EF Johnson Challenger 717x/718x microphonic fix

For reference, first generation EF Johnson Challenger radios are from the mid-1980s. Second (1.5 gen) generation Challenger radios are from the late 1980s.

In general, brands like Regency and EF Johnson make radios that are cheaper initially than Motorola Maxtracs that target the same low-mid two-way radio market. The key tradeoff is higher total cost of ownership in maintenance and reprogramming, as the Challenger may require opening the radio to mechanically tune for new, distinct frequencies.

These fixes require a 5/16" nutdriver, Phillips screwdriver and a scouring pad. Here are some issues EF Johnson synthesized radios have, tailored to the EF Johnson Challenger.

Microphonics are 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. Tuner Lube also fixes gaming TVs using a mechanical tuner permanently set to channel 3 that build up oxidation. The TV tuner self-cleaning mechanism that doesn’t get used when they’re always on one channel.

Heavy vehicles 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 the heat sink is loose. Tap the heatsink with a non-conductive object while transmitting to verify. For repeat trouble cases, use anti-seize compound on the screws, which makes them tack in better while still being removable.

The EF Johnson radios have a DC powered microphone amplifier, that under extreme dirty conditions can become shorted in the microphone jack. Usually a brush and electronics cleaner will cure this, unless the filth has gotten between the jack and board. A jack replacement may be warranted if the contacts are oxidized heavily.