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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)

Maxthon Chan
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  • I guess a BAT85 would be a better try. Maybe you have a G transistor left from which you could use the BE junction? – Wouter van Ooijen Jan 16 '15 at 16:42
  • I dumped and replaced all old transistors actually. The `3AG1`'s have a planar BE junction and I am a bit worried about its junction capacitance. Those old parts didn't have such a specification! – Maxthon Chan Jan 16 '15 at 17:01
  • @WoutervanOoijen That radio is older than me! I wouldn't trust anything in it at all (maybe except the PCB board itself) but I just didn't bother to replace all the caps and resistors. – Maxthon Chan Jan 16 '15 at 17:05
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    Why not just try it? – Andy aka Jan 16 '15 at 17:27

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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.

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Fizz
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  • +1 Lots of 1N34A diodes offered from China these days. Presumably they're not all rebadged 1N4148s. Maybe a BAT62 (0.35pF typical at 0V). – Spehro Pefhany Jan 17 '15 at 02:59
  • Yes [BAT62](http://www.infineon.com/dgdl/Infineon-BAT62SERIES-DS-v01_01-en.pdf?folderId=5546d4694909da4801490a07012f053b&fileId=db3a304314dca3890115181141bf0df3) has even better-looking stats capacitance-wise as you say (and about the same 0.2V drop at 25C). But the OP might not like that it is only a surface-mount part. Trying to adapt it to an old THT board might end up adding significant parasitic capacitance. – Fizz Jan 17 '15 at 03:15
  • Those selling "`1N34A`" offers rebadged `1N4148` at inflated prices and the 0.7V drop will probably eat away the signal completely, since the receiver resonator is pretty weak. The board is probably 25-30 years old so it was all single sided THT. I will give `1N5711` a go. – Maxthon Chan Jan 17 '15 at 13:41