[Hallicrafters] S20R BFO dead

[SX-25]

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