Activating AN/ARC5 Command Radio Set
Transmitters from WW2
Toby Haynes October, 2024
Link: VE7CNF Ham Radio Pages
The AN/ARC5 “Command Radio Set” was used in aircraft in WW2, and included “communication transmitter” components covering shortwave frequencies up to 9.1 MHz. They were used for CW, and for AM with a separate modulator/power supply unit. See the excellent Wikipedia article about “AN/ARC-5” for details of the radio set components and model numbers.
Command set components were sold as surplus after the war. They are of rugged construction so units in good condition can still be found. With a little restoration work, minor modifications, and a suitable power supply the AN/ARC-5 transmitters for 80m and 40m are reasonably stable and will produce over 50W of CW output power.
A few years ago I was given a BC-458-A transmitter, suitable for 40m, from the collection of VE7SL. I was impressed by the quality of construction and pristine condition inside the chassis. It was built in 1943 and remained unmodified. Due to its design simplicity and advanced age, I didn’t expect much performance from it. I put together a power supply, did a few repairs and modifications, and put it on the air. It put out more power than I expected, about 55W, with significant chirp and drift. Over time, and mostly with changes to the power supply, I tamed the chirp and drift somewhat while keeping the RF power output high.
I found aT-19 command set transmitter, which covers 80m, for sale online. The pictures looked OK, so I took a chance and bough it. It was in similar condition to my 40m unit, slightly battered outside but clean and unmodified inside the chassis, and it came with tubes.
This article describes my process of getting the T-19 80m transmitter on the air. A few of notes are added for the 40m version.
My unit had a few cobwebs and bug parts inside, and minor spots of corrosion where the chassis contacts the bottom cover.
Remove tubes and clean their glass, pins, and caps. Apply contact cleaner to tube pins and caps when they are re-installed. Don’t stress the glass-to-base or glass-to-cap joints, as these can be fragile on old tubes.
Clean the unit with compressed air, a vacuum cleaner, paintbrush, a cloth dampened with slightly soapy water, rubbing alcohol, etc. Clean inside the tube sockets, brush off variable capacitor plates, and blow out coil forms. Blow and vacuum air through the oscillator shield holes. Clean the through-chassis ceramic insulators and around terminals on the coil forms to avoid arcing. I used fine steel wool to clean corrosion from the bottom cover.
Apply lubricating contact cleaner to the rotating contacts for the variable coupling link on the plate tank coil, while rotating the link to work it in. Apply a little light oil to all shaft bearings. Apply a little synthetic grease to the worm gears.
Cleaning and de-oxidizing the roller inductor so that it works reliably is a separate subject. I don’t use the roller inductor, so just did some cosmetic cleaning.
The original 7-pin power connector on the rear is not compatible with any available modern connector. I chose to replace it with an octal tube socket. [ The T-19 and older BC-458-A use connectors of different sizes and pin connections. I wired the new connectors on both to match my power supply. ]
Unsolder wires from the connector, then remove it by prying the inner crimped flange open. Drill two holes for 6-32 screws and install the tube socket. [ The BC-458-A uses a smaller power connector than the T-19, so the chassis hole had to be filed larger for the tube socket to fit. ]
I like a ground stud on my equipment, so I drilled a hole right behind the bathtub capacitor and installed an #8-32 brass machine screw and nut. A brass thumb nut on the screw can be used to secure a chassis ground wire.
AN/ARC-5 transmitters are not designed for use in a modern ham shack with 50 ohm antenna systems and coaxial cables. They were designed to connect to a wire antenna that is much less than 1/4 wavelength long, with an aircraft fuselage as ground. Such an antenna is non-resonant and is electrically equivalent to a capacitance, typically around 100 pf, in series with a low resistance of 10 to 20 ohms.
The AN/ARC-5 transmitter output circuit uses a parallel LC tank connected to the final tube plates. Inside the final tank coil is a rotating variable output link coil that intercepts some of the tank’s RF magnetic field to produce an RF output voltage for the antenna. The rotating link has 4 turns for an 80m transmitter [ and 2 turns for a 40m transmitter ]. That connects through a series roller inductor, visible through the front panel window, to the antenna relay and connector on the front panel.
