EF Johnson Challenger 717x/718x microphonic fix

  • First generation EF Johnson Challenger radios are from the mid-1980s
  • Second (1.5 gen) generation Challenger radios are from the late 1980s.

EF Johnson radios are cheaper initially than Motorola Maxtracs, but the tradeoff is higher maintenance and higher programming cost for the Challenger. Pay now or pay later, like other things in life. Taking care of the simple problems below in the field, offers differentiation by fixing radios on the spot (even in the tractor) instead of doing a pull for the bench repair like most shops do.

These fixes don’t require much more than a 5/16” nutdriver, Phillips screwdriver and some Brillo, but they do require electronics experience. Here are the categories of issues first generation and 1.5 generation EF Johnson synthesized radios have, tailored to the EF Johnson Challenger.

Microphonics

This is a squealing sound on transmit and especially receive, with severe cases causing loss of lock and broadband splatter on transmit. It is related to oxidation of grounds after screws loosen from vibration. Some of the later synthesizers are packed with non-conductive foam to help mitigate this issue, with absorber (conductive) foam on the shield underside (not touching circuitry). For the Challengers, the following items are key suspects:

  1. shield over the final/pre-final amplifier is a key suspect
  2. shield over the mid-bottomside of the main PCB is also often the culprit
  3. shield over the component side of the main PCB is a secondary culprit
  4. in the worst cases or proactively, the main board itself can be cleaned to ground, taking care not to damage the RF cavity filters.

Scrape lightly on the chassis to remove oxidation, desolder and re-tin the ground, and apply an anti-oxidant suitable for electronics. The suitable anti-oxidant is Tuner Lube type products as are commonly available from GC Electronics and other sources. Yes, I also use this to fix the many rec/game room TVs using a mechanical tuner, perhaps permanently set to channel 3, that builds up oxidation from the damper basement environment. The TV tuners were designed with a self-cleaning mechanism, that doesn’t get used when they’re always on one channel.

Loose screws

In heavy equipment such as tractors and road graders, the constant heavy vibration can loosen the screws holding the power amplifier bolted onto the EF Johnson Challenger radios. This manifests as a scratching/hissing noise due to high resistance ground contact. Sometimes you can literally feel the heat sink as loose, most times you need to tap the heatsink with a screwdriver handle while transmitting to verify. I have not had to de-oxidize the heat sink, but if you have repeat case, use anti-seize compound on the screws (makes them tack in better while still being removable).

Bad microphone

The EF Johnson radios have a DC powered amplifier, that under extreme dirty conditions can become shorted in the mic jack. Usually a brush and electronics cleaner will cure this, unless the filth has gotten between the jack and board, in which case often a jack replacement is warranted because the contacts are usually oxidized heavily by that time.