[Hallicrafters] S20R BFO dead

[Carl]

I said that the large cap improved the INJECTION. The stability is a 
seperate issue but the way a SX-25 drifts its hard to tell.

Carl
KM1H


----- Original Message ----- 
From: "SX-25" <telegrapher at hotmail.com>
To: <Stevek0jw at aol.com>; <hallicrafters at mailman.qth.net>
Sent: Thursday, April 23, 2009 9:23 AM
Subject: Re: [Hallicrafters] S20R BFO dead


> Glad there might have been something meaningful in my response to your 
> post. Yes. the coil is a cantankerous component and, excepting for 
> one,  every SX-25 I have restored (6 so far) has had a problem with 
> the coil. The terminals are crimped/stapled into the core and, with 
> time, become loose. I wonder if the torque of rotating the adjustment 
> screw and/or knob shaft adds to the eventual loosening of those 
> terminals and attendant flexing/fatiguing of the wire of its coil. If 
> you don't have continuity through the coil, don't be afraid to 
> sacrifice a turn to produce enough wire to strip enough varnish and 
> resolder to the terminal. The parameters are not that tight. Once 
> everything is ship-shape I drop some Q-Dope on the terminal splays 
> inside the coil to keep it from rocking loose again.
>
> KM1H suggested a 50 pf cap and says it improves the stability of the 
> BFO. I haven't tried that but I've got an SX-25 on the bench now and 
> am going to see how that works. Maybe I learned something too from 
> YOUR BFO posting! The 2-5 pf cap that I came up with was based on my 
> measurement of the capacity of the original twisted pair coupling of 
> several SX-25s. If you have problems coming up with a small enough 
> capacitor for the job, two larger value capacitors in series works 
> too.
>
> You hear many derisive comments about the BFO being too unstable to be 
> practicable. Baloney! I suspect this is more due to guys who have 
> other issues with their Hallicrafters receivers. Dirty contacts on the 
> venerable band switch can make the beat note sound awful. The 6K8 
> mixer tube is often a culprit too.  I use my SX-25 nightly and am an 
> all CW op and copying CW with it is a pleasure. So it is indeed 
> possible to massage your receiver to a point of great BFO stability 
> with a little patience and care.
>
> WA9VLK
>
>
>
>
>
>
> Thanks for the tips! It looks like the BFO coil is the culprit, and I 
> appreciate the tip on the 2-5pf. cap where the pigtail is, I will try 
> that as well. I appreciate the suggestions!
> Steve
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