I purchased this homebrew Crystal Radio on Ebay. I have seen several of them and I believe it could have been a kit, but there is nothing to identify it. It works when there is a strong signal, but not well. I believe it is likely wired wrong since I can see some of the wires were recently soldered. Crystal radios seem simple, but I am confused by a double inductor coil in which one turns inside the other. Does anyone have a schematic for this puppy?

Ed

http://www.ebay.com/itm/320826592172?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649

Crystal radios such as your's will not have much gain unless they're used with a solid earth ground and a very large aerial. For the aerial of my crystal set, I will typically use my old VHF/UHF roof antenna as it is most convenient.

What you have is a variation on the "Loose Coupler" type of crystal set. It's also called a "variometer" because the coupling between the primary and secondary coils can be adjusted mechanically.

http://www.sparkmuseum.com/COUPLERS.HTM



One coil is connected to the detector circuit, the other to the antenna. The variable capacitor may be connected on either side, depending on the design.

Crystal radios don't have "gain" as there is no power supply or amplifying element. A long antenna and a ground will undoubtedly improve reception.

What is the detector... a galena crystal with catwhisker or a modern diode?

For more info on crystal sets, try Dave Schmarder's "Make a Radio" site.

Rich

Here is a modern-made version of what you have.

http://makearadio.com/crystal/14.php

Rich

Thanks guys !!! First, to answer the question: The circuit uses a galena crystal with a cat whisker. I put a 1N34 diode across it for diagnostic purposes rather than fighting the "sweet spot" on the crystal.

The schematic was not exactly the same, but close enough to allow me to understand the principle of the circuit. The unit was wired properly. I am not sure how many things can be wrong with such a simple circuit, but here is what I found and fixed today. There were 4 cold solder joints. The variable condenser was out of adjustment. The plates in the condenser had to be straightened. The variable condenser has a ferrous spring clip that holds tension and makes electrical connection. It was rusted and making intermittent contact.

It seems to be working as expected now.

Ed

Congrats!
I also use a 1N34 to get things "set up" before trying a cat whisker detector.

Rich

Hey Ed,

Nice find. Sounds like you've got it working. Most crystal sets are simple enough to figure out the schematic simply by physically back tracing the wiring by eye.

K