Re: Crystal Tester


From: Mike McShan (
Date: Sat Nov 28 1998 - 15:49:09 EST


I'm glad that someone raised this question since I was about to ask a
similar one. I don't have one of the six-pack boards or kits so I decided
that I ought to be able to kludge together a crystal tester with the help
of my junk box and the ARRL HB. I'd also seen articles/schematics in the
past for making these thingees, so I had an idea where to start. Got one
together; amazing, it works. Ugly. Won't tell you some of the intermediate
steps to "it works." ;-) Uses two 2n2222: one for the xtal oscillator and
the other to turn on the LED when oscillation occurs. Used whatever
resistors and capacitors I had on hand that were in the right ballpark for
biasing the transistors, etc.

Here are my observations and questions:

I have found that xtals in the 80M - 40M range light the LED but higher
frequency ones do not. BUT, the oscillator circuit works up to at least 18
MHz since I can hear the output nicely in my HF radio, even from across the
room. What is limiting the 2nd part of the circuit (LED driver) so that
only the lower frequencies trigger the LED? Would a choice of different
transistors, as Ed suggests
>Off hand, my favorite transistor up into that 70
>MHz rage and even up into 300 MHz is the 2N5179.
>And it's brother the 2N5109. With an Ft of 900+
>Mhz they give you gain into parts of the UHF
be the way to improve frequency range? What other design parameters should
be considered (understand that this is the simplest possible Colpitts
circuit)? Would a metered indicator have more sensitivity?

Anywho, building and debugging this little tester was a good exercise for
starting the Elmer200 course.

Mike N5JKY
Edmond, OK

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