Microphonic 800 MHz EF Johnson Repeaters

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). The EF Johnson Viking 242-2008 175 Watt 800 MHz repeaters typically have five in a 7-foot rack including transmitter combiner.

Diagnosing

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. Three of the five EF Johnson 800 MHz NPSPAC repeaters had microphonics.

Repairing

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!

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Written by Michael Hirsch, Ph.D. //

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