----- Original Message -----
Sent: Tuesday, May 17, 2005 7:34 PM
Subject: LF: Modification of the blower
circuit and VFO drive for the ROPEX LF TRANSMITTER
To the members of the rsgb LF group
.
Those of you who own the ROPEX TX will
have the following experience :
The blower of the set is making so much
noise that it is sometimes difficult to copy the rather weak manual CW sigs
that you hear on LF .
I made the following change : You need
the following parts 1 transistor pnp 2 N 2905 a tagstrip and a few pieces of
wire . Disconnect the black minus wire of the blower and connect it to the
emitter of the transistor , connect the collector to the chassis and the base
to the collector of the BC547 antennarelaydriver this point is also
connected to the minus connection of the transmit Led . The result of this mod
is that the blower is off during reception and on during transmit with the
same delay as the keying circuit , so you can have a quiet shack for
listening to weak signals . As the finals do not generate any heat during
reception the cooling will be ample .
The second modification is more time
consuming :
The ROPEX is Xtal controlled and after a
few QSO,s you will get the urge to connect a VFO for QSY . For the
modification you will need the following parts : 1 miniature switch double
pole double throw , a BNC chassis female type plug,a 10
nf capacitor and a short piece of RG174 miniature coax or something
like it and a few pieces of hookup wire .
Now the most difficult part follows
: in order to remove the crystal you have to remove the complete case
including the heatsink .
Carefully remove the crystal with solder
wick . Make a hole for the switch and the BNC connector on the rear of the
chassis on the right side of the blower , this completes the most difficult
part of the modification , Now the soldering work . Connect the previous
connections of the Xtal on the cicuit board to the poles of the switch by
means of two pieces RG174 and the Xtal to one side of the switch ( if you
switch to this side the original circuit comes into action and the TX is Xtal
controlled as it used to be ) . Thereafter connect the BNC with the 10
nf capacitor to that side of the switch which switches to
terminal 10 of the oscillator IC HCT4060 . Thats
all.
Requirements for the VFO : Any VFO giving
about 200 to 500 millivolts on the same frequency as the Xtal will do for
manual CW.
IMPORTANT! The VFO has to be on or the
switch set to Xtaldrive otherwise the TX will take off at random
.
You can prevent this by leaving the
switch out and just connect the VFO to terminal 10 off the oscillator ,
more or less as in Dicks PAØSE transmitter ( described in the LF experimenters
handbook ) with the slight disadvantage that you cannot use the VFO on
the Xtal frequency .
73,William
PAØWFO
A good article on the Ropex William. It
is a very useful TX for LF considering its size. I have been using mine
recently and it performs very well considering the low power, although I have
a good LF antenna system.
One thing to mention, if you use a
grounded antenna matching system for 50 ohms at the base of the antenna the
Ropex will not work on the high pwr setting, this is a well known problem but
it can be overcome by using an extra LPF at the antenna output socket feeding
the 50 ohm coax to the antenna.
73 cul de Mal/G3KEV
