[R-390] Below 500 kHz mod, again?

Sun Sep 26 20:33:50 EDT 2004

Hi

At least from what I have seen the R-390 does a very good job from 6 
MHz on down. I would not be concerned about the 390 limiting the 
process of up conversion down there at all. As far as a reference is 
concerned it's pretty easy to divide what ever you have handy down to 
what ever you need. A reference TCXO out of an old cell phone should be 
up in the > 10 MHz region. You could divide it down to just about 
anything you needed.

To keep from changing bands on the 390 I would say there is a 
significant advantage to converting everything into a single 1 MHz 
span. Doing everything at exact 1 MHz multiples probably will get you 
in spur trouble so I would not go to great lengths to make that part of 
it happen.

A reasonable cell phone TCXO should be in the 1 ppm range over normal 
operation temperatures. That would give you a 4 Hz error / drift with a 
4 MHz conversion oscillator. At 12 MHz you would get 12 Hz. In this 
case lower is better for drift.

Depending on the antenna you may not be able to cover the whole band 
from DC to 500 KHz in one swoop. On the 390 they work pretty hard to 
only cover an octave (2:1 frequency range) with any one set of RF 
coils. The guys who did the 390 did a pretty good job of it. 
Duplicating the same thing probably isn't a bad idea ...

If we stick with octaves then we get something like 512 to 256, 256 to 
128, 128 to 64, 64 to 32, 32 to 16. Setting up five front ends is a 
construction project of the first order.

Once you get to about 110 KHz LORAN-C at 100 KHz is about all you will 
hear.  There isn't a lot of stuff below that,  that is very interesting 
to listen to. Dedicated receivers seem to work better for LORAN, WWV-B 
and that sort of thing. That would eliminate all but the top two 
octaves and get the construction project back to some kind of 
reasonable level.

Of course you could always just pick up a BC-453 and do it the easy way 
...

	Take Care!

		Bob Camp
		KB8TQ

On Sep 26, 2004, at 7:33 PM, Michael Murphy wrote:

> John,
>
> Never heard of this R390A sub-500 KHz mod either. The up-converter 
> idea Bob
> is talking about should work great with the R390A. Active voltage probe
> antennas and active loops work fantastic with an upconverter. There are
> plenty of antenna projects out there.
>
>  In the R390A I would try upconversion to a band like 10 MHz (as the 
> tunable
> IF). A few advantages here: First the R390A is dual conversion 8 MHz 
> and up
> and that means more stability, less noise and potentially fewer 
> birdies than
> at 2 MHz where it is triple conversion. (Bob?). Secondly, 10 MHz 
> crystals
> and 10 MHz stabilized references are easy to find for your converter.
> Finally, WWV lives here, so we can always "zero" everything.
>
> Tuning will be normal - upward, that is 10.200 MHz = 200 kHz.
>
> Of course we want to use a nice vacuum tube circuit with a 6BA6 
> preamplifier
> and a couple of 6C4s, one as a mixer and one as the 10 MHz oscillator 
> and
> some serious tuned circuits up front. Anybody got a circuit? It seems 
> that
> everybody uses boring solid state upconverters! These usually are 
> simple
> circuits with a low pass filter followed by single or double balanced 
> ICs
> like the MC1496/1596 or the NE602 or SA612. Passive switching 
> converters
> using CMOS gates or analog switches can also be utilized. Double 
> balanced
> mixers used backwards are very effective with an active antenna ahead 
> of
> them.
>
> Here is a simple circuit: http://jacksonharbor.home.att.net/lfconv.htm
>
> Here is a link to a circuit which is very similar to the first LF 
> converter
> that I built back in the late 1980's. Mine was a bit simpler, but used 
> the
> diode mixer idea as shown and it worked very nicely. I think I used a 
> 4 MHz
> tunable IF.  http://www.lwca.org/library/articles/kf5cq/lfconvtr.htm
>
> Mike Murphy WB2UID
>
> ----- Original Message -----
> From: "Bob Camp" <ham at cq.nu>
> To: "John Seboldt" <k0jd-l at seboldt.net>; "R-390 HF Receiver List"
> <r-390 at mailman.qth.net>
> Sent: Sunday, September 26, 2004 9:49 AM
> Subject: Re: [R-390] Below 500 kHz mod, again?
>
>
>> Hi
>>
>> I have seen a no change mod for the Racal 6790 but I don't think there
>> is one for the R-390. The way the front end of the R-390 works it 
>> would
>> be a major mechanical mod to get the poor beast running below 500 KHz.
>>
>> The best bet on an R-390 is to use an outboard up converter to boost
>> the DC to 500 KHz band up to something like 2 to 2.5 MHz. There are a
>> number of designs out there for these converters. Some of them do the
>> up conversion at the antenna so they can combine a whip antenna 
>> pre-amp
>> with the up converter. A bunch of the guys that show up in the 
>> archives
>> here have a neat little whip pre amp they came up with. It would make
>> an interesting starting point for a high performance antennal mounted
>> converter.
>>
>> Enjoy!
>>
>> Bob Camp
>> KB8TQ
>>
>>
>> On Sep 25, 2004, at 8:34 PM, John Seboldt wrote:
>>
>>> I read and deleted the simple no-damage mod for opening up the R-390
>>> below 500 kHz, before realizing that it probably would apply to my
>>> R-392 also! Could somebody repost it again?
>>>
>>> Thanks
>>>
>>> John K0JD
>>> Milwaukee, WI
>>>
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