Microphonic 800 MHz EF Johnson Repeaters

From before the EFJ 8600 800 MHz radios of the late 1980s through the late 1990s, EF Johnson radios have a tendency to microphonic behavior. Under severe cases, the radio will not be able to receive at all because the microphonics disrupt the PL/DPL/LTR decoding. The Summit public safety radios are a new generation design and don’t seem to have this problem (although the 35 Watt 800 MHz EFJ Summits have a tendency to burn out the power amplifier output PCB trace).

inside of EFJ Viking repeater
EF Johnson Viking 242-2008 175 Watt 800 MHz repeater. Five would go in a 7-foot rack incl. transmitter combiner. Photo courtesy Terapeak.

Diagnosing microphonics

The “2008” repeaters we’re having issues with are about 5 years old, and if I tap on a different rack 6 feet away, I can hear it in the repeater receive audio. That was only after I unplugged the fan. 3 of the 5 repeaters had microphonics.

Repairing microphonics

I fixed two microphonic repeaters on-site by

  1. complete teardown
  2. scrub off all mating surfaces (where ground pads meet chassis)
  3. blowoff any filings
  4. apply antioxidant.

The fix would usually last 3-5 years that way. If the oxidation was heavy on the tinned pads, I would “freshen” them by heating, desoldering, and applying a very thin layer of new solder.

So one 242-2008 EF Johnson repeater needed to go back to the factory. It may have been a bad component on the board as even swapping chassis didn’t help.

Microphonics happen in microwave circuits as well–even particle accelerators can have microphonics!

Leave a Comment