Howes Radio Sidetone Circuit

BRINGING HIGH 500 southlake boulevard

TECHNOLOGY Richmond. Virginia 23236

DOWN TO EARTH 804-794-2500 FAX: 804-794-8284

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73 Amateur Radio Today * April, 1994 75

Mike Bryce WB3VGE 2225 Mayflower NW Massiilon OH 44646

While doing a review on the Howes transceiver kits, I found they lacked even the simplest T/R switching—it's done manually with a front-panel-mounted switch. The lack of a sidetone also proved frustrating to me. The kit did have a module for RF sensed sidetone generation, but that required a second speaker; one speaker (or headphone) for the receiver and another for the sidetone I toyed with ttie idea of using a pair of stereo headphones—one side for sidetone* the other tor receive audio. I trashed this idea based solely on my experience with stereo headphones and amateur receivers,

Stereo headphones have an audio response much too wide for pleasant listening. A 10 kHz beat note can really be appreciated after hearing it through a quality stereo headset Reducing the audio bandwidth is especially important with a direct conversion receiver.

So. to fist both problems, I built up the circuit shown m Figure 1 It's a combination QSK module with sidetone generator. It also has a reed relay for keying the emitter lead of the driver transistor used in the Howes transmitter Today's electronic Keyers normally use a transistor pulling the key line to ground. This method worfcs very well . . , most of the lime. But, Ihe emitter-collector junction, with its 0.7 volt drop, will not pull the key line ail the way to ground. This may cause trouble when keying a rig using emitter keying, such as in the Howes transceiver.

Another drawback with ihe Howes system of T/R control is Ihe ability to key the Iransmitter without switching the antennas. This could destroy the PA transistor in the transmitter, or cock the receiver. My QSK module prevents this from happening.

The QSK Module

A multi-pole relay does the switching between the receiver and the transmitter. The relay switches antennas, grounds the receiver's front end, and has several contacts left over for other tasks.

The reed relay keys the transmitter while the sidetone is injected into the receiver's audio chain, You can adjust the delay between transmit and receive with a front panel control. This coniroi replaces the manual T/R switch on the transceiver, it s possible to gel 'liH QSK. if you don't mind the clicking of the main relay as you key, tn our bells and whistles department, a red LED glows when the module goes into transmit mode I installed this LED behind the translucent face of the meter, it looks nice and, best of

76 73 A ma teur Radio Today »April,

Photo 8 Controller inside the bot torn of the Howes transceiver

Photo A The T/R controller ts built on a smalt piece of pert board The retay is mounted so its contacts are toward the edge of the board.

output of U1A. With Q4 on, Q1 is turned off. When 04 goes off, 01 turns on and keys the rig via the contacts of the reed relay. Transistors snd Q1 follow the keying at the Ri R2 junction. The reed relay provides a direct-to-ground keying for the Howes transmitter

Construction

Although when first looking at the schematic the QSK module seems complicated. In tact it's really two 10s and some transistors. You could use fewer components, but 1 think you'll get sloppier operation, too.

This module is built on a piece of copper-clad pert board available from Radio S hack. The circuit is simple, so no PC board is available. If you re so inclined, lay one ouï if you wish. The relay is mounted on its side using a piece of ctoubie-skJed tape. Diode D4 is mounted across the coif pins and not on the pert board. I used IC sockets for the LM324 and the 555 timer chip.

I placed the LED inside an LED lens before I giued the combination to the back of the meter s face with a drop of super glue. The lens does little to make the LED brighter, but its flat face makes the glue hold better.

Ifs best to build this circuit and test as you go. Testing as you go when building on pert bo arc can make troubleshooting easier. The reference voltage source would be the first to go on the perf board, followed by the delay, sidetone and finally the keying components.

After assembly, test the module out before you install it in your Iransceiver. I left the connections between the antenna, receiver and transmitter up to you. It s simple to do. Using miniature coax, solder directly to the pins on the reîay. it pre-asscmbied the coax before I mounted the QSK module inside the

Howes transceiver The QSK module had to be mounted on the bottom of the chassis. The 50k pot came with the kit and is placed in the front panel hole meant for the T/R switch.

Final Notes if you have more than S10 in this project, you've spent too much money. The project has junk box priority! You can substitute parts without much concern. You don't have to use a 7S05 either. A 78LOO5 is fine, any 5 volt regulator would work as well. Why, a zener diode and resistor may work, too.

The 50k delay control is way too now in value lor proper use. Capacitor C4 had to be a rather large value to allow enough delay. Why use the 50k pot to begin with? It was a leftover from the Howes transceiver kit. A 470k pot and 22 cap for C4 would be a good starting point if you want to experiment.

Although this project began as a fix to the manual T/R switching in the Howes transceiver, there is no reason why you can't use the basic module in your own QRF transceiver. It sure is simple, cheap and packs a lot of features for the money.

Photo A The T/R controller ts built on a smalt piece of pert board The retay is mounted so its contacts are toward the edge of the board.

Photo 8 Controller inside the bot torn of the Howes transceiver

Low Power Operation all. you don't have to drill any more holes in the front of the rig.

How It Works

A stable +5 volt reference voltage is supplied by U3, a 7805 regulator. A small load Is placed on the 7805 by R15. This helps keep the regulator stable. One section of an LM324 Is used to buffer the output of the regulator. In a circuit like this, the use of a buffer for the reference is overkill but since the amplifier was available, I look advantage of if. The +5 volt reference is used by the delay circuit, Capacitors CG through CQ are required to ensure stability. The entire QSK module is protected from reverse polarity by D1. a 1N4002 diode.

The QSK module is keyed by grounding the junction of R1 and R2. Normally, this junction is 3 volts. Resistor R3 and Cl help to remove any noise on the key line. Amplifier U1A buffers this key line before sending It out., The output of U1A is normally high. Keying the QSK module pulls the junction oi Ri and H2 to ground. The result is a low at U1 As output

With U1A output sitting hrgh tun-keyed). it goes to three different sub-circuiis sidetone generator, delay driver, and transmitter keying The sidetone generator is a 555 timer (what else?) and is kept off by Q2. This keeps the timer's reset pin held to ground. When the QSK module is keyed, Q2 turns off, allowing the timer's resel pin to go high. The result Ing output of the 555 is filtered by Ri 5. R16 and C9. This filter clips off the edges of the square wave to make it easier to lister to. DC blocking is provided by C10. The output level is set by trimmer R17. The resulimg 800 Hz tone is fed to the receiver board via the center terminal of the volume contra L

The delay circuit takes the high from UlA and compares il lo the reference voiiage when the QSK module is unkeyed, U1C outputs a high, charging up C4 via D2, This output is compared against the reference voltage by U1D. As long as the input is higher than the reference Q3 remains off.

Keying the QSK module changes things. U1C turns off, and C4 begins to discharge via R8 and R9t the delay control. When the charge on C4 is lower than the reference on U1D pin 13, it outputs a high turning on Q3, a power MOSFEX The relay then closes and the antennas switch. Diode D4 protects Q3 from the EMF caused by the relay coirs collapse.

Transistor Q4 Inverts the

Pre Tonal Com Lm324

LA/VV

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