This document was originally posted by Bill, G4WJS as part of a long thread.
Assuming you are running on MS Windows and have installed WSJT-X, this explains how to set Tx audio levels correctly:
The issue you may have with Windows WSJT-X Tx audio levels is due to the "Pwr" slider in WSJT-X no longer being linked to the Windows mixer control for the Tx audio device. This means that you must set the mixer slider manually, this should be a one-time operation.
Setting the mixer control is a two step operation. Firstly you must set the mixer control to maximum. The mixer control is only visible when transmitting so use the "Tune" button in WSJT-X. It is advisable to reduce the "Pwr" slider in WSJT-X to minimum while doing this to limit the output power, or do this part with the rig switched off. The mixer slider will look something like this:
Once the mixer is set to maximum the second step is very important. You must now adjust the master level for your Tx audio device to 0dB. The level control for that is in the audio playback device properties for the Tx audio device which looks like this:
If your slider scale is not in decibels then right-click the slider and select decibels. Your audio device may not have gain, in which case 0 dB will be fully to the right. Some devices do have gain, in those cases it is critical that the gain be reduced to 0 dB otherwise a severely distorted transmit signal will be produced.
Once you have completed this one time setup you can increase the "Pwr" slider back to maximum for full level audio output from WSJT-X. I recommend rechecking your Tx levels elsewhere, e.g. data modulation level adjustments on your rig, audio interface level controls etc. to ensure that you are not over-driving the audio input of your rig. A good method is to use a 1,500 Hz Tx audio offset "Tune" tone with the " ;Pwr" slider at maximum and your rig at maximum power into a good dummy load. With that arrangement reducing the interface or rig's data modulation input level until little or no ALC is indicated on the rig is about right and should give around 90% of the maximum output of your rig.
If your interface and rig do not have any Tx audio level controls then it is OK to use the operating system audio device master level control as shown above to reduce the audio level to a correct amount so long as the critical 0 dB level is never exceeded.
Once this setup is complete you can make routine Tx power adjustments using power control on the rig or the "Pwr" slider in WSJT-X.