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.
Click on any picture to enlarge it
Installed product detector.
A surplus plug in module was used.
Backside of the detector.
Side view of the detector.
Schematic of the detector.
Modified HRO-60 block diagram.
Modifications of the HRO-60.