Restoring a surplus military coaxial switch

I bought years ago a rugged military electically-operated 2-way 50 Ohm coaxial rotary switch with N-female connectors, model SA-185/U (MIL-S-3928/1A) produced around the year 1965

This switch acts as a (slow) latching relay, the supply voltage is 28V DC with a 3-way connector. If the A-C pins are energized, the common RF input is connected to the first output. The second output is selected energizing the B-C pins. At the end of the commutation, the supply circuit is open, so one can leave the voltage applied without power absorption.

The switching time is about 500-700ms, it’s too slow to be used as a tx/rx relay, for example in a PA.

I don’t have any specs of this object, please write me at my qrz.com email address, if you know something about it.

In my current configuration I use it for switch the antenna between local (TS-590S + tube PA) and remote (TS2000 + SS PA) 50 MHz stations. Unfortunately, the past few days I am noticing some failed changeover and a good contact is reached only after a few repetitions, so I decided to open the box to solve the problem.

A rotating stepped electromagnet is connected to a barrel that holds the RF contacts. When energized the barrel rotates for 180 degrees with a sound like a “machine gun”.

The problem was some dry grease on the contacts, easily solved with isopropanol and a Dremel with a felt wheel.

Now that it’s fully functional I will do some RF measures and I’ll post here the results.

2 Responses to “Restoring a surplus military coaxial switch”

  1. The switch was used to switch receivers in airborne ECM equipment, the mechanism is know as a step or “Ledex” relay. This particular switch is rated to 12.4 GHz.

    regards,
    Scott W7SVJ

  2. Hi Scott, thanks for info, very appreciated. I’m googling for Ledex now…. 73

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