[Hallicrafters] S20R BFO dead

[Carl]

Halli had a great idea when they had variable BFO injection on the 
SX-17. Then they dropped it. Ive found on my SX-25 and others that a 
50-100pf mica cap raises the injection to a point that improves CW and 
SSB reception without having to ride the RF gain as much.

Ive done that to other brands also with good results.

Carl
KM1H


----- Original Message ----- 
From: "SX-25" <telegrapher at hotmail.com>
To: <hallicrafters at mailman.qth.net>
Sent: Wednesday, April 22, 2009 5:10 PM
Subject: Re: [Hallicrafters] S20R BFO dead


> Steve,
>
>     I have never worked on an S-20R but I have worked on a lot of 
> SX-25s which were descendants from the S-20R.
> There are two areas I have found to be problematic in the BFO 
> circuitry of the SX-25 (and I would presume similarly your S-20R). 
> Obviously do the housekeeping first: 6J5 good, switch SW-3 good, IF/RF 
> alignment good (or at least fair).
>
>     Start with your connection from the BFO coil going to the 6SQ7 
> second detector. Hallicrafters used  an arrangement of two wires 
> twisted together to induce the beat oscillation into the second 
> detector. I typically can go from a strong beat note to none at all by 
> moving this twisted pair arrangement around. So I get rid of it in my 
> SX-25s. If you measure the capacitance of that twisted pair of wires 
> it runs around 2 to 5 pf. So I eliminate it and insert a capacitor of 
> between 2 and 5 pf in its place.
>
>     Next, go over the oscillator with a fine-tooth comb. You will 
> often find the terminals wiggly and wobbly and making dubious contact 
> with the wire in the coil. Remove the oscillator coil from its can and 
> give it a good once over. Draw diagrams as you do this and it will be 
> easier when it's time to put it back together; but it looks more 
> complicated than it really is. Once you have the BFO coil re-assembled 
> and all connections intact, solder the coil's shroud can to the 
> mounting bracket. This too has a stabilizing effect on lessens the 
> BFO's variance-caused frequency wobble.
>
>     Hope this helps. If I am speaking out of turn here because I have 
> no S-20R experience and if I am leading you astray, I apologize. 
> However this has been my experience with SX-25s.
>
>     Rotsa ruck!
>
>     WA9VLK
>
>
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