[Collins] wider audio for 51J4?

[Dr.Gerald N. Johnson]

Dr. Gerald N. Johnson, Electrical Engineer
----- Original Message -----
From: kirklandb at sympatico.ca [mailto:kirklandb at sympatico.ca]
Sent: 6/13/2006 10:59:41 AM
To: ehertz at tcaf.org;collins at mailman.qth.net
Subject: Re: [Collins] wider audio for 51J4?

> j-4 still has 2 stages of IF selectivity after the mechanical
> filter. the j-4 is a j-3 with the 1st IF transformer replaced
> with the mechanical filter kit.
> 
> Several folks have indicated that lab based j-4's didn't use a
> 6 KHz mechanical filter, they used some form of capacitor coupled
> network. I don't have a schematic for this but would greatly
> appreciate one.  Conceivably one could make up their own
> tuned circuit to use in place of the 6 KHz filter.  Anyone
> know the terminating impedances for the mechanical filters??

It could be as simple as a double tuned IF transformer. Probably with 100 pf for tuning capacitance which is what most tube impedance mechanical filters run. It would be possible to design a capacitor coupled filter using miniature IF transfomrs. I did a three coil one for the Ditto radio a few years back. I'm not home but I recall, I used three IF transformers from Mouser with a selected coupling capacitor or tow to achieve the desired bandwidth and it worked reasonably well.
> 
> If you want to open up the BW even more then you would need
> to change the coupling caps in the IF transformers as well.
> This was done in certain j-2's.

And maybe stagger tune all the IF transformers.


> 
> You are not the first to ask for dead mechanical filters.
> I've asked previouly to no avail.
> 
> Bill, Ve3jhu
> 
> 
73, Jerry, K0CQ, Technical Advisor to the CRA