Chatterbox Transmitter and Receiver
This Webpage has been added to my website at the request of many Radio Amateurs,
who have listened to my transmissions on TOP BAND. I am told that my signal
doesn't sound like the original Chatterbox design and that they would like to know
what my Modifications are, so that they can incorporate them into their own version.
Unfortunately, this is not an easy task to do, as some of the modifications made were
taken from a number of unknown sources. Some of the modifications were suggested
by the students who may have received their information from copyright material.
I have therefore not reproduced the circuit diagrams here, so as not to breach any copyright.
(Although I feel that the Authors would give their permission if asked, unfortunately they are not all
known to me and so I have made this decision)
I have however, given details of my Low-Pass Filter, which if not fitted to the output
of the Transmitter, would have allowed a large number of harmonics to be radiated,
giving possible rise to interference - also, a BFO for the reception of SSB and AM
on the Receiver. The BFO is very stable, and I use it when SSB or CW stations call in
to make contact.
I wish to point out that the Chatterbox Transmitter and Receiver was originally
designed by G3RJV (The Rev. George Dobbs) and G3ROO (Ian Keyser). But for
their efforts, I would not have started the project. The information given below
is only given as a guideline. If anyone would like a copy of the original article, then
they should either contact the authors or write to Practical Wireless. (Printed Circuit Boards for this Project can be obtained from the PW PCB Service). The original
articles appeared in Practical Wireless Magazine Aug/Sept 1991.
I became personally involved with the Chatterbox Transmitter and Receiver in October 1993. My Construction class for the Radio Amateur's Course had just started at the Local College and I was looking for a project - one to get the students involved in building their own Transmitters and Receivers.
To cut a long story short, there were many problems with the Transmitter.
I decided that the only way to resolve the problems quickly, was for me to build
a prototype at home, where I could work without interruptions.
The result was very time consuming, but very rewarding.
The Chatterbox Transmitter
The major modifications were made to the main Transmitter Board.
The Transmitter as a whole was unstable. This was corrected by re-designing the layout of the tracks
on the printed circuit board, followed by fitting additional decoupling capacitors.
The Modulation was very poor and distorted. I have been told by other Radio Amateurs, that it would
have been better to use a larger Audio Power output Integrated Circuit. This would have required a
larger supply voltage - the original design uses +12Volts.
The audio level was improved by applying modulation to both the Driver and the Power Amplifier stages.
Many Radio Amateurs have told me that they have tried commercial modulation transformers with
success. I am still using a home wound Modulation Transformer which has given no problems at all.
Note: I am aware that a Pye Westminster ex-PMR Model W15 A.M Modulation Transformer is
very similar in size to the 'home wound version' and fits on the Printed Circuit Board without any
problems. (Part No. AL21219).
|A very important addition to the output stage
is that of a Low Pass Filter. Having built a Spectrum Analyser,
I discovered strong harmonics on eighty and forty metres. The LPF above has a 'Cut-off'' at 2.5MHz.
Tests now show that harmful harmonics (those which cause interference) are completely eliminated.
| Output from Low-Pass Filter
home built Spectrum Analyser.
|The circuit above shows the switching arrangements.
Position 1. is to NET. Position 2. is to Receive.
Position 3. is OFF. (It also provides a PAUSE between Receive and Transmit (Position 4.). This allows
time for the Receiver to go dead before the Transmitter activates. (Plop in the Loudspeaker)
|The Speech Processor used with the Chatterbox|
|Even after boosting the audio in order to
increase the level of modulation, the modulated output signal was sub-standard.
This has been overcome by using a Home Built Speech Processor.
The Speech Processor was designed around six BC108 Transistors. The measured frequency response of the completed unit at the -3dB points is 350Hz to 3KHz. The power is taken from the 12Volt supply used by the Chatterbox. The Speech Processor requires a 9Volts d.c. supply, and
this is provided by the regulated power supply shown below.
|When the Speech Processor is used, R12
on the Main Transmitter Board is set to the '1/3rd on'
position, and the modulation contains all the extraneous sounds of the children in the living room.
In fact, I have to turn the audio gain down, as the Transmitter output could easily become over
|The Chatterbox Receiver|
The Chatterbox Receiver is excellent. The only problem being that it doesn't have a BFO fitted.
The Circuit below is that of a Beat Frequency Oscillator which is stable enough for the reception
of Single Sideband. (The Transformer was taken out of a scrap Transistor Radio Receiver.)
It is worth noting that:
The Intermediate Frequency is 455 KHz. The Oscillator Frequency is 1345 - 1545 KHz The Receiver Tuning Frequency is 1800 - 2000 KHz
The output from this BFO is quite high. So as not to overload the Receiver
it is necessary to limit the coupling. This is achieved by connecting a 2pF capacitor to TR1 Collector, and then connecting the other end to a short length of wire supported about 1cm above IC4 on the Receiver PCB using insulated wire supports.
When building Transmitters, especially when FETS are
used in Power Amplifier Stages, it is important to check
for Harmonics. FETS are prone to generating Harmonics.
The best way to check, is to use a Spectrum Analyser.
Block Diagram showing outline of Transmitter, Low Pass Filter and Receiver.
The Power Supply is 13.8 Volts and is also Home Built. This is described elsewhere on this Website.
The Maximum Measured Power Output from the G3NGD version is 6.25Watts [7.9dBW]
Currently, September, 2010, G3NGD is using his New Six-Band Trapped Dipole (14MHz - 52MHz) with the
feeders strapped, Series Tuned against Ground as the Aerial. This seems to work well and makes the whole set-up a 'Stand Alone' Home Constructed Station.
|The set-up could be used
as a portable station and a Kite could be used as the Aerial.
The Aerial could be tuned through the Tuning Unit already used by G3NGD.
This is basically, 29 Turns of 20SWG Copper Wire wound on an 8cm dia. former, tuned in series with a
2x500pF tuning capacitor (1000pF) taken from an Old Valve Radio Receiver of fifty years ago.
John built his own Box Kites in the 'Sixties' and transmitted to Ireland and the I.O.M. during the daytime.
The advantage of using this set-up is the fact that it will work off the 12V car battery.
In the 'Sixties', the equipment needed 200V D.C. and G3NGD built an Inverter, 12V in and 200V out.
The unit used OC35 Power Transistors and the equipment is shown elsewhere in this website.