An old AM radio that passed down to me from my grandparents finally ceased to function after I accidentally dropped it and old parts inside got damaged. I ripped all damaged parts out and replaced them with modern equivalents. There is one thing I am having trouble replacing: the detector diode.
The original diode is 2AP9 (Chinese part, was THE detector diode in Chinese radio designs, similar to 1N34A,) a germanium detector diode that got shattered. I don't have any germanium parts any more (the germanium PNP transistors was replaced with silicon ones, but the amplifier is adjusted accordingly to make them work correctly after a few resistors get replaced)
However I do have a few Schottky rectifiers 1N5819. Are those good enough as detector diodes in AM radio? Do I need to adjust some caps and resistors to compensate the 110pF junction capacitance? (2AP9 have 1pF)
Best Answer
If 2AP9 is similar to 1N34A, you can surely still find those, although I'm not sure about availability in China (your location).
As for using Schottkys as substitute, 1N5711 (a ~0.2V [@ 25C] UHF Schottky) does work as substitute in some designs; 1N5711 has only 2pF capacitance. YMMV if it will actually work in your radio, but it seems to have a better chance than 1N5819.
There are some research papers on using (CMOS-fabricated) Schottkys as ultrawide-band AM detectors: "The input and output matching of the detector is better than -10 dB from 0-10.3 GHz". I'm guessing that a worry might be getting too much band and thus possibly HF noise when using a Schottky instead of a Ge diode for an AM radio in the classic AM radio band.