 ms: no RF output due to latency in sound card
- 144 MHz RF 350 ms RF envelope emerges instantly at full amplitude (Blue).
When Tune is stopped, WSJT-X halts the audio stream and releases PTT after 400ms. SEND goes high at the same instant, allowing the sequencer to drop back to receive mode and safely complete the remaining stages.