Ahh... the old R390 was the first radio I ever learned to repair
I could tweek that thing to an mds of -127 dbm
I owned a R390. Sadly I sold it. Big, clunky, but an amazing receiver.
Mechanical tuning mechanism was unique.
The PTO on the S-line is also unique. Slug tuned, a real challenge to tweak. Had to buy at Bristol (splined) allen wrench to accomplish some fine tuning. Mine has the best mechanical CW filter. Wonderful quiet receiver. Not that sensitive on higher frequencies, but can really handle front end overload.