Lab599 wrote this in a FB response. "
The ALC in the Discovery TX-500 transceiver is essentially protection against high SWR and is configured to prevent the failure of the output stage. At high SWR, the ALC will lower the output power to a level that is safe for the transceiver. The ALC parameters are selected by engineers and do not require additional adjustment by the user.
The ALC graphic scale allows you to visually assess the decrease in output power, which in turn indicates a high SWR and, accordingly, that your setup as a whole is not optimally tuned.
There is no external control of the ALC in the TX-500.
"
