[amsat-bb] Solid State conversion (tube equivalents)? END

Robert Bruninga bruninga at usna.edu
Wed Mar 1 23:26:21 UTC 2017

Here is the conclusion to this thread with great feedback from James
Duffey and Diane Bruce...

> I picked up an old boatanchor LM-21 Frequency Meter at a Hamfest over
the weekend.  I original asked people here about converting the 3 tubes to
solid state.  But they said, why bother.  Just use it with the tubes...
and were curious how well it was calibrated now compared to its 85 year
old design age (1930's)?

So today I just powered it up with the tubes and it was within 150 Hz at 2
MHz after not being used since 1976.  Not bad.  In fact when I first
compared it to the modern sig gen, I thought it wasn't even working
because the 150 Hz was so close to zero beat I didn't hear it at first
until I dithered the dial!

I have no idea why I am doing this, but it sure brings back the good ole


-----Original Message-----
From: James Duffey [mailto:jamesduffey at comcast.net]

... why bother converting to solid state? The LM/BC-221 frequency meters
are, as you note,  very nice frequency meters/sources. They don't drift
when warmed up and are very solid mechanically. They age well and you are
likely to have performance now very close to what they were new.

If you insist on converting it to solid state, here is one article
describing how:
< http://www.hanssummers.com/images/stories/bc221t/bc221.pdf >
< http://hanssummers.com/bc221t.html >

including schematics and the manual.

But I doubt performance will be substantially better, if any, than the
tube unit you start out with.

Enjoy it. Technologically it is a wonderful thing to have just the way it
is. It is still a very useful laboratory quality instrument.  - Duffey

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