[ARC5] The not-so humble BC-221/LM-xx frequency meters.

[Leslie Smith]

>From Ken Gordon:  "I am curious, Les. Why is it that you feel the need
to dig into the BC-221? Is something wrong with it, or is it simply your
innate curiosity?

Hello Ken,
Both and more.  I also have an LM-something that has white "powder" on
the tag-terminals and a sad-looking green liquid staining the nice
white-ceramic crystal socket.  I want to make it "go".   Perhaps I
should say I want to stop the rot.

More generally I'm interested in the specification, accuracy and
long-term stability of this type of instrument.   

Beyond the set itself, the manual is a cornucopia of useful info for the
person willing to read the well set-out manual.  (The same can be said
for the ARC-5 document.)  I leaned more about how a good manual should
be set out by studying the ARC-5 manaul than by reading a book on how to
write a good manual.  

The temperature stability for the crystal oscillator (holder, crystal &
circuit) in the LM series is given as 0.0001 percent per degree C, with
respect to +10C.  In my book that's 1Hz per degree C.  The first (and
most obvious question) is "how could "they" measure this, with the gear
available in the time before octal tubes became available?"  What was
their primary standard?  I have a very common Fluke frequency counter -
but can I trust it?   (I have access to a rubidium standard, in case
any-one want's to suggest that might be a useful point of reference.) 
But rubidium standards have only been available for the past few
decades.  So one obvious question that the LM/BC-221 sets raise is "What
was their primary standard?"

This raises the question of how did they use their standard; what was
their method.  For my Fluke, I probe the circuit and read the dial - but
it isn't so easy without digital electronics.  Then there is the problem
raised by connecting my Fluke to any circuit.  Unless the connection is
well buffered the Fluke will "pull" the oscillator.  That isn't a
problem with a LM-xx meter.

According to the specs the worst-case drift for the VFO is 0.02 percent
(on the higher band).  At 10,000kHz that's an in-accuracy of 2kHz - not
too spiffy by todays's standards - but useful of you're making contact
with a distant station that must spend minimum time actually on the air. 

I have never used WWV to check the accuracy of either the Fluke or the
military heterodyne frequency meters.  I want to learn how to do that.  

The frequency of the 1000kHz crystals were specified to a close
tolerance.  Now they are 70 years old (at a minimum).  How much have
they drifted?  If I want to KNOW the REAL long-term drift I must
calibrate my Fluke frequency count against some primary stanard - and
I've never done that before.

So - all in all Ken, my humble heterodyne frequency meters are
instruments of some capability (even today).  That's why I'm interested
in them.

Best Christmas greetings to you over there in cold old Moscow!

   73 de Les Smith
   vk2bcu at operamail.com

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