An AN/ARC-5 system includes an antenna current meter for tune-up. When transmitting, the series roller inductor is adjusted to be resonant with the antenna, as indicated by maximum antenna current, and so the coil cancels out the antenna’s capacitance. The antenna’s remaining low resistance is driven from the variable output link in the final plate tank coil. Link coupling is adjusted using a front panel knob, and the load resistance on the final tube plates drops as the link coupling is increased. The coupling is adjusted for matched loading of the final tubes, as indicated by a maximum reading on the antenna current meter.
To drive a 50 ohm resistive load, the series roller inductor is not necessary and can be left unused. Turns must be added to the output coupling link on the final tank coil so that proper tube loading and maximum output power can be achieved with a load resistance near 50 ohms. Finally, a coaxial output connector can be added.
With the modifications described here, an AN /ARC-5 transmitter can be used in a modern shack with 50 ohm coaxial cables feeding matched antennas. It may be necessary to use an external antenna tuner to correct for high SWR, as with any modern transmitter.
I mounted a BNC connector on the front panel in place of the original antenna connector. This required making some custom parts from sheet aluminum. Your solution may vary, depending on the materials and tools you have. Or, you could just drill a new hole in the front panel to mount a BNC connector.
Remove the antenna connector from the front panel by loosening the relay contact behind it. Also remove the lower grounded relay contact just below the antenna connector, and it’s front panel screw.
To mount a BNC connector into the antenna connector hole, I made a front washer, inner centering washer, and rear plate.
Front washer 0.063” AL
sheet, ID 0.370” for BNC, OD 0.76”
Inner centering washer 0.042” AL sheet, ID 0.370” for BNC, OD
0.625”
Outside has a
notch for the alignment tab in the front panel hole
Inside hole has
a flat to prevent the BNC from turning.
Rear plate 0.042” AL sheet
Hole 0.370”
dia, with a flat for the BNC connector
Hole 0.093” dia
for screw through panel,
for ground
lug and grounded relay contact.
Install the BNC connector, front washer, and inner centering washer from the front. Loosely install the back plate with the BNC lockwasher and nut. Install the lower screw from the front, through a solder lug, to the grounded relay contact on the rear. Tighten the screw and the BNC nut.
The variable output coupling link coil is located inside the final plate coil, on a shaft with rotating contacts. The rear rotating contact for the link is grounded, while the front rotating contact connects through the roller inductor to the antenna relay.
Disconnect the coupling link by unsoldering the grounding wire from the rear rotating contact and bend the wire out of the way. Unsolder the wire from the front rotating contact to the antenna roller coil and bend the wire out of the way.
Use solid copper wire, around 20 ga. and with 600V or higher-rated insulation, to add turns in series with the variable coupling link. The winding direction of the new turns is important. Add 2.5 turns for 80m. [ Add 1 turn for 40m .] Solder the wire to the right lug on the rear rotating contact, then wrap it around the outside of the plate coil clockwise as seen from above. Connect the other end of the added turns to a ground lug. The new turns must be tight to the plate coil form, underneath any other wires, and near the same level as the coupling link coil itself.
Use RG58 cable to connect the front rotating contact of the variable output link to the front panel BNC connector. The shield braid must be soldered to a ground lug at each end of the coax cable.
I checked the molded mica and tubular capacitors for leakage with a multimeter and found no problems, so left them as-is. However, be suspicious since even molded mica capacitors may fail after 80 years.
A triple bathtub capacitor contains three 0.05uF foil capacitors in a single metal can. These capacitors may fail with age. Modern film capacitors can be put into the old can to preserve the original appearance.
Use a hacksaw to carefully cut around the can just below the top ring. Open the capacitor, cut the wires, and remove the old wax, capacitors, and insulators. The old wires can be removed by heating the terminals and the back of the can with a soldering iron, then pulling out the wires. Clear solder from inside the old terminals and from the hole in the bottom of the can. Clean the inside of the can by scraping, then with solvent to remove the wax.
