On 26 Sep 2007 at 15:21, Kevin Purcell wrote:
> Adding an attenuator to the Argosy is perhaps one of the best things
> you can do.
>
> The RF amp is always in circuit in the Argosy in front of the mixer
> which for the low bands is just not needed. The noise figure of the
> antenna is much higher than the noise figure of the front end. But
> due to some cunning design the RF amp is reused as a post-mixer
> driver in the transmit chain so it's difficult just to take out of
> circuit.
>
> The nice feature of the 1980s TT rigs is they're separate boards
> linked together at 50ohms so it's easy to grab a signal and
> manipulate it rather than hacking on a PCB track.
>
> There is a nice rx only link from the LPF board to the Mixer board
> that you can use install a attenuator.
>
> WA3IFY, how did you add yours?
>
> 73 Kevin
>
I've tried several different approaches and have settled on switching a
resistive pi type attenuator with a really tiny dry circuit DPDT relay. It
simply
inserts in series with the coax cable that runs from the LPF/SWR board to
the first mixer board. Very easy to do. I switch it with the power on/off
switch of the volume control. ( after I remove the 120 volt wiring of course !)
The mixer end of the circuit is connector #33. I build a very small circuit
board, roughly the size of a postage stamp, and use MTA connectors just
like TT uses in the Argosy. Makes it a snap to remove or do other service
on the radio if need be. I've never had occasion to remove it though.
73,
-Lee-
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