----- Original Message -----

From: PA0WFO

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 .

William you forgot to say, the PA explodes without drive !!!!!!!!!!!!!! not just TAKES OFF

The pa fets in my model are IRF1010N, maybe they have already been changed before !!

A protective cct is necessary so that the the TX cannot come on except there is already drive. I have such a cct on my home built TX as a precautionary measure.

Ropex modification operators beware because if the switch got moved accidentally from the Xtal position and there was no drive via an ext VFO you will end up with a heap of ashes!!

Beware de MAL/G3KEV

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