Friday, 21 August 2009

Compressor limiter circuit

Below is a circuit for a limiter compressor
that can be used with any transmitter.
If not being used with the series mod just omit
everything after C15 around the U1:D section.

Dave Martin

Wednesday, 17 June 2009

The Corsette Low power SW Transmitter.

The idea behind the Corsette was to come up with a simple QRP AM TX for 6 to 7 MHz, that could be built in a very short space of time.
The circuit below is basically the first 2 stages of a Corsair,it must be noted that there is no low pass filter on the output. You should add one as even at these low powers harmonics can be a problem.A suitable filter will follow

Alternative oscillator for Corsair and Commando.

One disadvantage of the original oscillator circuit was the lack of available frequency shift.
The circuit below addresses that problem for the versions that run between 5 and 7 MHz.
Depending on crystal used you may find a tunable range of 7 KHz.
Just replace the first 2 stages from the normal circuit with those below.
To use on other bands the windings on L1/L2 would need changing.

Tuesday, 16 June 2009

The Commando Transmitter by Dave Martin.

The "Commando" is the higher power,high level modulated version of the "Corsair"
and was designed by Dave Martin in 2003.

Below is the circuit for the basic 48M version.
The table below it gives the changes that are needed for other bands.
(Click on pictures for larger size.)
Commando TX

Next is the circuit for the modulator, it is based around a
TDA2003 stereo power IC running in bridge configuration.

The Transformer, AL21219 above, is from a PYE Westminister W15AM PMR set. It may be available from "Garex" . As an alternative you may construct your own using a small transformer such as mains to 12v 1amp.Remove all the original winding and wind the primary with 80 turns of 24 SWG enamelled wire, this will be the audio side that connects to the TDA2003's pin 4's. The secondary winding is 180 turns of 22 SWG enamelled wire,
this is the side that gives the modulated VDC.

The above modulator is
needed if you intend
running portable with a 12 Volt car battery.
If you are to run from hom
e with mains AC you can
construct the series modulator below instead.


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The Corsair Transmitter by Dave Martin

The CORSAIR transmitter was designed by
Dave Martin of W.N.K.R. in 2004.

It is a 10 watt AM short wave transmitter that does not need a modulation transformer,
thereby much simplifying the design and still only needing 12VDC allowing it to run from a vehicle battery.
The "Pierce" oscillator uses a common FeT, the BF245.
The driver stage is a BFY 51 and is series modulated by a LM386 Audio IC.
The output stage is a very robust MoSFeT,
the IRF530.
It is possible to achieve more
output power than 10 watts.

But to give excellent audio reproduction, the power has been kept to the lower level.
On these pages we will outline versions of the "CORSAIR" for use on different bands.
At present versions have been made and used on air on medium wave through to 32 metres (9.3Mhz)

A LM386 audio IC is used to series modulate Q2. The LM386's output on pin 5, sits at half the supply voltage. But is swung plus and minus with the audio being fed to it's input.In the high current version Q4 allows Q2 to draw more current without damaging the LM386. The PA, Q3, is a IRF530 which is configured as a linear amplifier. To set up you should, by adjusting RV2, set the DC voltage at the junction of L8 & R7 to 2.5 VDC. Then by moving the turns on L6 further apart or closer together, get the RF power to read 10 Watts. It must be noted that Q2,Q3 & Q4 must have heatsinks and that Q3 & Q4 should be mounted using a TO220 insulation kit.
The table below shows the component changes needed to the PA and Filter section to use the CORSAIR on other bands.

All the above coils are wound using 22 - 24 SWG,
enamel covered copper wire.

Click on any picture to view large size.

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