LM-21 Heterodyne Frequency Meter

The LM series of frequency meters have been around since the second world war, from the LM-1 to the LM-22. They seem to have all had a similar circuit, maybe with a minor modification or two. This is an LM-21 that I've had so long I can't remember where I found it. I think I've had it for 30 years or so. It's actually my second LM. I had one even before this one but "lost" it. The first one came from Fair Radio, I think this one most likely came from a hamfest because I got everything as one set, but I just can't remember.
The meter works great but the paint is flaking and someone cut short the post on the knob for zero beating the meter. This unit has a black front panel but the housing has been painted battleship gray. Both are in need of a new paint job.

The unit also needs recapped and the crystal probably should be checked and adjusted after all these years.
On top are the name plates and serial number. Two plates say it was reconditioned at the Mare Island Naval Shipyard, but don't say when.
It still has its original calibration book, which is as important to have as the meter itself. These books are serial numbered to match the unit they belong with and are not interchangeable.
I have an original power supply and the interconnecting power cable. It definitely needs a new paint job, too.
The power supply is listed as a type R.B.-20104A. It's pretty simple inside, a pie filtered high voltage supply providing about 250 volts, and a filament supply.
I recently fired the meter up (almost literally) to use it to calibrate my signal generator the old fashioned way. Since I've had it for so long I just assumed I had already recapped it at some time in the past. But it turns out I never recapped it. It was still running on 1940's capacitors. After being on for about half-an-hour the power supply blew a fuse. When I pulled the supply, the pictured bathtub capacitor, housing two 0.1uf capacitors that were directly across the AC line, was still spraying hot oil all over the adjacent components. What a mess, especially considering that the oil could very well contain PCBs that you don't want to get on you.
Two sets of rubber gloves, about a roll of paper towels, and a lot of degreasing spray cleaner later, I had the mess safely cleaned up. I had to remove the filter cans too, to get the oil that had been sucked in under them. After I was sure it was clean enough to work on, I recapped it. But it's a good example of why those leaky old paper caps have to go. Thankfully there was a fuse that blew before something worse happened.
The one item I didn't originally get with the power supply was the AC input connector, so origiinally I just removed the AC socket, tossed it into a junk box, and ran the power cord straight into the unit. After the above mishap I wished I hadn't done that and started digging through junk drawers looking for the original recepticle. And amazingly, I found it. I reinstalled it and then made my own connector plug for the AC connection. It's crude but it works just fine, and it keeps the power supply looking original.
Prepping the meter's cabinet for a new paint job. The metal label plates are attached with press-in rivets, which can be tapped out from the inside with a hammer and punch. They will be reused after cleaning and polishing them.

​The cabinet originally had a black wrinkle finish but I guess the Navy repainted it gray. I'm tempted to go back to the black finish, but haven't really decided yet.
Before and after of touching up the front panel. The before photo is worse than the top photo because I cleaned the panel before starting, which caused even more paint to flake off. The whole front panel wrinkle finish was dry brushed; I've just started in the bottom corners but the glare of the flash makes it hard to see any real detail (and the photo being a JPEG doesn't help either). I used a very small brush to work around the engraved lettering. It's not perfect but it'll do.

​I also fixed the cut off tuning rod on the Corrector knob and replaced the incorrect knob on the Modulation switch with a chicken-head knob.
The top rear of the unit, with V102 pulled so the bathtub capacitor hiding up in front can be seen. It has two 0.1uF 400VDC capacitors inside that will need replaced.
On the bottom there are a couple more capacitors to replace, and someone has replaced the crystal trimmer capacitor. I can't fathom why it would have been necessary to replace it. Other than the trimmer, the meter appears to all original.
This is a closeup of where the crystal trimmer was after removing the tacked in replacement. Only the bracket which is the rear plate is left. Short of physical damage, which I see no signs of, I really can't figure what possesed someone to remove the original.
Here's my planned replacement trimmer, if everything works out right. The original trimmer is listed as 2-3.3pF on the schematic. The replacement the was tacked in was 1.5-7pF and measures as set around 3-4pF. My trimmer is approx 2-9pF so it should work.
Page created 10/15/2020
Last update 1/23/2021