I’ve suspected for a long time that the BFO signal is weak. Today I made an experiment that strengthened that hypothesis but also encouraged me to do something about it.
While listening to a strong station on SSB I have to turn the RF gain way down to prevent terrible distortion. That makes the sound in the speaker rather low, even with AF gain at max. I can get much louder sound on AM.
While listening to a strong local station I turned on an RF signal generator and tuned it for zero beat. I could now turn up the RF gain and listen to loud and clear sound. That’s encouraging and promises that if I could get a stronger BFO signal injected I would significantly improve SSB reception.
When I first looked at the schematic a while ago I couldn’t find what path the signal takes from the BFO to the IF chain. I suspected a misprint of the schematic. However, the newer HR-10B schematics are exactly the same. You’d expect such a mistake to be corrected. In fact, Heathkit used to deliver errata sheets and updates to their manuals.
As it happens the BFO triode is one part of a tube (6EA8) that contains the last IF amp pentode. That is where the BFO signal is supposed to be mixed in. But there is no input for the BFO signal:
Best guess is that there is internal leakage in the tube that gets the signals mixed.
So what to do?
One simple idea is to connect a capacitor from the anode of the triode (pin 1) to the screen grid of the pentode (pin 3). The two .005 capacitors may swallow a lot of that signal, but there should still be more left than just internal leakage. I think.
Another idea would be to twist together wires to the anode and the screen grid so that the wires makes a simple transformer. That transformer would also unavoidably become a capacitor. I would have to think through the phasing in that case.
This is where I would like to hear your opinion.
Manual, including schematic for HR-10(B) Searchable