DIY: T/R Interface for Hallicrafters HT-44 & SX-117 (Updated)

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If you’re running vintage "twins" like the Hallicrafters SX-117 receiver and HT-44 transmitter, you know the challenge of managing antenna switching and receiver muting. While these rigs were designed to work together, a robust external T/R (Transmit/Receive) relay is the best way to protect your receiver’s front-end and provide a seamless operating experience.

This project covers a "fail-safe" design that handles RF switching and receiver muting in one shielded enclosure. This DIY design uses an external RF relay for your antenna and utilizes the HT-44’s internal circuitry to automatically mute the SX-117.

The Design Strategy

We are using a HiLetgo 12V 1-Channel Relay Module. Unlike a bare relay, this module is opto-isolated. This means it only draws a tiny amount of current from the HT-44's vintage internal contacts, ensuring they stay clean and arc-free for years to come.

Parts List

  • Relay: HiLetgo 12V 1-Channel Relay Module (with High/Low trigger jumper).
  • Plug: Amphenol 86-PM11 (11-Pin Control Plug).
  • Wire: 22 AWG Stranded hookup wire.
  • Connectors: 1x SO-239 (Antenna), 2x RCA Phono Jacks (RX/TX).
  • Enclosure: Small shielded project box.

Step 1: Configure the Relay Module

Before wiring, locate the 3-pin jumper on the HiLetgo board labeled "High/Low." Move the jumper to the "L" (Low) position. This ensures the relay triggers when the HT-44 pulls the signal to ground.

Step 2: The Control Wiring (11-Pin Plug)

We are tapping directly into the HT-44’s Control Outlet for power and switching. No external "wall-wart" power supply is needed!

HT-44 Pin Destination Function
Pin 3 Module DC+ +12.6V Power
Pin 11 Module DC- Ground
Pin 1 Module IN Switched Keying Signal
Pin 9 SX-117 REC STBY Lug Muting Loop Path A
Pin 10 SX-117 GND Lug Muting Loop Path B

Step 3: RF Connections (The "Business End")

Wire the blue screw terminals on the relay module to your RF jacks:

  1. Center (COM): To the Antenna (SO-239).
  2. Normally Closed (NC): To the SX-117 ANT jack.
  3. Normally Open (NO): To the HT-44 ANT jack.

Station Interconnect Diagram (ASCII)


       +-------------------------------------------------------+
       |         HT-44 TO SX-117 INTERFACE SCHEMATIC           |
       +-------------------------------------------------------+
                 |                 |                 |
            [HT-44 P3]        [HT-44 P11]       [HT-44 P1]
              (+12V)             (GND)           (KEYED)
                 |                 |                 |
           +-----V-----------------V-----------------V-----+
           |           HILETGO RELAY MODULE (SET L)        |
           |    [DC+]            [DC-]             [IN]    |
           +-----------------------|-----------------------+
                                   |
           +-------(NC)----------(COM)----------(NO)-------+
           |           |           |           |           |
       [SX-117 ANT]    |       [ANTENNA]       |      [HT-44 TX]
                       |                       |
                       +-------[SHIELD]--------+

       +-------------------------------------------------------+
       |           AUTOMATIC RECEIVER MUTING LOOP              |
       +-------------------------------------------------------+
        [HT-44 PIN 9]  <------------------->  [SX-117 REC STBY]
        [HT-44 PIN 10] <------------------->  [SX-117 GND LUG]

How it Works

  • On Receive: The relay is relaxed. Your antenna is physically connected to the SX-117. Pins 9 and 10 of the HT-44 are "shorted" internally, providing the ground path the SX-117 needs to stay active.
  • On Transmit: As soon as you hit the PTT or VOX, the HT-44 pulls Pin 1 to ground. The relay module clicks instantly, moving the antenna to the HT-44. Simultaneously, the HT-44 "breaks" the connection between Pins 9 and 10, which ungrounds the SX-117's standby lug and silences the receiver.

Final Bench Tip

When soldering the 11-pin Amphenol plug, use 22 AWG stranded wire and apply a small piece of heat-shrink tubing over each pin. These pins are close together, and a single "whisker" of wire can short out your 12V rail!

73, and see you on the air with the "Twins!"

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