Since the 1.610 MHz received signal was noisy, a 2.5 mH RF choke was connected across the output - to no avail. Then, on an impulse, when it was connected across the OA90 diode there was an enormous increase in signal strength.
Tuning up later with the converter inadvertently switched off, an equally strong signal was found on 1.830 MHz.
A real effective tripler had been stumbled upon making the oscillator redundant.
So out it went, leaving only diode and choke to do the job.
The result - a chance minimalist project!