[R-390] A 6kc. mechanical filter for a R-390a?

[Bob kb8tq]

Hi

To measure something like the input to the old style filters, you need to dig out the ever popular “admittance bridge". It’s sort of
the forerunner to the modern S parameter test sets. One in good shape will give you the data. You *can* do the measurement with
more modern stuff, it is a bear of a measurement to calibrate properly (unless you cheat …). 

Bob


> On Jun 15, 2017, at 11:01 PM, Al S. <racalra17 at gmail.com> wrote:
> 
> The TM 11-856a Army technical manual for the R-390a says the input and output impedances of the filters are in excess of 20k ohms and resonating capacitances of between 120 to 140 pf. How to measure the impedances accurately.
> Maybe its a known value. 
> 
> Al
> 
> On Jun 14, 2017 11:24 PM, "Alan Victor" <amvictor at ncsu.edu <mailto:amvictor at ncsu.edu>> wrote:
> First you need to establish the current terminating Z for the mechanical filters in the 390A. Then you need to know the required termination Z for the ones you wish to install. Designing a matching system to handle this problem is pretty straight forward. I do not believe the current 390A mechanical filters are that high a Z. At least the ones I have worked with at 455 kHz and built some receivers around were only about 3k//?? Cpf. I would have to look them up in my notes. 
> 
> Alan W4AMV
> 
> On Wed, Jun 14, 2017 at 7:41 PM, Al S. <racalra17 at gmail.com <mailto:racalra17 at gmail.com>> wrote:
> I seem to recall an article somewhere?, maybe the Y2k manual or notes by
> Mr. Dallas Lankford on using the modern Collins torsional filters as
> roofing filters in the receiver. Using toroidal matching transformers.
> 
> Al
> On Jun 14, 2017 10:04 PM, "Bob kb8tq" <kb8tq at n1k.org <mailto:kb8tq at n1k.org>> wrote:
> 
> > HI
> >
> > You will have to use some sort of transformer match. Doing really wide
> > range
> > impedance matching with networks is a pain. You probably are going from
> > megohms
> > to hundreds of ohms …
> >
> > Bob
> >
> > > On Jun 14, 2017, at 8:57 PM, Al S. <racalra17 at gmail.com <mailto:racalra17 at gmail.com>> wrote:
> > >
> > > I have one of the more modern blue cased Collins mechanical filters here.
> > > It was apparently used in the Racal 6790 receiver BW is 6.0 kc. 526 9957
> > > 040.
> > >
> > > This filter is designed to be used in that SS receiver and for lower
> > > impedance, than the higher impedance the stock 390a filters see.
> > >
> > > I would like to experiment with this filter in a 390a. Any ideas on
> > > toroidal matching to the IF deck in the 390a.
> > >
> > > Thanks
> > >
> > > Al VE3AJM
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