PRC-47 Remote Control Interface

This blog post describes a Remote Control Interface I designed for my PRC-47 Transceiver.

The PRC-47 is a Vietnam War era transceiver designed to operate from 2.000 to 11.999 MHz in 1 KHz steps. It is USB only (but can be modified for LSB operation -- see here).  It can also operate CW or FSK, and it's designed to be powered by either a DC supply (from 24 to 28 VDC) or a 115 VAC, 400 Hz supply.

Transmit power (into a 50 ohm load) is rated at 100 watts PEP (High Power Position) or 20 watts PEP (Low Power Position).

Antennas supplied with the radio were either a 15 foot whip (AS-1320/PRC-47) or a 45 foot long wire (with two alligator-clip jumpers to select the frequency range -- the AS-1321/PRC-47)).  

The image below shows the PRC-47 with its 15 foot whip installed.  Note the handset location, and that the operator would typically be right next to the antenna.

I wanted to try operating the radio outside as a "portable" radio, but I did not want to be anywhere near the antenna while transmitting (I'm always concerned about the possible effects of near-field high-power RF).  So I decided to design some circuitry that would let me sit off at a distance and use the radio through a "remote control" interface.

The image below shows the "Remote" end of my Remote Control Interface.  Note that I'm not using a standard military handset, but instead I am using a Collins MM-1 Microphone (through a custom mic preamp) and stereo headphones.

The image below shows the "Local" side of the control interface, connected to the PRC-47.

Remote-operation of military radios is not something new -- the military has long had remote-control interfaces, such as the AN/GRA-6, which consists of a C-434/GRC Local Control Unit and C-433/GRC Remote Control Unit.  These two units would be connected together with two-wire telephone line (see the image, below) for operation up to two miles apart:

But I don't have these control units, so...why note make my own?

And rather than using two-wire telephone cable to interconnect the local and remote interfaces, which complicates the design (essentially you need to add a telephone "hybrid" circuit at each end to separate RX and TX audio), why not use eight-wire CAT5E cable?  CAT5E cable is ubiquitous and I can purchase it preassembled in long lengths.  That seemed to me to be a simple way to go.

So the specs were:

Remote Unit:  

• Two U-79/U Connectors to support both Audio In (via my Mic Preamp) and Audio Out (e.g. LS-166 Speaker)
• 3.5mm Headphone Jack for modern Stereo Headphones
• Switch to Turn the Radio ON/OFF Remotely
• Switch to Enable TX with Microphone PTT switch or Force TX by placing switch in the opposite position.
• RJ-45 Connector for CAT5E interconnecting cable.

Local Unit (connected to the PRC-47 radio):

• U-77/U Connector for all signals (that are used) 
• RJ-45 Connector for CAT5E interconnecting cable.

Below are my Remote Unit (left side) and the Local Unit (right-side):

Note that the signals on the two PRC-47 U-79/U connectors are:

There are 10 pins, of which I use 7.  Pin D is not connected within the radio, so no need to wire it up, and I'm also not using pins A (RCVR AUDIO -- same as the SPEAKER signal but with a 15K resistor in series), nor pin K (CW KEY).

Here's the schematic:

Schematic Notes:

1. R1 and C1 (to the headphone jack) serve two purposes:  The resistor knocks down the receive signal for my modern stereo headphones, and the capacitor provides a bit of low-pass filtering (corner-frequency is 1 KHz) to reduce the receive audio's high-frequency "hiss".
2. My custom Mic preamp uses "MIC GND" (pin E) as as both the Microphone and the PTT return path, because the Collins MM-1 Mic has a common-ground for the PTT and MIC signals.  I decided to use the MIC GND for the PTT return rather than the perhaps better choice: GND, because, if the Mic Return were instead routed on GND, I thought there could possibly be noise injected within the PRC-47 into the Mic audio signal.
3. The "Remote ON/OFF" toggle-switch will turn the radio on and off in lieu of the radio's front-panel power switch.  However, for the "Remote ON/OFF" switch to work properly, the radio's front-panel Power switch should be in its OFF position (full left).  
4. Although I only use 7 of the PRC-47 signals, there are 8 wires in the CAT5E cable, so I use the eighth wire to double-up the interconnecting wire for the "GND" signal (pin H).

FYI -- here's an image of CAT5E cable wiring:

Here's a picture looking at the front corner of the Remote Unit:

And here's a picture of the back showing the RJ45 connector:

The image below shows the two switches and headphone jack on the left-hand side of the Remote Unit's front panel:

And here's a photo of the Local Unit.  It consists only of the RJ-45 and the U-77/U connector that would attach to the PRC-47.

Operation:

I tested remote operation with the radio mounted in its normal operating position, on a shelf in the shack, attached to a G5RV antenna feed via coax from the shack's station.

The Remote Unit was connected to the Local Unit via 50 feet of plain, unshielded, CAT5E cable.  The two ends of the CAT5E cable were each wrapped around their own FT240-43 Ferrite Toroid core with 12 turns, for RF suppression on the CAT5E cable.

The 50 feet of cable was half unspooled (approximately) from the coil it was shipped as and routed across my garage lab from the Local Control unit at the PRC-47 operating position at one end of the cable to the Remote Control unit at my workbench at the other cable end.

Testing took place during a morning net on 75 meters, with the radio putting out about 170 watts on SSB peaks.

The "Remote ON/OFF" switch functioned as it should, turning the PRC-47 ON when the switch was flipped ON.

Transmit audio was reported to be of good quality.

Receive audio was good, too.  Audio via the headphones sounded good, as well as audio via an LS-166 speaker connected to the second U-79/U connector on the Remote Control unit (the other U-79/U connector was assigned to the microphone and its preamp).

I did notice, while listening through the headphones, that if I attached the LS-166 speaker to the second U-79/U connector there was a slight decrease in the level of the headphone audio -- this is most likely due to additional signal loss from the increased current required to also drive the LS-166 (in addition to the headphones) and subsequent loss due to the resistance of the CAT5E cable.  But, although noticeable, it did not significantly affect performance, in my opinion.

Future testing will be in the field, with a 45-foot wire antenna connected directly to the PRC-47's antenna connector.

Other Stuff:

For reference, here's a schematic of the Mic Preamp I use with the PRC-47 and mics such as my Collins MM-1 hand-mic:

Standard Caveat:

As always, I might have made a mistake in my equations, assumptions, drawings, or interpretations.  If you see anything you believe to be in error or if anything is confusing, please feel free to contact me or comment below.

And so I should add -- this information is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.