[Boatanchors] LM-13 Frequency Meter

[Bill Cromwell]

Hi Brian, and anyone else interested,

The heterodyne frequency meters (LM, BC221, others) can be used to 
measure the frequency of our oscillators and other signals. These days a 
digital frequency counter can do that of course. But the LMs can do 
tricks the digital counters cannot do. The LMs can detect and measure 
the frequency of other oscillators OR the LM can supply the marker for 
your receiver - not at 100kc or 25 kc "steps" but right on the frequency 
you want to tune.

Hams and experimenters who do not have one of these should get one, 
maintain it, and use the bejeepers out of it. I already have two LMs and 
I have waited for others who might not have one to speak up for the one 
being offered by Jack. If the hacking disturbs you then maybe make an 
offer for one of Brian's working machines. Don't be put off by an LM 
without the original book (matching serial numbers). You can generate a 
new book or tuning chart with a little effort and a computer. Does 
everybody here have a computer? <evil grin>

For an example of the utility - a short, true story. I have a RAK-7 - a 
large, low frequency regenerative receiver that was once use by the U.S. 
Navy and some merchant vessels. The beast has extensive shielding inside 
the metal cabinet and two rf amplifiers. The rf amps are not to increase 
sensitivity but to isolate the oscillating detector from the antenna and 
to control incoming signal levels. It has a log scale that can resolve 
incoming signals to a few cycles per second with care. For CW operation 
that is a great plenty. I wanted to check the calibration of of the dial 
and make a new tuning chart (the old one was decades out of date). I 
poked the end of a wire inside a small hole in the case with the other 
end connected to the digital counter. Nothing. I connected the antenna 
terminal directly to the counter. Nothing. I listened for the RAK on the 
LM. Nothing. Then I listened for the LM with the RAK. I got my results! 
Try that every few kc across the dial with your digital counter!

73,

Bill  KU8H


On 07/22/2014 07:59 AM, Brian Clarke wrote:
> Hello Jack, and anyone else interested,
>
> The LM and BC-221 heterodyne frequency meters are designed to produce 
> very many harmonics. Consequently, they are not good for VFO service. 
> Sure they are stable, and you can dial up a frequency quite 
> accurately, at least within 10 to 15 Hz of the on-board calibration 
> xtal (the limit of resolution is the lowest frequency your headphones 
> can reproduce and that your ears can discern - to get 10 to 15 Hz, you 
> need really good cans). As a driver for a VLF, MF or HF transmitter, 
> you would need to add filtration to attenuate the harmonics. To 
> produce a shop standard, I have upon occasion added a 6AK5 and a short 
> antenna.
>
> I have a few LM and BC-221 series with the correct frequency book, the 
> calibrated crystal and all tubes. I even have one with all the spare 
> tubes and a spare crystal. There are no extra holes, unlike Jack's one 
> illustrated in his photobucket library. All the original components 
> and wiring are still there - especially that wonderful Cardwell 
> variable capacitor. I have one or possibly two mains power supplies 
> for them. I have no need for so many het freq meters. So, if someone 
> would like to make me an offer ....
>
> 73 de Brian, VK2GCE.
>