Calibrating Audio Tones for MFJ TNCs

Properly calibrating audio tones on a TNC can make a huge difference when it comes to efficiency of a packet link. While some TNCs do not need nor allow calibration, the "workhorse" of most packet networks do. The MFJ-1270b, 1270c, 1274 and 1276 radios offer both 1200 and 300 baud operation as well as optional modems to run at 2400, 4800 or 9600 baud. We will only discuss 1200 and 300 baud here, but the same principles should apply to any speed.

Connecting the TNC to your computer

The procedures outlined here only apply to select few TNCs. With proper documentation of the TNC, these should be applicable to any TNC where you can monitor their audio out. For the MFJ TNC2 clones, there is a 1/8" mono jack in the back of the TNC. This means a simple mono (or stereo) audio jumper cable between the TNC and the Line In or Microphone in will suffice. For others, one could wire up a connector to fit the TNC, sending TXA to the left channel (tip) and ground to the shaft of a standard 1/8" mono plug suitable to plug into a computer soundcard.

The MFJ-1270B and MFJ-1274 are nearly identical on the inside with the exception of the tuning indicator on the front of the MFJ-1274. The tuning procedure for both are identical. To start, open up the TNC, they have 4 screws total holding the cover on. A small phillips screwdriver works nicely. You will want to locate a small standard screwdriver or non-metallic tuning tool to actually adjust the potentiometers which govern audio tone.

Once the cover is removed, with the TNC facing you, you will see 4 sets of potentiometers bundled together in the center, then two more off to the left.