Path: utzoo!attcan!uunet!samsung!usc!apple!fox!portal!cup.portal.com!ISW
From: ISW@cup.portal.com (Isaac S Wingfield)
Newsgroups: sci.electronics
Subject: Re: Crystal radios - wasn't it a 'galena' crystal?
Message-ID: <24441@cup.portal.com>
Date: 24 Nov 89 08:07:11 GMT
References: <5386@internal.Apple.COM>
Distribution: na
Organization: The Portal System (TM)
Lines: 51

Mike Smith writes:

>wasn't the detector in old crystal radios made of 'galena'?
>Someone posted that the crystal was a germanium lump, but I
>seem to remember 'galena' as the stuff...  If so, it
>raises an interesting point.

>Galena is PbS (Lead Sulfide).  Taking a look at the periodic
>chart it looks like Pb and S are in about the right places to
>make a semiconductor (like Indium Phospide, Gallium arsenide, etc.)
>Is PbS a semiconductor (if properly doped)?  If so, what are it's
>properties?  I seem to remember some 'old timers' saying that a
>good crystal was far better than those new fangled diode things...

Nope, galena is right. Typically, a cubic crystal about 1/4" on the
side was cast into a base of some low melting point alloy (anybody
know what?). The "cat's whisker" (moveable lever with a spring wire
on the end) hung over the galena; you moved it around 'till you found
a "hot spot", a place where the junction rectified.

It was (is) better than "new fangled diode things" because the forward
conduction drop is much less (FCD is how much signal it takes to
"break through" the junction); what you get to listen to is anything
over this voltage. Silicon diodes are about 0.6 V, germanium are about
0.3 V, and as I remember it, galena is about 0.05 V; this means that
you can listen to weaker stations with galena.

BTW, if you are considering building a crystal radio, be aware that
you *really* have to look for germanium. I know for a fact that many
1N34's (classic germanium diode) are now made from silicon; last time
I required germanium was about 15 years ago, and out of all the 1N34
manufacturers, only ITT's were Ge. Nowadays, I dunno.

There's a thing called a "backwards diode" that I always thought
would be good for a crystal radio. It's a silicon diode that's doped
so heavily that it leaks like a sieve in the reverse direction; it
can't block more than about 25 - 50 mV. But oddly, the forward conduction
drop is still 0.6 V! So, you just turn it around (backwards diode, remember?)
and Hey, Presto! You have a diode with 25 mV FCD, and 0.6 V reverse
blocking - plenty good enough for a crystal set.

It used to be said that the oscillator and converter noise in a typical
superhet AM receiver was responsible for considerable signal degradation,
and a good TRF rig (just a crystal set with an RF amp) sounded much better.
It would be fun to build a "modrun" receiver using filter synthesis
techniques for optimum passband shape in the tuned circuits, and a low
loss detector.

Isaac
isw@cup.portal.com
 


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