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. I haven't seen a reference to manufacture dates, but I did see an LM-13 online that was dated to 1941. They seem to have all had a similar circuit, maybe with minor modifications. 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. The first one came from Fair Radio, maybe this one did too, I just don'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 face but the housing has been painted battleship gray. It's about time for a new paint job.

The unit also needs recapped and the crystal probably needs checked and adjusted after all these years.
On top are the name plates and serial number. One plate says it was reconditioned at the Mare Island Naval Shipyard, but doesn'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 to and are not interchangeable.
I have an original power supply and the interconnecting power cable. It definitely needs a new paint job.
The power supply is listed as a type R.B.-20104A. It's pretty simple inside, a 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 less than 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 degreaser 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 the whole power supply. 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 I 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, and it keeps the power supply looking original.
Page created 10/15/2020
Last update 10/17/2020