Fw: [Premium-Rx] RE: Mods for Racal 6790GM

[Michael O'Beirne]

----- Original Message -----
From: "Michael O'Beirne" <michaelob at tiscali.co.uk>
To: "Dan Seven" <danseven at sasktel.net>
Sent: Thursday, February 17, 2005 11:11 PM
Subject: Re: [Premium-Rx] RE: Mods for Racal 6790GM


> Dear Dan,
>
> I don't have an RA6790 but I do have the similar RA1792.  Here are some
> thoughts:
>
> 1.    Sensitivity - you can either enable or bypass the RF amp.  There is
an
> internal link inside the screened module.  With the RF amp in, you do not
> need any external gain.  It's very sensitive already.  Mine has a measured
> MDS of -131dBm with the SSB filter, which is about average.
>
> 2.    Preselector - In most locations and with most normal antennas you
> should not need an external (passive) preselector.  The front end and
mixer
> are pretty bombproof.  However, it is a fully wideband system and
sometimes
> preselection can be a good idea.  I use the ex-British Army "Acceptor
Unit"
> made by Marconi in the 1960s.  It's a simple, manually-tuned, double tuned
> circuit covering 2 to 30MMz in 4 bands and provides an ultimate
selectivity
> down to about 50dB.  I used these "for real" in the 1970s with a large
> mobile station called the D11 / R234 working FSK with full two frequency
> duplex, and they are very good.  They used to cost about £6 at hamfests in
> the UK because they were boring green boxes for 19 inch racks with
military
> coax connectors and no one had any idea what they did or how good they
were
> (except those in the know of course and we bought them pronto!!).
>
> Racal made a special automatic passive preselector for this set called the
> MA1101(from memory - or 1011).  This is tuned via the local oscillator
from
> external LO output on the RA1792.  The LO signal is applied in the
> preselector to some memory logic and a look-up table that in turn
activates
> a motorised bandswitch and a motorised capacitor to tune the device.  As
you
> tune the receiver the preselector's capacitors are adjusted to follow.
> There are 4 cascaded chunky tuned circuits and no active devices in the
> signal path, so the Q value should be pretty good.  The unit is about the
> same size as the RA1792 but weighs a ton.  It's all massive diecastings
> inside.  I have one bought cheaply on spec but it has never worked as I
> could not find a circuit or a manual, and I'm hopeless with digital
control
> circuitry, though it is a most interesting talking point.  I believe the
> original cost was more than the 1792 (some folk say a lot more).  These
are
> rare devices.  I guess the main use would have been for warships.  Mine
has
> some traces of salt deposit on the metalwork.
>
> For MW/LW and VLF I use another passive preselector, this time made by
> Plessey called the PV157 (made originally for the PR155).  This tunes
15kHz
> to 3 MHz in about 6 bands (and also "DIRECT") and additionally has a
> switched ladder attenuator giving 0, 10, 20 and 30dB loss.  It is useful
> below 60kHz because the RA1792's synthesiser is not so clever down there,
> and is fairly useless with excess phase noise at 16kHz.  A broadband input
> drowned the 16kHz station at night when I could also hear half the very
> powerful French Long Wave stations as well at my QTH using an AD370 active
> dipole!!!  [16kHz used to be the VLF station from Criggion in Wales used
by
> the MoD to communicate with submerged UK nuclear submarines.  It has an
> identifiable, very narrow shift RTTY signal, like a warble.  Criggion
could
> be heard all over the world.  Now the place has been abandoned and the
huge
> masts resembling the Eiffel Tower pulled down, and the transmissions come
> from other stations in the North of England.]
>
> For VLF a better approach is to use a dedicated upconverter using a
crystal
> controlled LO.  A good one is the Datong product.  This converts the band
> 0 - 1MHz up to 28 - 29MHz.  Thus you would tune Criggion as 28,016kHz. It
is
> very sensitive and overloads if you apply a long wire aerial to it.  10
feet
> of wire is often enough.  A true professional would use the dedicated
Racal
> product, the RA137.  These are ancient technology but excellent, and I
have
> seen a special mod to the RA1792 by our listening boys at GCHQ to use the
> RA1792s with them.
>
> I hope this helps.  Contact me directly if you need any more info.
> 73s
> Michael
> G8MOB
>
> ----- Original Message -----
> From: "Dan Seven" <danseven at sasktel.net>
> To: <premium-rx at ml.skirrow.org>
> Sent: Thursday, February 17, 2005 8:23 PM
> Subject: [Premium-Rx] RE: Mods for Racal 6790GM
>
>
> >
> > Hello Everyone:
> >
> > I am interested to know if there are any documented mods available to
> > improve the performance of the Ra6790GM.
> > For instance, has anyone tried a preselector with this unit, or outboard
> > preamp?
> > Any help would be appreciated.
> >
> > Thanks, Dan Seven
> >
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