Contributed by Scott McKenzie, Avionics Tech Rep
At Duncan Aviation, we recently have had a number of components sent in for a KHF-950 HF system, with a KFS-594 Miniature Control Unit installed, that have been squawked as being stuck in transmit mode or the frequency is changing on the controller every time a transmit is attempted. After bench testing, it was determined that there were no faults with the suspected units, but a misunderstanding of the proper operation of this system.
Here’s a quick refresher course.
figure 1. Direct Frequency Mode.
figure 2. Channel Mode.
figure 3. Program Mode.
Direct Frequency Mode
The first thing that technicians should do when testing the KHF-950 HF system is select channel 0 (direct frequency mode) on the HF controller and enter the desired frequency. (figure 1. Direct Frequency Mode)
This ensures the frequency that is selected via the controller is the frequency that will be both received and transmitted (simplex operation) on when the microphone is keyed. If any other channel is selected (1 through 19), it is possible that the frequency displayed on the controller will change when attempting to transmit.
Channel Mode
By selecting a channel (1 through 19), the user can store preset frequencies using the white STO button. To program a preset channel frequency, first select the channel. Next, enter the desired frequency and press the STO button. (figure 2. Channel Mode)
Program Mode
Each channel can also be programmed for duplex operation. Duplex operation is receiving on one frequency, while transmitting on a different frequency. To program a channel for duplex operation, first select the desired channel, then enter the desired receive frequency and press the STO button. The display on the control should now be in program mode. Program mode is annunciated by a flashing dash in the space adjacent to the channel number. (figure 3. Program Mode photo)
Also note that the TX annunciation is in the upper right hand corner. You may now enter the desired transmit frequency, and press the STO button again. The selected channel is now programmed for duplex operation.
STO Button
The STO button can perform a couple of other functions in addition to being used to store preset frequencies. If the STO button pressed while the microphone is keyed, a 1000Hz tone will be transmitted. This is used to break the squelch of some stations.
Also, pressing the STO button will clear any error conditions, which are annunciated by a flashing "E" being displayed for more than three seconds. If the display shows a TX annunciation upon power up, and you are not able to control the cursor, pressing the STO button three times should clear this indication and allow for full function of the HF controller.
If you have any questions or need assistance when troubleshooting a KHF-950 HF system, contact Duncan Aviation's Avionics Tech Reps, available 24 hours a day, every day.
Scott McKenzie is an avionics tech rep located in Duncan Aviation's Lincoln, Nebraska, facility. He specializes in troubleshooting the latest in avionics systems installed on aircraft today. His aviation career began in 1995.
