An Automatic CW T/R System for Vintage Stations
by Greg Latta, AA8V
Main Page and Exterior Views

Overall View - Small
Click on the image for a larger view.

Click here for a super detailed view.

Background Material and Exterior Views
 Design Specifications
 Overall Circuit Description
 The Circuits Are Independent
 Exterior Views
Schematics and Circuit Descriptions
 Timer/Hold Circuit
 Sidetone Circuit
 Negative Keying Inverter
 AC Wiring Circuit
 Switch and Input/Output Wiring
 Power Supply

I love old gear, especially tube gear. As a novice, I started with a Hallicrafters S-120 receiver and an Ameco AC-1 transmitter. To handle the transmit receive switching I used a ceramic knife switch I purchased at the local hardware store. Over the years I progressed to better equipment and started using a Dow-Key antenna changeover relay actuated by the transmitter. Eventually, I graduated to a modern transceiver with completely automatic transmit/receive switching. Just touch the key, and I was transmitting.

Even though I had modern equipment, I still preferred to use the vintage tube gear. To me it was more fun and more of a challenge. However, the homebrew and vintage transmitters I was using sometimes lacked any kind of transmit/receive switching circuit, and I was back to manual T/R switching. I quickly found that automatic T/R switching had spoiled me, and I decided to design and build an automatic T/R system that would work with ANY transmitter and receiver combination. The result was this unit.

Design Specifications:
The specifications of the unit are as follows:
1. The unit is small in size.
2. It connects between the key and the transmitter. The key plugs into the unit, and the unit plugs into the transmitter.
3. It is positively keyed, so keyers without negative keying outputs can be used with it.
4. Though positive keyed, it can key both positive and negative keyed transmitters.
5. It runs on 120V AC. No batteries are required.
6. It has a timer/hold circuit that actuates the moment the key is pressed, and stays engaged for an adjustable hold time after keying stops.
7. A switch can disable the timer/hold circuit or set it to hold indefinitely.
8. The timer/hold circuit controls a 120V outlet, into which an external T/R relay, such as a Dow-Key coaxial relay, is plugged.
9. The timer/hold circuit changes the state of normally closed (NC) and normally open (NO) outputs to facilitate receiver muting and other similar functions.
10. The unit has a sidetone generator and self contained speaker with adjustable volume and pitch controls.
11. A switch can turn the sidetone off and on.
12. It is solid state and uses readily available parts. It does not use any programmable devices or other specialized parts.

Overall Circuit Description:
The Automatic CW T/R system contains four circuits:

1. Timer/Hold Circuit: The timer/hold circuit is the heart of the system. Whenever the key is pressed the timer is activated and a relay is closed. The timer remains activated and the relay closed until the operator stops sending. The relay remains closed for an adjustable hold time after the operator stops sending, as in a modern transceiver.

2. Sidetone Circuit: The sidetone circuit generates an audio tone with adjustable pitch and volume whenever the key is pressed.

3. Keying Inverter: The keying inverter allows the unit to key negative keyed transmitters in addition to positive keyed transmitters.

4. Power Supply: The power supply runs on 120V AC and provides regulated power to run the other circuits in the system.

The Circuits Are Independent Of Each Other:
The circuits in the T/R system are independent of each other. This means that if you only want the timer/hold function, you do not have to build the sidetone or keying inverter circuits. Likewise, if you just want to add a sidetone to your keying, you can just build the sidetone circuit and so on.

An Automatic CW T/R System for Vintage Stations
Exterior Views

Overall View:
The T/R system is built in a small sloping cabinet that I happened to have on hand. It was a tough job fitting everything inside this cabinet, but worth the effort. However, it is literally crammed full.

I would recommend a slightly larger enclosure if you have one available. This would give you a little more room to move around in. You have to fit in two relays, the circuits and power supply, and still have room for the speaker and connectors.

 Overall View - Small
Click on the image for a larger view.

Click here for a super detailed view.

Top View:
In this top view all of the switches and knobs are clearly visible.

At top left is the switch that turns the sidetone off and on.

Below the sidetone switch is the timer function switch. This has three positions: Off, On, and Hold. The timer can be turned off when only the sidetone or keying inverter functions are desired. When placed in hold, the unit is placed in transmit mode indefinitely. This can be useful when tuning up.

The top two controls in the middle control the volume and pitch of the sidetone generator. The bottom knob controls the delay time (hold) time, the amount of time the unit stays in transmit after the operator stops sending.

Seven holes were drilled to accomodate the speaker.

 Top View - Small
Click on the image for a larger view.

Click here for a super detailed view.

Rear View:
The rear panel of the T/R system contains the various outputs. The 120V AC outlet at top left is activated in transmit. This is used to activate a 120V Dow-Key antenna changeover relay. If a cube tap is used here, an "On The Air Light" could also be activated in transmit mode. Current should be limited to about 1A.

The state of the connectors at bottom right is changed when in transmit. For example, I use the NC (normally closed) connector to mute my receiver. I have the receiver mute circuit of my 6x2 (or Hallicrafters SX-96) connected to the NC connector. When the system goes into transmit, the NC connector is opened, muting the receiver.

Likewise, the NO (normally open) circuit closes in transmit, and could be used for a variety of functions.

The keying outputs are connected to the key jack on the transmitter. The XMTR+ output is used with positive keyed transmitters, and the XMTR- output is used with negative (grid block) keyed transmitters.

 Rear View Small
Click on the image for a larger view.

Click here for a super detailed view.

Front View:
A key, keyer, or bug is plugged into the key jack on the front of the unit. The unit is positive keyed, so it is compatible with any keyer. The power switch turns the unit off and on, with a green pilot light showing when the unit is on. A red LED lights whenever the system is in the transmit mode.

 Front View - Small
Click on the image for a larger view.

Click here for a super detailed view.

meterBack to Dr. Greg Latta's Electrical Engineering and Amateur Radio Pages

Questions, Comments, and E-Mail

LetterIf you have any questions or comments, you can send E-Mail to Dr. Greg Latta at

Thanks for stopping by!