The Sony ST-SB920 Tuner as a dx receiver

(for weak-signal long-distance FM reception)


main PCB

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


tuner module with lid removed











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

Block Diagram









graph of filter responses










Showing the IF response including the effect of 'compensation' capacitors


For RDS dx-ing, we might as well forget the bandwidth switching and modify the 920 for 4 x 150's at all times, so I rebuilt the circuit from CF203 to CF231 on stripboard without any switching, and mounted it carefully. This made a useful improvement in the filtering.

IF on stripboard










The alignment of the 'front-end' module has a big effect on performance, which may explain differences of user satisfaction. The alignment can be optimised with a little care, to improve the off-channel rejection and overload products.
The tunable core of the IFT is a crucial adjustment and can be adjusted through the access hole in the front-end lid. Carefully choose a screwdriver with an undistorted blade to fit accurately, or better use the correct platic or copper trimmer tool. Tune to a medium-strong local station somewhere near the centre of the band, say 94-102MHz (we need constant signal level with no other strong signals on nearby channels) and adjust for maximum signal level on the '920 display. This can be done more accurately and faster by connecting a voltmeter to D221 or the 'SIGNAL' adjustment control. Remember that the '920 signal level detector circuit is fed through only one filter, so the signal meter will not accurately reflect the signal level for weak signals with stronger adjacent channels.
If you feel adventurous you can also ajust the three self-supporting coils which are revealed when you take the lid off the front-end (but release the rear mounting screw from the front-end to release the rear flange of the lid). Use wooden cocktail sticks, and plastic tweezers if you can find a pair (Maplin have discontinued theirs, but RS have several types). This requires great care and patience, especially for the coil near the centre which is actually two coils in close proximity. A voltmeter is essential for this, preferably one with an analogue or bargraph display, and a probe clip so you do not have to hold the probe with your third hand.

J.Hardstone March 2015