[R-390] Coil forms and more

[Jim Brannigan]

John,
The SB-620 has a mixer in it. (now set for a 3395kc input) I was trying to
find the frequency of the oscillator Heath used to mix 455kc. to the 350kc.
IF of the SB-620.

My "guess" was confirmed, it is 805kc.

Certainly, another solution is an external mixer for 455kc. to 3395kc.

I plan to use it with a 75S-3B.  Actually my R-390A had a "panadapter"
output on it.  A 6BE6 used instead of the 6C4 in the third mixer.
I removed it last winter in a general maintenance session.

Jim

> Hi Jim,
> if I read your post correctly you are attempting to mix down 455 kc to
your
> Scanalyzer IF for use with a 390 rx. Cool!
>
> In this case you want to use a low-side LO injection frequency, because if
> you use a high-side LO you will end up with a spectral inversion which
will
> be very evident on your Scanalyser display!
>
> The unwanted mixer products should be far enough way because with the 390
> you'll only be able to see a narrow slice of the band anyway:
> - if you are monitoring the IF output then the scanned bandwidth will be
the
> 390 IF bandwidth
> - if you are tapping off the 3d mixer (recommended) the bandwidth will be
> only 35 to 40 kc max due to all
>    the cascaded selectivity in the RF and 2nd IF stages (yes, it is that
> narrow).
>
> Also, try to use a balanced or double balanced mixer, you'll get far less
> troublesome RF and especially LO feedthrough into your scanalyzer IF.
>
> John
> KA1XC
>
> ----- Original Message ----- 
> From: "Jim Brannigan" <jbrannig@optonline.net>
> To: <R-390@mailman.qth.net>
> Sent: Wednesday, June 11, 2003 7:45 AM
> Subject: [R-390] Coil forms and more
>
>
> > Thanks to all for the suggestion and help with my SB-620.  After many
> years
> > the 455 kc. conversion project might get off the ground.
> >
> > Does anyone know the mixer oscillator injection frequency?  It is not
> > stipulated in the manual or on-line sources.
> >
> > Here is my guess:
> > The IF of the Heath "Scanalyzer" is 350kc.
> > To convert 455kc., the oscillator may be either 805 or 105kc.
> > The "high" injection of 805kc produces first order products of 350 and
> > 1260kc.
> > "low injection of 105kc produces 350 and 560kc.
> >
> > The "High" injection products are further apart and it would be easier
to
> > filter the unwanted sum frequency.
> > So my "guess" is 805kc.
> > I know the value of the parallel capacitance.  If the oscillator
frequency
> > is known, I can calculate the coil inductance.
> >
> > Any help or ideas greatly appreciated.
> >
> > Jim