[turretslug]

Hmm, intriguing! I do recall a Murata data sheet somewhere sometime where they were quite stringent that there should be no DC whatsoever on input or output pins, I assumed maybe something to do with electrolytic degradation of the interfacing bonds? Perusal of quite a few tuner circuits suggests that this guidance doesn't always get followed, though.

For quite a long time now, I've used a basic compact old Rotel tuner (RMT-80L) as an aux hifi source, basically for R4 LW and R5 MW. It developed crackling and intermittency on the AM bands, traced to the little SFU455A between IF amps. Sure enough, this had output pin "live" to bias/AGC, making me wonder if this had caused filter failure. I took the opportunity to replace it with a CFUM450E, fitted "dead bug" style under the PCB and now isolated with 22n to IF amp base. (The shift to 450kHz to get rid of the 909kHz whistle!) Looking through many Rotel tuners of the era, the same AM topology crops up again and again without ceramic filter isolation...

I wonder if Sony have also left the filter "live" to DC- I suppose something like corrosion or a tin whisker-type syndrome could develop in time, but usually well out of warranty, for the sake of farthings' worth of isolating capacitor!

Colin