The ST-SB920 is a popular component-audio AM/FM tuner with RDS, manufactured by Sony from 1998 until about 2008, often used with the TA-FB940R Amplifier. It has been chosen by many FM dx-ers because of its wide availability while offering switchable IF-bandwidth, direct frequency entry and two antenna inputs for FM, and quite commendable RDS decoding
It is normal to modify the tuner by fitting narrower IF filters. Unfortunately the 3rd filter and the optional 'narrow' filter are largely ineffective because of signal leakage past the filters.
Investigating this showed that removing the 'narrow' filter CF231 had almost no effect on reception (in the 'narrow' setting), due to the signals passing straight through the amplifier section and bypassing the filter due to the PCB layout. I estimated the leakage as only about 10dB less than through the filter.The first modification required for dx-ing is to disable the tuning mute, which can be done by snipping wire jumpers JW72 and JW75 adjacent to the 'Left Channel' socket
The block functions of the receiver are shown here - there is nothing very unusual about the design, but the IF bandwidth switching for FM is of interest for dx use, and the front-end module can be seen to have bandpass filtering (two adjacent coils) and seems to have been designed with the option for two bandpass stages
Showing the IF response including the effect of 'compensation' capacitors