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A friend and I have been trying to build an AM radio. We got a few working, like the basic "crystal" (germanium diode) one with an earpiece, generally picking u
by drivers99 2y ago
A friend and I have been trying to build an AM radio. We got a few working, like the basic "crystal" (germanium diode) one with an earpiece, generally picking up 1 or 2 stations very faintly. Then some attempts with an AM radio IC (TA7642), some that use a couple diodes and LM386 op-amp but they're generally terrible, if they work at all. It seems that following random schematics off the web or a youtube screenshot doesn't work very well.
I do have an RF design book I haven't started (by Chris Bowick) as well as this PDF now, which should be even more practical, so I'm hopeful I can figure it out. I also have some test equipment such as nanoVNA, tinySA, and an oscilloscope which makes it possible to get visibility into how stuff behave beyond "I don't hear anything; no idea what's wrong." I was able to see how the tank circuit was behaving as you tune it.
- wbl 2y agoApplication notes can be pretty informative. Also ARRL handbook
- drivers99 2y agoApplication notes as in the examples on datasheets? Cool, I do have the ARRL handbook (100th edition). I also got the 50th (1973) and 66th (1989) editions partly out of curiosity (for the 1973 one) and because someone told me a late 80's edition might be good for info about building stuff.
- buescher 2y agoRonald Quan’s Build Your Own Transistor Radios is pretty good but I have only read it, not built any of the radios in it. The Bowick book is solid for fundamentals like impedance matching and basic filter design. The old brown edition is more concise than the new one.
- rjsw 2y agoI worked through this [1] book (PDF) as a child, everything worked fine but have no idea if component availability has changed. [1] https://www.worldradiohistory.com/BOOKSHELF-ARH/Technology/Making-a-Transistor-Radio-Ladybird%20Books-1972.pdf https://www.worldradiohistory.com/BOOKSHELF-ARH/Technology/M...
- drivers99 2y agoNice, I ran across that a month or two ago and it was great. I got to the OC71 germanium transistor and found out they're obsolete.
- kazinator 2y agoSilicon PNP transistors should work in those circuits. It's also trivial to reverse all the polarities in the circuits and switch to NPN. Use a positive supply voltage rather than negative, flip the rectifier diode around as well as all the electrolytic caps.
- kazinator 2y agoIn that most advanced circuit where are two diode drops against the BE junction of the transistor, you almost certainly want silicon diodes to go with the silicon transistor. The diode type is not otherwise critical. Other than that, I don't see any transistor biasing that depends on a detailed model of the germanium transistor.