With careful arrangement and bending of leads, three 0.047uF 630V film capacitors can fit into the old can with their top leads lined up with the three terminals and the bottom leads all in the center. Trim the top leads to fit into the cap terminals by about 0.2 inches, then solder them into the terminals. Drill out the hole in the bottom of the can to fit all three ground leads, then pass the ground leads through and assemble the capacitors into the can. Align the top cap to give good contact with the can, and with proper orientation of the cap terminals with the mounting posts. Solder the cap back onto the can in a few places around the perimeter, but leave gaps to allow venting. Trim and bend the capacitor ground leads and solder them to the bottom of the can.
Later transmitter models like the T-19 use an RF choke in the final plate supply, probably to allow for plate-modulated AM. [ The earlier BC-458-A feeds the plate supply to the tank coil and uses a bypass capacitor to ground on the DC end of the coil. ]
I added a bypass capacitor (1.9 nF 1kV film type, but not critical) from the plate choke high voltage feed-through to a ground lug. This removes any RF voltage from the plate supply wire under the chassis.
Some of the old carbon resistors will probably have failed. Carbon resistors drift upward in value with age. The resistance may be unstable and vary with mechanical stress and temperature changes. Unstable resistors may cause changes in oscillator frequency and output power. Series plate and screen dropping resistors, grid leak resistors, and cathode bias resistors should be close to their specified values to get full transmitter power output.
Measure all resistors and note the values. Then, push the resistors to bend the leads a bit. Use a soldering iron to tin the resistor leads a short distance from the resistor bodies to heat them. Measure the resistances again.
Resistors that change value due to stress or heating will be unstable and should be replaced. I also replaced resistors that had drifted more than 10% above their specified value.
I added a 680 ohm resistor in parallel with the eye tube filament’s parallel resistor to bring the oscillator tube’s filament above 12V.
I don’t use the antenna relay, so I disconnected it’s coil by leaving it’s black wire unconnected.
The CW keying relay is under the chassis. The shorter contact connects plate power to the oscillator, and the long contacts ground the cathodes of the final tubes. Bend the short contacts closer together, and the long contacts farther apart. This will cause the oscillator to turn on before the final amplifier, and so reduce chirp.
Across the keying relay’s 24V coil, connect a 390 ohm resistor in series with a reverse-biased diode (1N4148, 1N4001, or similar with anode to the black wire). This suppresses the high-voltage transient when the relay turns off, and the resistor absorbs energy from the coil so the relay turns off faster. [ For the BC-458-A, the resistor reduced the turn-off time from 16ms to 8ms. ]
Test the tubes for cathode emission on a tube tester. Select two 1625 final tubes that have similar emission values, to give good load sharing since they are connected in parallel.
With a 24V filament supply connected and the tubes warmed up, measure the tube filament voltages to check that they are all close to half the supply voltage.
Screws for the bottom and top covers are #3-48 binder head, about 0.2” long, with internal-star lock washers. There are 31 screws.
A manual switch can be used to switch the antenna between the receiver and transmitter. Be sure it’s a shorting-type switch with high enough isolation to protect the receiver. It should still be possible to hear the transmitter in the receiver, to monitor your own transmission.
Connect a 50-ohm load to the transmitter. Start at the minimum output coupling setting (zero), which will give low plate current. Set the VFO frequency. Set the link coupling for maximum output power, which is at a knob position around 4 on my transmitters. Reduce coupling slightly to drop the plate current by about 10 mA, and the output power should drop very little. This gives efficient operation, at a final plate current around 135 mA with my transmitters.
My high voltage power supply is still a prototype. Choke L1 reduces ripple on the plate supply, to prevent 120Hz hum modulation. The plate supply is 555V key-down, and a meter monitors plate current. Tube screens are supplied through series dropping resistors at 280V key-down. Zener diodes D1-D4 limit the screen voltage to 400V key-up to protect the screen bypass capacitor in the transmitter. Two gas regulator tubes, V1 and V2, keep the oscillator supply near 208V. A cable runs from the power supply to an octal tube-base plug that connects to the transmitter.
A separate 24VDC switching power supply provides low voltage for the tube filaments and relay.
(c) Toby Haynes, VE7CNF. 2024