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 days...

Bob, WB4APR

-----Original Message----- From: James Duffey [mailto:jamesduffey@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 KK6MC