First the electrical differences.

The R390 employs 2 RF amplifier stages and a more complex frequency
management system but has the same triple conversion layout as the R390-A.
The tubes used are also different in many cases such as the use of 6AJ5
in place of 6AK5 (see elsewhere on this page) and 6BJ6 in place of 5749.
The IF system uses L-C type filtering to obtain essentially the same
bandwidths but the crystal filter for the two narrowest bandwidths is
essentially the same in both radios. Many consider that this makes for
a smoother audio sound than in the R390-A which employs mechanical filters.
The audio amplifiers in both radios are the same but the R390 employs a
much more complex voltage regulator system for the B+ supply. This employs
6082 tubes which generate a great deal of heat which can lead to problems.
The first and second crystal oscillators are contained in a separate
sub assembly which is driven by a shaft connected to the main gearbox
via a small Oldham coupler.
The calibrator is also on a separate sub assembly mounted underneath
the chassis.
The internal coax connections are made via standard BNC connectors.
In the R390-A these are replaced by a mini variety which is difficult
to handle sometimes.

Mechanically there are great differences although the system used is similar
with slug racks and cams. The first and second converters are placed in line
with the RF racks rather than behind them as in the R390-A.
There will be a more detailed discussion on the mechanics of the R390 elsewhere
on this page.
Suffice it to say here do not attempt any adjustments to the mechanical
or electrical alignment without adequate instructions. There are a couple
of traps that will result in instant foul up of the mechanical sync!!!!