For AM the balance of the product modulator (Schottky diode ring D73) is disturbed by a small DC voltage/current (R260=12k, C209=1u, R261=1k) applied to the AF port. From there the carrier + sidebands take the same TX path as the DSB-SC signal, with the difference that the AM IF-filters are selected instead of the SSB IF-filters. The exact carrier level is set by a trimmer (VR5=10k) on the switch unit (F).