The BFO circuit is mounted on a small terminal strip in the upper center of this photo.  

     The only thing that might be critical is where to place the end of BFO output wire.  I found that everything works best if you locate the end of the wire near (but not too near) the wire that goes from the 1st IF transformer to the grid of the IF amplifier tube.  If you place the wire too close and couple the signal too strongly, you will "swamp" out the IF amplifier tube.  If your off the air signals appear weak and the normal static goes quiet when you turn on the BFO, it's because the IF tube is being "swamped" and you need to move the output wire further from the grid wire.  Actually, turning on the BFO will tend to make the normal static (white noise) noticeably louder when the BFO is turned on, if everything is located properly.

     Finally, where does the power for this module come from?  In other words, how to power it up?  This module draws only a tiny, tiny amount of current and so it will not load down any circuits it gets its power from.  This module will work well with any voltage from 5 to 15 volts.  Almost all tube type radios have a source of low voltage DC.  In the case of my S-120 radio, I simply tapped the +6 VDC that appears at the cathode of the audio amplifier tube (its biasing circuit) and for my Fairbanks Morse, I simply wired the module in series with the cathode of the IF tube and it works splendidly without any other coupling. 

Connecting the module directly to the anode of the IF amplifier tube.
This not only supplies the module (shown in the tiny box) with DC
power, but it injects the signal at the cathode too.
The BFO's carrier wave mixes with the IF signal to produce an
"artificial" (or recovered) AM signal that can be detected by the V3 tube.
This module was built on a small piece of perfboard.
Note the switch that grounds out the AVC line when the BFO is active.

Tapping the audio amplifier tube for 6 VDC to run the BFO module

This module draws only a tiny amount of current and will not load down the
cathode circuits shown above.


Interfacing the BFO module with the receiver

     This module has been designed to operate with radios having an intermediate frequency (IF) of 455 KHz and all the values shown are for oscillation on this frequency.  Be aware that the basic circuit be redesigned for other IF frequencies such as 100 KHz (sometimes used in dual conversion radios).

     First a little discussion of what a BFO signal does and why it is needed for SSB and CW, but not for ordinary AM broadcasts.  Amplititude Modulation (AM) is the oldest method of impressing the voice (or music) onto a radio wave so that it may be received tens to hundreds of miles away.  As simple as the AM transmitter is in theory, the radio wave it puts out is rather complex.  We will not go into the complexity of the AM signal (called its "modulation envelope") except to say that there are two sideband waves and one carrier wave that goes out over the air.  It is in the sidebands where the audio is located, but the audio doesn't make any sense unless there is a blank carrier wave present.  We say that the sidebands must have a carrier wave to "heterodyne" against, but we won't go into heterodyining here.  

     To concentrate all a transmitter's energy into JUST the wave that carries the audio, circuits in a SSB transmitter first eliminate the blank carrier wave and then filter out either the upper sideband or the lower sideband so than only one sideband remains.  This one sideband, the single sideband, is then amplified to hundreds of watts and then that power is coupled to an antenna for broadcast.  This is an extremely effective and efficient way to transmit from an energy standpoint, but SSB has other benefits when it comes to such things as "selective fading" and many other technical things that we won't go into here.  A big problem comes in when that signal is received and we want to turn it back into voice audio we can listen to.  Without a BFO it is impossible to make out what is being said because it sounds just like Donald Duck squawking.  The question remains, how to recover the original audio even though the original carrier wave is missing?

     This is where the BFO and its "artificial carrier wave" comes in.  In the "front end" of our receiver, the off-the-air SSB signal we want to listen to has been transformed into a series of waves, all around 455 KHz, through the superheterodyne process (another thing we won't go into here), but because it is a SSB signal, it is without a carrier wave and those signals are now residing in the IF stage of the receiver.  The BFO, oscillating at 455 KHz, puts out a blank wave that can substitute for this missing carrier if it is "mixed" (or heterodyned) with the IF signal.  In radios designed specifically for SSB, the BFO's signal is mixed (heterodyned) with the signal in a device called a "product detector" (yep, we won't go into that either).  The product detector then puts out an audio signal that can be amplified and sent to a speaker and the recovered voice sounds much more natural and much less like Donald Duck.  Most old radios are very unlikely to have a product detetector stage, but for sure it will have an AM diode detector stage and this kind of detector is quite good enough to turn an AM signal into sound (audio) that can be amplified and heard.

     If we mix the BFO's artificial carrier wave in the IF's tube, we will create a signal that is so close to being identical to an ordinary AM signal, the diode detector stage won't know the difference and it will create audio that we can amplify and send to our speaker.  The careful introduction of the BFO's signal into the IF tube' input and making the IF tube act as a mixer will create an artificial AM signal that the diode detector turns into sound.  The phrase "careful introduction of the BFO's signal" means that we can't just jam the BFO's signal into the input of the tube (its signal grid), but it must be done with the BFO wire with signal on it, not too close to the grid wire so that the BFO's signal doesn't overwhelm the IF tube (swamp it).  Since the BFO's signal level is so high (volts) and the IF grid can only handle microvolts, the "coupling" to the grid must be very light.  By the way, this coupling can also be through the cathode circuit too as was done with my Fairbanks Morse receiver in a kind of special way.

My S-120's BFO
Note the very light coupling of the BFO's signal to the grid
wire of V2 (IF amplifier tube).  This causes the signal in V2 to
mix (or heterodyne) with the artificial carrier wave produced
by the BFO.  The output of T6 then is demodulated by V3's
diode detector and is turned into sound.
Notice the switch circled in red.  This switch disables the
automatic volume control (AVC) when the BFO is in use.

     There is one final thing about using a BFO with a radio with a standard AM (diode) detector and that is the necessity to disable the Automatic Volume (or Gain) Control also known as the AVC (or AGC).  The reason the AVC must be disabled is because the BFO puts out a strong signal on the radio's IF frequency and the AVC circuits (usually a Hazletine type) interprets the BFO as a very strong AM signal (it is the same as a strong carrier wave) and so causes a large negative voltage to appear on the AVC line and that drastically reduces the ability of the IF amplifier to ... well ... to amplify.  When the IF amplifier is "cut off" by too much AVC voltage, the radio essentially goes dead and so there must be some way to "short" the AVC line to circuit ground.

     If you wish to add this BFO to a radio that has never had any kind of BFO and does not presently have a way to disable its AVC, you must install a switch to disable the AVC or this module won't work for you.  Many radios such as the S-120 already have a disable switch, but my Fairbanks Morse didn't, so I had to add one to ground out its AVC line.