I was hoping you'd remember the article, Peter, and perhaps have a copy on hand, as all I have
are the Technical Topics Scrapbooks, and I'm not sure which box they got packed into. It
did generate a lot of feedback on whether the old receivers actually needed a product detector
or not. Many have marginal BFO drive for good demodulation at high signal levels, so backing off
the RF gain control was an important part of the process for CW.
I'm assuming that the R-390 has a BFO, but the injection level and detector linearity are not
optimum for clean SSB, which is why an external product detector is desirable.