Interesting cheat

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.

BFO schematic. Note that there is no output path! The on/off switch is connected to the 125 V DC supply.

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:

2nd (last) IF amp. Screen grid gets clean DC stepped down from the 125V supply line. T3 connects to diode detector.

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.

In depth

Manual, including schematic for HR-10(B) Searchable

2 thoughts on “Interesting cheat”

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