I have restored a Hallicrafters S77A . I could not receive SSB signals no
matter how I twiddled the CW tone control and even CW tones were weak. I
guess it was not designed to receive SSB. Any way, I had this idea to change
the injection point for the BFO signal. Originally the BFO was coupled to
the plate of the second detector by a 2 pf 'gimmick'. I moved the BFO
injection to the screen on the second IF tube by adding a 680 ohm resistor
between the screen and screen supply and connecting a 100 pf cap from screen
to BFO output. After recentering the BFO in the IF bandpass I tried it on
the 40 meter band. It worked! Sound is great and not too difficult to tune.
It drifts and has to be readjusted periodically but never the less it is
usable for SW listening. I have looked around for schematics and none that
I have found use this circuit. Could some of you pros explain why my change
works so well and also why it wasn't used way back when.


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