Or try these...
Does the LSB/USB configuration of your radio match that of your TNC?
Are you on the correct EXACT center (not dial) frequency? The main frequency chart is listed as center frequency. In the lower right hand corner of the Terminal screen
on the AirMail Terminal form, you will find the actual dial frequency. The dial frequency varies with modem type and other factors, while the center frequency is always the same, regardless of mode of operation or sideband used.
The TNC to computer baud rate should be set as high as possible, given the specs of your computer and TNC.
Over modulation of the transmit audio? You should barely see any ALC with full output to your transceiver. This can be set up in AirMail. Read the AirMail help files. Reduce the transmit audio gain level. Less is Better.
Turn off all processors, compressors and filters on both the receiver and transmitter.
Is your receive bandwidth filter wide enough for the mode used? Pactor 3 needs 2.4 KHz.; Pactor 2 needs 0.7 KHz; Pactor 1 needs 0.6 KHz.
Some radios do not perform well with both the microphone and the TNC connected
simultaneously to the radio. Disconnect the microphone.
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Maybe you have an “RF or Audio” problem due to RF feedback into your system. A symptom on this problem would be a “better” signal, when using a very low power
output. Use ferrite chokes and/or less power output. This feedback can also be reduced by Increasing the audio level from the TNC and decreasing the transmit audio gain at the transmitter.
If you are using a battery
for power, is the battery
voltage too low? In this case maybe a lower power output setting may work better.
Listen to your transmitted signal with another receiver. Does it sound like that of a public RMS Pactor station? See above.
Maybe you need a better antenna?
Install a better earth ground connection or counterpoise