May 032017

It’s about time that I wrote a quick blog post about the 6m fox hunt transmitter that I made for Military Radio Collectors Group events. I made it out of an AN/PRT-4A transmitter. From the outside, it looks unmodified. But on the inside it has a few changes!

Transmitter and Battery Pack

Transmitter and Battery Pack

I started with a refurbished AN/PRT-4A transmitter, which I purchased from C&H Surplus a few years ago. I don’t know if they have more left.

I designed a small board which mounts on the bottom of the main housing in place of the original battery connector. This board includes a microcontroller with enough flash memory to store a short audio message. There’s no built-in message recording capability; it’s necessary to use arcane magic to include the message in the firmware image for the microcontroller.

The Brains

The Brains

I also built a new 15V battery pack based on two 4-cell AAA holders and one 2-cell AAA holder. It was tricky to find holders which just barely squeeze into the original battery box. I used these parts:

  • Memory Protection Devices BH4AAAW (x2)
  • Memory Protection Devices BH2AAAW (x1)
Frankenstein's Battery Pack

Frankenstein’s Battery Pack

Modifications to the radio included:

  • Remove original battery connector.
  • Remove original chassis wiring harness.
  • Remove return spring from the butterfly switch, so it does not return to the off position when released.
  • Build new wiring harness.
  • Install new PCB on bottom of the chassis, and wire everything together.

I have shared all of the PCB design files and microcontroller firmware on GitLab:

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