Adding Digital to the HTX-10
The HTX10 is a neat little rig which has AM, USB, LSB, and even FM (with one PL). A few years ago I found an article by Jim (N5IB) for an outboard circuit which plugs into the mic jack and injects a CW tone on transmit. I built it and was very happy to make CW contacts with it.
A few years later I was inspired to develop a way to run digital modes on the HTX-10. My task was simpler than the one faced by Jim, who needed to generate a CW tone, because software (such as WJST by Joe Taylor) does the work encoding data (CQ, QTH, etc...) into tones for transmitting. On the receive side the same software decodes received tones and displays the messages.
My goal was to build as simple an interface as possible with as many parts “off the shelf.” I had an old HP which had both RS-232 and USB, but I opted to use the mic and headphone jacks. This not only simplified the circuit and reduced part count, but also provided a solution which can be used with almost any laptop... with minimal system changes.
The receive side (rig-to-PC) uses a standard cable with 1/8” jacks connected from the speaker jack of the HTX-10 to the mic jack on the laptop. The rig's volume control controls the input to the laptop. Some software packages (like the K1JT's popular FT8 protocol) includes a virtual meter, as seen on the right.
Unfortunately, on the transmit side we cannot directly connect the output of the laptop speaker directly to rig's mic plug. Jim explained why in his article on the CW circuit: “The PTT switch on the microphone does double duty. One pole is used to switch the audio line, disconnecting the mic from the rig when in receive mode. The other pole grounds a control line to place the rig in transmit or receive mode. In receive mode, it grounds the common side of the speaker, thus serving as a receive mute.”
Since the rig must stay in the above state for the duration of the transmission, I chose a VOX circuit. Oh, and there was a bonus. Because the relay is energized for the duration of the transmit string, the circuit is protocol agnostic and will work as well for formats such as PSK or MFSK (with indeterminate transmission lengths) as it would for FT8 (always 13.5 seconds.
The VOX in action
In the above picture is the HTX-10, above an IC-718. Via the headphone jack, audio from the PC goes into the circuit, (installed in a plastic case), which sits above the MFJ tuner. Extending from the circuit is a bundle of leads to the mic plug. From behind the HTX 10 is a standard audio cable from the speaker jack to the mic input of the PC.
The circuit worked really, really well. It even ran F/H mode, which is critical for many DXpeditions. With just 25 watts into a humble dipole I logged 117 entities including St. Brandon Islands, Japan, New Zealand, Australia, Ducie & other Pacific islands. As for WAS (my ham obsession) I have a funny story... by September 2022 I had 49 states but one stubbornly refused to give up an RR73. If you guessed the state was Alaska or Hawaii you would be wrong. Those were... well, I hesitate to say it but yea, they were easy. (Hell, I even got North Dakota early in the chase.) What I needed was for the %$#^@ ionosphere to cooperate so I could get short skip into Delaware. Then, on February 1, 2023, the ionosphere proved it has a sense of humor. Within five minutes, I got not one but TWO Delaware stations!
The ionosphere has a laugh