[orbanp1]

Hi Pete,

I take you have the schematics of the radio!
Also have the TBA570 datasheet at hand!

That indicator is a very curious way of signal strength reporting.
Certainly the TBA570 application does not suggest it.

Pin 5 is the demodulated AM audio output, it is an emitter follower output after a peak-detector in the IC. It has the signal average as well as the full audio signal on it. The indicator shows the signal average, the meter itself does the "audio filtering".
Pin 14 is the same emitter follower output (through a different resistor), now filtering is applied to it, and that signal is used internally as the AGC-signal in AM mode. In FM-mode the AGC is shorted to the ground, there is no gain regulation in the IF-amp, in FM-mode the IF-amp works in a "limiting" mode.
In the FM-IF there is another amp stage in addition to the first differential amp. The AM-IF uses just one diff-amp stage. Pin 5 still gives the rectified signal strength (now FM-IF) from the output of the first IF diff-amp stage.

There is no provision to set individual indicator sensitivities for each of the bands.
R638 sets the zero of the indicator, R639 sets the sensitivity.
It sure looks like a compromise solution to signal strength reporting.

Regards, Peter