Communication receivers designed prior to the 1960's typically had only a simple
diode detector. This was fine for AM and CW reception as the receiver's AVC was
turned off during CW reception since the BFO would upset the audio derived AVC
voltage. To receive SSB, the BFO was needed, so AVC wasn't available. In addition, in
order to get the proper BFO injection, the receiver's RF gain would have to be
adjusted, depending upon the signal strength of the SSB signal.
The product detector resolved this problem, allowing AVC use with the BFO on.
Thus a product detector is typically a very worthwhile addition to older receivers.
We built this product detector specifically for a National HRO-60 receiver, but the
circuit and the modifications to the receiver can be adapted to many earlier
receivers.
The detector plugs into the HRO's NFM adapter socket. To receive SSB signals
through the detector, the HRO-60 mode switch is simply placed in the NFM
position. A few receiver circuit modifications are necessary in order energize the
BFO in the NFM mode position and to route the BFO signal to the NFM socket.