Using a Heathkit HD-15 phone patch as a DVK interface  

The sound card audio is fed into the line input of the patch unit. The mic audio is taken from one of the 2 outputs, 600-ohm or Hi-Z. I have tried this interface on the TS-180S, IC-451A, IC-751 and FT-221 radios. To use the interface with the station mic and logging software, add some impedance matching to the sound card output and switching of the audio and PTT line. If you wish, you can put the mods inside the box.

The 8-ohm audio from the sound card is fed through an impedance matching transformer into the line input (600-ohm). This interface allows the audio level to be monitored on the VU meter and the TX mic gain can be adjusted using the front panel control. The sound card volume can be adjusted to match the station mic level when the `Trans gain` control is mid range. The sensitivity of the VU meter can be increased if necessary by reducing R1 (3600-ohm), which is connected between S1-A to S2-4. The VU meter allows you to set all your wave files to the same level. After recording your files and removing the leading and trailing `dead-air` using sound card software, play each one into the DVK interface and reset the volume of each accordingly.

A small control relay switches the audio between the station mic and the DVK. The newer radios power the mic (8V) from the front panel connector. A 2ufd capacitor blocks the mic power dc. Use the 600-ohm audio for new radios and the Hi-Z audio for older radios using a high impedance microphone. Shielded audio cables are used for all mic connections. I used one of the RCA phono jacks for the mic output to the radio.

Another small control relay switches the PTT line. The control line from the pc interface (ground on TX) can be connected at the SPKR terminal for example. The wire going to this terminal can be removed. While not necessary when using a RIGblaster interfaced to the logger, a bypass switch should to be wired in series with the pc interface line to drop the PTT control, if you hit the wrong F-key for example. Connection to the radio's PTT line can be made via the other RCA phono jack. I used a single DPDT relay to switch both the audio and the PTT line.

I connected to the 8-ohm speaker output of my sound card since the line-output level was not high enough to drive the DVK interface. Now there is more than enough audio available. If you want to add an 8-ohm audio attenuator to make the volume adjustment less sensitive, attach a T-pad at the line terminals going into the 8-ohm/1K-ohm transformer. 10dB T-pad: 2 x 4.3-ohm series resistors and a 5.6-ohm shunt resistor to ground. 20dB T-pad: 2 x 6.8-ohm series resistors and the shunt resistor is 1.5-ohms.