I was listening to the 20 meter band today, while the s-meter was not
giving the same signal strenght as a reference FT-757, the TS-120 could
copy every weak signal the 757 could.
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If you want to increase the indication of the S-meter (for example because
you wish it to be the same as on your other transceiver) ,there highly
likely is a trimpot near the meter or in the associated AGC circuit which
you can manipulate. Almost every S-meter derives its signal from the AGC.
If you have a signal generator and a good quality attenuator you can set the
S-meter indication to S9 with a receiver input of 50 microvolts @ 50 Ohms
(-73dBm).
Of course it doesn't mean that the S-meter is then correct over its entire
scale ,
1 S-point meant to be a change of 6 dB.

Frank GM0CSZ / KN6WH